Additional Questions - Chapter 7 Thermodynamics 11th Chemistry Guide Samacheer Kalvi Solutions - SaraNextGen [2024-2025]

Updated By SaraNextGen

On April 24, 2024, 11:35 AM

Additional Questions Solved
I. Choose the correct answer from the following:
Question 1.
When a liquid boils, there is .........
(a) increase in entropy
(b) a decrease in entropy
(c) increase in heat of vaporization
(d) an increase in free energy
Answer:
(a) Increase in entropy
Question 2.
if $\Delta \mathrm{G}$ for a reaction is negative, the change is
(a) spontaneous
(b) non-spontaneous
(c) reversible
(d) equilibrium
Answer:

(a) spontaneous
Question 3.
In which of the following process. the process is always non-feasible?
(a) $\Delta \mathrm{H}>\mathrm{O}, \Delta \mathrm{S}>\mathrm{O}$
(b) $\Delta \mathrm{H}<\mathrm{O}, \Delta \mathrm{S}>\mathrm{O}$
(c) $\Delta \mathrm{H}>\mathrm{O}, \Delta \mathrm{S}<\mathrm{O}$
(d) $\Delta \mathrm{H}<\mathrm{O}, \Delta \mathrm{S}>\mathrm{O}$
Answer:
(c) $\Delta \mathrm{H}>\mathrm{O}, \Delta \mathrm{S}<\mathrm{O}$
Question 4.
Change in Gibbs free energy is given by
(a) $\Delta \mathrm{G}=\Delta \mathrm{H}+\mathrm{T} \Delta \mathrm{S}$
(b) $\Delta \mathrm{G}=\Delta \mathrm{H}-\mathrm{T} \Delta \mathrm{S}$
(c) $\Delta \mathrm{G}=\mathrm{H} \times \mathrm{T} \Delta \mathrm{S}$
(d) none of these
Answer:
(b) $\Delta \mathrm{G}=\Delta \mathrm{H}-\mathrm{T} \Delta \mathrm{S}$
Question 5.
Which of the following process is feasible at all ternperatures?
(a) $\Delta \mathrm{H}>\mathrm{O} . \Delta \mathrm{S}>\mathrm{O}$
(b) $\Delta \mathrm{H}>\mathrm{O} . \Delta \mathrm{S}<\mathrm{O}$
(c) $\Delta \mathrm{H}<\mathrm{O} . \Delta \mathrm{S}>\mathrm{O}$
(d) $\Delta \mathrm{H}<\mathrm{O} . \Delta \mathrm{S}<\mathrm{O}$
Answer:
(c) $\Delta \mathrm{H}<\mathrm{O} . \Delta \mathrm{S}>\mathrm{O}$

Question 6.
Calculate the entropy change during the melting of one mole of ice into water at $0^{\circ} \mathrm{C}$ and $1 \mathrm{~atm}$ pressure. Enthalpy of fusion of ice is $6008 \mathrm{~J} \mathrm{Mole}^{-1}$.
(a) $22.007 \mathrm{~J} \mathrm{~K}^{-1} \mathrm{Mole}^{-1}$
(b) $22.007 \mathrm{~J} \mathrm{~K}^{-1} \mathrm{Mole}^{-1}$
(c) $220.07 \mathrm{~J} \mathrm{~K}^{-1} \mathrm{Mole}^{-1}$
(d) $2.2007 \mathrm{~J} \mathrm{~K}^{-1} \mathrm{Mole}^{-1}$
Answer:
(a) $22.007 \mathrm{~J} \mathrm{~K}^{-1} \mathrm{Mole}^{-1}$
Hint:
Enthalpy of fusion of ice $6008 \mathrm{~J} \mathrm{~mol}^{-1}=\Delta \mathrm{H}$
$
\Delta \mathrm{S}=\frac{\Delta \mathrm{H}}{\mathrm{T}_{\mathrm{m}}} \mathrm{T}_{\mathrm{m}}=0^{\circ} \mathrm{C}=273 \mathrm{~K}
$
$
\text { Entropy of fusion }=\frac{\text { Enthalpy of fusion of ice }}{\text { Melting point (Kelvin) }}
$

$
\Delta \mathrm{S}=\frac{6008}{273}=22.007 \mathrm{~J} \mathrm{~K}^{-1} \mathrm{~mol}^{-1}
$
Question 7.
Calculate the entropy change of a process $\mathrm{H}_2 \mathrm{O}_{(1)} \rightarrow \mathrm{H}_2 \mathrm{O}_{(\mathrm{g})}$ at $373 \mathrm{~K}$. Enthalpy of vaporization of water is $40850 \mathrm{~J} \mathrm{Mole}^{-1}$.
(a) $120 \mathrm{~J} \mathrm{~K}^{-1} \mathrm{~mol}^{-1}$
(b) $9.1 \times 10 \mathrm{~J} \mathrm{~K}^{-1} \mathrm{~mol}^{-1}$
(c) $9.1 \times 10 \mathrm{~J} \mathrm{~K}^{-1} \mathrm{~mol}^{-1}$
(d) $109.52 \mathrm{~J} \mathrm{~K}^{-1} \mathrm{~mol}^{-1}$
Answer:
(d) $109.52 \mathrm{~J} \mathrm{~K}^{-1} \mathrm{~mol}^{-1}$
EnthaLpy of vaporization of water is $=\Delta \mathrm{H}=40850 \mathrm{~J} \mathrm{~mol}^{-1}$
Boiling point $=373 \mathrm{~K}=\mathrm{T}_{\mathrm{b}}$
$
\Delta \mathrm{S}=\frac{\Delta \mathrm{H}}{\mathrm{T}_{\mathrm{b}}}=\frac{40850}{373}=109.517=109.52 \mathrm{JK}^{-1} \mathrm{~mol}^{-1}
$
Question 8.
The final temperature of an engine whose initial temperature is $400 \mathrm{~K}$ and having efficiency $25 \%$.
(a) $200 \mathrm{~K}$
(b) $400 \mathrm{~K}$
(c) $300 \mathrm{~K}$
(d) $450 \mathrm{~K}$
Answer:
(c) $300 \mathrm{~K}$
Initial temperature $=\mathrm{T}_1=400 \mathrm{~K}$

$
\begin{aligned}
& \text { Final temperature }=\mathrm{T}_2=? \\
& \text { Efficiency }=25 \% \\
& \eta=\left[\frac{T_1-T_2}{\mathrm{~T}_1}\right] \times 100 \\
& 25=\left[\frac{400-\mathrm{T}_2}{400}\right] \times 100 \\
& \frac{400-T_2}{4}=25 \\
& 400-\mathrm{T}_2=100 \\
& -\mathrm{T}_2=100-400 \\
& \therefore-\mathrm{T}_2=-300 \\
& \mathrm{~T}_2=300 \mathrm{~K}
\end{aligned}
$
Question 9.
When solid melts there is
(a) an increase of entropy
(b) a decrease in entropy
(c) an increase in free energy
(d) an increase of heat of fusion

Answer:
(a) an increase of entropy
Question 10 .
The unit of entropy is .........
(a) $\mathrm{J} \mathrm{K}^{-1} \mathrm{~mol}^{-1}$
(b) $\mathrm{J} \mathrm{mol}^{-1}$
(c) $\mathrm{J} \mathrm{K} \mathrm{mol}^{-1}$
(d) $\mathrm{J}^{-1} \mathrm{~K}^{-1}$
Answer:
(a) $\mathrm{JK}^{-1} \mathrm{~mol}^{-1}$
Question 11.
If $G=0$. then the process is
(a) equilibrium
(b) spontaneous
(c) non-spontaneous
(d) none of these
Answer:
(a) equilibrium
Question 12 .
The standard conditions for $\mathrm{G}^{\circ}$ are
(a) $1 \mathrm{~mm} \mathrm{Hg} / 25^{\circ} \mathrm{C}$
(b) $1 \mathrm{~atm} / 25 \mathrm{~K}$
(c) $1 \mathrm{attn} / 298 \mathrm{~K}$
(d) $1 \mathrm{~atm} / 0 \mathrm{~K}$
Answer:
(c) $1 \mathrm{~atm} / 298 \mathrm{~K}$

Question 13.
The efficiency of engine working between 100 to $400 \mathrm{~K}$
(a) $25 \%$
(b) $75 \%$
(c) $100 \%$
(d) $50 \%$
Answer:
(b) $75 \%$
Solution:
$
\begin{aligned}
& \eta=\left[\frac{\mathrm{T}_1-\mathrm{T}_2}{\mathrm{~T}_1}\right] \times 100 \\
& \mathrm{~T}_1=400 \mathrm{~K} \\
& \mathrm{~T}_2=100 \mathrm{~K} \\
& \therefore \eta=\left[\frac{\mathrm{T}_1-\mathrm{T}_2}{\mathrm{~T}_1}\right] \times 100 \\
& =\frac{300}{400} \times 100=\frac{30000}{400}=\frac{300}{4} \\
& =75 \%
\end{aligned}
$

Question 14.
Entropy is a function.
(a) state
(b) path
(c) defined
(d) undefined
Answer:
(a) state
Question 15 .
An efficiency of an engine is always
(a) $=0 \%$
(b) $>100 \%$
(c) $<100 \%$
(d) $=100 \%$
Answer:
(c) $<100 \%$
Question 16.
if system moves from ordered state to disordered state, its entropy
(a) decreases
(b) increases
(c) become zero
(d) increases then decreases
Answer:
(b) increases
Question 17.
In a reversible process, the entropy of Universe is ..........
(a) greater than zero
(b) less than zero

(c) equal to zero
(d) remains constant
Answer:
(c) equal to zero
Question 18 .
In which of the following entropy increases?
(a) Condensation of water vapour
(b) Liquid freezes to solid
(c) Sublimation
(d) Gas freezes to a solid
Answer:
(c) Sublimation
Question 19.
Which of the following is a state function?
(a) $q$
(b) $\Delta \mathrm{q}$
(c) $\mathrm{W}$
(d) $\Delta \mathrm{S}$

Answer:
(d) $\Delta \mathrm{S}$
Question 20.
Which of the following will have highest $\Delta \mathrm{H}_{\text {vap }}$ Value?
(a) Acetone
(b) Ethanol
(c) Carbon tetrachloride
(d) Chloroform
Answer:
(b) Ethanol
Question 21.
Which of the following is not a state function?
(a) $\mathrm{S}$
(b) $\mathrm{H}$
(c) G
(d) $q$
Answer:
(d) $\mathrm{q}$
Question 22.
The net work done by the system
(a) $\mathrm{w}-\mathrm{P} \Delta \mathrm{V}$
(b) $\mathrm{w}+\mathrm{P} \Delta \mathrm{V}$
(c) $-w+P \Delta V$
(d) $-\mathrm{w}-\mathrm{P} \Delta \mathrm{V}$
Answer:
(d) $-\mathrm{w}-\mathrm{P} \Delta \mathrm{V}$

Question 23.
$-\Delta \mathrm{G}$ is the net work done by the system except
(a) electrical work
(b) expansion work
(c) chemical work
(d) photo chemical work
Answer:
(b) expansion work
Question 24.
The enthalpy of vapourisation of a liquid is $30 \mathrm{~kJ}^{-1} \mathrm{~mol}^{-1}$ and the entropy of vaporization is 75 $\mathrm{JK} \mathrm{mol} \mathrm{m}^{-1}$ The boiling point of the liquid at $1 \mathrm{~atm}$ is .........
(a) $250 \mathrm{~K}$
(b) $400 \mathrm{~K}$
(c) $450 \mathrm{~K}$
(d) $600 \mathrm{~K}$
Answer:
(b) $400 \mathrm{~K}$
Solution:

$
\begin{aligned}
& \Delta \mathrm{H}_{\text {vap }}=30 \mathrm{~kJ} \mathrm{~mol}^{-1} \times 1000=30000 \mathrm{~J} \mathrm{~mol}^{-1} \\
& \Delta \mathrm{S}_{\text {vap }}=75 \mathrm{~J} \mathrm{~mol}^{-1} \\
& \mathrm{~T}_{\mathrm{b}}=\text { Boiling point }=? \\
& \Delta \mathrm{S}_{\text {vap }}=\frac{\Delta \mathrm{H}_{\text {vap }}}{\mathrm{T}_{\mathrm{b}}} \\
& \therefore \mathrm{T}_{\mathrm{b}}=\frac{\Delta \mathrm{H}_{\text {vap }}}{\Delta \mathrm{S}_{\text {vap }}}=\frac{30000}{75}=400 \mathrm{~K} \\
& \mathrm{~T}_{\mathrm{b}}=400 \mathrm{~K} \\
& \mathrm{~Tb}=400 \mathrm{~K}
\end{aligned}
$
Question 25 .
In a reversible process $\Delta \mathrm{S}_{\mathrm{sys}}+\Delta \mathrm{S}_{\text {surr }}$ is
(a) $>0$
(b) $<0$
(c) $\geq 0$
$(\mathrm{d})=0$
Answer:
$(\mathrm{d})=0$
Question 26.
Which of the following does not result in an increase in the entropy?
(a) Crystallization of sucrose from solution
(b) Rusting of Iron
(c) Conversion of ice to water
(d) Vaporization of camphor
Answer:
(a) Crystallization of sucrose from solution

Question 27.
The standard free energy change $\left(\Delta \mathrm{G}^{\circ}\right)$ is related to equilibrium constant $(\mathrm{K})$ as
(a) $\Delta \mathrm{G}^{\circ}=-1303 \mathrm{RT}$ in K
(b) $\Delta \mathrm{G}^{\circ}=2.303 \mathrm{RT} \log \mathrm{K}$
(c) $\Delta \mathrm{G}^{\circ}=\mathrm{RT}$ in $\mathrm{K}$
(d) $\Delta \mathrm{G}^{\circ}=-2.303 \mathrm{RT} \log \mathrm{K}$
Answer:
(d) $\Delta \mathrm{G}^{\circ}=-2.303 \mathrm{RT} \log \mathrm{K}$
Question 28.
enropy change involved in the conversion of I mole of liquid water at $373 \mathrm{~K}$ to vapour at the same temperature will be $\left(\Delta \mathrm{H}_{\text {vap }}=2.257 \mathrm{~kJ} \mathrm{~g}^{-1}\right)$
(a) $0.119 \mathrm{~kJ}$
(b) $0.109 \mathrm{~kJ}$
(c) $0.129 \mathrm{k}$
(d) $0.120 \mathrm{~kJ}$
Answer:
(b) $0.109 \mathrm{~kJ}$
Solution:
$
\begin{aligned}
& \Delta \mathrm{H}_{\text {vap }}=2.257 \mathrm{~kJ} \mathrm{~g}^{-1} \\
& 1 \mathrm{~mole}^{\mathrm{H}} \mathrm{H}_2 \mathrm{O}=18 \mathrm{~g} \\
& \therefore \Delta \mathrm{H}_{\text {vap }} \text { for } 1 \mathrm{~mole}=2.257 \mathrm{x} 18=40.626 \mathrm{k} \mathrm{J} \mathrm{g}^{-1} \\
& \mathrm{~T}_{\mathrm{b}}=373 \mathrm{~K} \\
& \Delta \mathrm{H}_{\text {vap }}=\frac{40.626}{373}=0.1089 \mathrm{KJ} \mathrm{g}^{-1} \\
& =0.109 \mathrm{~kJ} \mathrm{~mol}^{-1}
\end{aligned}
$

Question 29.
Which of the following units represent largest amount of energy?
(a) calories
(b) Joule
(c) erg
(d) $\mathrm{eV}$
Answer:
(a) calories
Question 30.
The intensive property among these quantities is
(a) mass
(b) volume
(c) enthalpy
(d) mass / volume
Answer:
(d) mass /volume

Question 31 .
System in which there is no exchange of matter, work or energy from surrounding is
(a) closed
(b) isolated
(c) adiabatic
(d) isothermal
Answer:
(b) isolated
Question 32 .
Which of the following is not an intensive property?
(a) Pressure
(b) Density
(c) Volume
(d) Surface tension
Answer:
(c) Volume
Question 33.
A gas can expand from loo $\mathrm{ml}$ to $250 \mathrm{ml}$ under a constant pressure of $2 \mathrm{~atm}$. The work done by the gas is .........
(a) $-30.39 \mathrm{~J}$
(b) $25 \mathrm{~J}$
(c) $5 \mathrm{~kJ}$
(d) $16 \mathrm{~J}$
Answer:
(a) $-30.39 \mathrm{~J}$
Solution:
$\Delta \mathrm{V}=$ expansion in volume $=100$ to 250
$\Delta \mathrm{V}=\mathrm{V}_2-\mathrm{V}_2=250-100$

$\Delta \mathrm{V}=150 \mathrm{ml}=0.15$ litre
Work done $=$ ?
Pressure $=2 \mathrm{~atm}$
$
\begin{aligned}
& \mathrm{w}=-\mathrm{P} \Delta \mathrm{V} \\
& =-2 \times 0.15 \text { litre } \times 101.3 \mathrm{JL}^{-1} \mathrm{~atm}^{-1} \\
& =-30.39 \mathrm{~J} .
\end{aligned}
$
Question 34.
An ideal gas expands in volume from $1 \times 10^{-3} \mathrm{~m}^3$ to $1 \times 10^{-2} \mathrm{~m}^3$ at $300 \mathrm{~K}$ against a constant pressure at $1 \times 10^5 \mathrm{Nm}^5$. The work done is .........
(a) $-900 \mathrm{~J}$
(b) $900 \mathrm{~kJ}$
(c) $270 \mathrm{~kJ}$
(d) $-900 \mathrm{~kJ}$
Answer:
(a) $-900 \mathrm{~J}$

Question 35.
Identify the state quantity among the following
(a) $q$
(b) $q-w$
(c) $q+w$
(d) $q / w$
Answer:
(b) $q-w$
Question 36.
In general, for an exothermic reaction to be spontaneous
(a) temperature should be high
(b) temperature should be zero
(c) temperature should be low
(d) temperature has no effect
Answer:
(c) temperature should be low
Question 37.
Heat of neutralization of a strong acid by a strong base is a constant value because
(a) only $\mathrm{OH}^{+}$and $\mathrm{OH}^{-}$ions react in every case
(b) the strong base and strong acid react completely
(c) the strong base and strong acid react in aqueous solution
(d) salt formed does not hydrolyse
Answer:
(a) only $\mathrm{OH}^{+}$and $\mathrm{OH}^{-}$ions react in every case

Question 38 .
The heat absorbed at constant volume is equal to the system's change in
(a) enthalpy
(b) entropy
(c) internal energy
(d) free energy
Answer:

(c) internal energy
Question 39.
Heat of neutralization is always
(a) positive
(b) negative
(c) zero
(d) positive or negative
Answer:
(b) negative

Question 40.
The heat of formation $\mathrm{CO}$ and $\mathrm{CO}_2$ are $-26.4 \mathrm{Kcal}$ and $-94 \mathrm{Kcal}$ respectively. Heat of combustion of carbon monoxide will be
(a) $+26.4 \mathrm{KCal}$
(b) $-67.6 \mathrm{KCal}$
(c) $-120.6 \mathrm{~K} \mathrm{Cal}$
(d) $+52.8 \mathrm{~K} \mathrm{Cal}$
Answer:
(b) $-67.6 \mathrm{KCal}$
Solution:
$
\begin{aligned}
& \mathrm{C}+1 / 2 \mathrm{O}_2 \rightarrow \mathrm{CO} \Delta \mathrm{H}=-26.4 \text { K.cal } \\
& \mathrm{C}+\mathrm{O}_2 \rightarrow \mathrm{CO}_2 \Delta \mathrm{H}=-94 \mathrm{~K} . \mathrm{cal} \ldots .
\end{aligned}
$
Heat of combustion of $\mathrm{CO}$ is $\mathrm{C}+1 / 2 \mathrm{O}_2 \rightarrow \mathrm{CO}_2 \Delta \mathrm{H}=$ ?
Equation (1) is reversed.

$
\Delta \mathrm{H}=-67.6 \mathrm{~K} \text {. cal }
$
Question 41.
For the reaction $\mathrm{H}_2+\mathrm{I}_2 \rightleftharpoons 2 \mathrm{HI}, \Delta \mathrm{H}=12.40 \mathrm{Kcal}$ the heat of formation of $\mathrm{HI}$ is
(a) $12.4 \mathrm{Kcal} \mathrm{mol}^{-1}$
(b) $-12.4 \mathrm{Kcal} \mathrm{mol}^{-1}$
(c) $-6.20 \mathrm{Kcal} \mathrm{mol}^{-1}$
(d) $6.20 \mathrm{Kcal} \mathrm{mol}^{-1}$
Answer:
(d) $6.20 \mathrm{Kcal} \mathrm{mol}^{-1}$
Solution:
$
\begin{aligned}
& \mathrm{H}_2+\mathrm{I}_2 \rightarrow 2 \mathrm{HI} \Delta \mathrm{H}=12.40 \mathrm{~K} . \mathrm{cal} \\
& 1 / 2 \mathrm{H}_2+1 / 2 \mathrm{I}_2 \rightarrow \mathrm{HI}=\Delta \mathrm{H} / 2 \\
& \therefore \frac{12.40}{2}=6.20 \mathrm{~K} . \mathrm{cal} \mathrm{mol}^{-1}
\end{aligned}
$
Question 42.
Heat capacity is
(a) $\frac{d q}{d T}$
(b) dq.dT
(c) $\sum \mathrm{q} \cdot \frac{1}{d T}$
(d) none of these
Answer:
(a) $\frac{d q}{d T}$

Question 43.
The relation between $\mathrm{C}_{\mathrm{p}}$ and $\mathrm{C}_{\mathrm{v}}$ is
(a) $\mathrm{C}_{\mathrm{p}}-\mathrm{C}_{\mathrm{v}}=\mathrm{R}$
(b) $\mathrm{C}_{\mathrm{p}}+\mathrm{C}_{\mathrm{v}}=\mathrm{R}$
(c) $-285 \mathrm{KJ}$
(d) $\mathrm{R}-\mathrm{C}_{\mathrm{v}}=\mathrm{C}_{\mathrm{p}}$
Answer:
(a) $C_p-C_v=R$
Question 44.
The heat required to raise the temperature of a body by I $\mathrm{K}$ is called
(a) specific heat
(b) thermal capacity
(c) water equivalent
(d) none of these
Answer:
(b) thermal capacity
Question 45 .
Heat liberated when $100 \mathrm{ml}$ of $\mathrm{NN} \mathrm{NaOH}$ is neutralized by $300 \mathrm{ml}$ of in $\mathrm{HCl}$
(a) 22.92 Ici
(b) $17.19 \mathrm{~kJ}$
(c) $11.46 \mathrm{~kJ}$
(d) $5.73 \mathrm{Id}$
Answer:
Base $=\mathrm{V}_1=100 \mathrm{ml}$
$\mathrm{N}_1=1 \mathrm{~N}$
Acid $=\mathrm{V}_2=300 \mathrm{ml}$
$\mathrm{N}_2=1 \mathrm{~N}$
Enthalpy of neutralization of $1000 \mathrm{ml}=57.3 \mathrm{~kJ}$.
$\therefore$ Enthalpy of neutralization of $100 \mathrm{ml} \mathrm{x} 100=\frac{5.73 \mathrm{~kJ}}{1000} \times 100=5.73 \mathrm{~kJ}$.
Question 46.
In order to decompose $9 \mathrm{~g}$ of water, $142.5 \mathrm{~kJ}$ of heat is required. Hence enthalpy of formation of water is ..........
(a) $-142.5 \mathrm{~kJ}$
(b) $142.5 \mathrm{~kJ}$
(c) $-285 \mathrm{~kJ}$
(d) $285 \mathrm{~kJ}$
Answer:
(c) $-285 \mathrm{~kJ}$

Solution:
$
\begin{aligned}
& \mathrm{H}_2 \mathrm{O} \rightarrow \mathrm{H}_2+1 / 2 \mathrm{O}_2 \\
& 18(\mathrm{~g})
\end{aligned}
$
$9 \mathrm{~g} \mathrm{H}_2 \mathrm{O}$ is decomposed by $-142.5 \mathrm{~kJ}$ amount of heat.
$\therefore 18 \mathrm{~g} \mathrm{H}_2 \mathrm{O}$ will be decomposed by $\frac{142.5}{9} \times 18^2=+285 \mathrm{~kJ}$.
$\therefore 18 \mathrm{~g}$ of $\mathrm{H}_2 \mathrm{O}$ is formed by $-285 \mathrm{~kJ}$ amount of heat. (Evolution of heat $=-$ ve sign)

Question 47.
Assertion (A) : Combustion of all organic compounds is an exotherinic reaction ............ Reason (R) : The enthalpies of all elements in their standard state are zero. Which of the above statement isare not correct?
(a) both $A$ and $R$ are true and $R$ is the correct explanation of $A$
(b) both $A$ and $R$ are true and $R$ is not correct explanation of $A$
(c) both $A$ and $R$ are false
(d) $A$ is false but $R$ is true Answer:
(b) both $\mathrm{A}$ and $\mathrm{R}$ are true and $\mathrm{R}$ is not correct explanation of $\mathrm{A}$
Question 48.
Assertion (A) : Spontaneous process is an irreversible process and may be reversed by same external agency.
Reason (R) : Decrease in enthalpy is a contributory factor for spontaneity.
(a) both $\mathrm{A}$ and $\mathrm{R}$ are true and $\mathrm{R}$ is the correct explanation of $\mathrm{A}$
(b) both $\mathrm{A}$ and $\mathrm{R}$ are true and $\mathrm{R}$ is not correct explanation of $\mathrm{A}$
(c) both $A$ and $R$ are false
(d) $A$ is false but $R$ is true
Answer:
(b) both $A$ and $R$ are true and $R$ is not correct explanation of $A$
Question 49.
Assertion (A) : A liquid crystallizes into a solid and accompanied by decrease in entropy. Reason (R) : In crystals molecules are organised in an ordered manner.
(a) both $\mathrm{A}$ and $\mathrm{R}$ are true and $\mathrm{R}$ is the correct explanation of $\mathrm{A}$
(b) both $A$ and $R$ are true and $R$ is not correct explanation of $A$
(c) both $A$ and $R$ are false
(d) $A$ is false but $R$ is true
Answer:
(a) both $A$ and $R$ are true and $R$ is the correct explanation of $A$
Question 50.
Thermodynamics is applicable to
(a) macroscopic system only
(b) microscopic system only Thermodynamics
(c) homogeneous system only
(d) heterogeneous system only

Answer:
(a) macroscopic system only
Question 51
An isochoric process takes place at constant
(a) temperature
(b) pressure
(c) volume
(d) concentration
Answer:
(c) volume
Question 52.
For a cyclic process, the change in internal energy of the system is
(a) always + ve
(b) equal to zero
(c) always - ve
(d) none of the above
Answer:
(b) equal to zero
Question 53.
Which of the following properties is not a fùnction of state?
(a) Concentration
(b) Internal energy
(c) Enthalpy
(d) Entropy
Answer:
(u) Concentration
Question 54 .
Which of the following relation is true?
(a) $C_p>C_v$
(b) $\mathrm{C}_{\mathrm{v}}>\mathrm{C}_{\mathrm{p}}$
(c) $\mathrm{C}_{\mathrm{p}}=\mathrm{C}_{\mathrm{v}}$
(d) $\mathrm{C}_{\mathrm{p}}=\mathrm{C}_{\mathrm{v}}=0$
Answer:
(a) $\mathrm{C}_{\mathrm{p}}>\mathrm{C}_{\mathrm{v}}$

Question 55.
Which of the following always has a negative value?
(a) heat of reaction
(b) heat of solution
(c) heat of combustion
(d) heat of formation
Answer:
(c) beat of combustion
Question 56.
The bond energy depends upon
(a) size of the atom
(b) electronegativity
(c) bond length
(d) all of the above
Answer:
(d) all of the above
Question 57.
For an endothermic reaction
(a) $\Delta \mathrm{H}$ is -ve
(b) $\Delta \mathrm{H}$ is $+\mathrm{ve}$
(e) $\Delta \mathrm{H}$ is zero
(d) none of these
Answer:
(b) $\mathrm{AH}$ is +ve
Question 58.
The process depicted by the equation.
$
\mathrm{H}_2 \mathrm{O}_{(\mathrm{s})} \rightarrow \mathrm{H}_2 \mathrm{O}_{(1)}
$
$\Delta \mathrm{H}=+1.43 \mathrm{kcal}$ represents
(a) fusion

(b) melting
(c) evaporation
(d) boiling
Answer:
(a) fusion
Question 59
Which one is the correct unit for entropy?
(a) $\mathrm{KJ} \mathrm{mol}$
(b) $\mathrm{JK}^{-1} \mathrm{~mol}$
(c) $\mathrm{JK}^{-1} \mathrm{~mol}^{-1}$
(d) $\mathrm{KJ} \mathrm{mol}^{-1}$
Answer:
(c) $\mathrm{JK}^{-1} \mathrm{~mol}^{-1}$
Question 60.
A thermodynamic state function is a quantity
(a) used to determine heat changes
(b) whose value is independent of path
(c) used to determine pressure volume work
(d) whose value depends on temperature only
Answer:
(b) whose value is independent of path
Question 61.
For the process to occur under adiabatic conditions, the correct condition is
(a) $\Delta \mathrm{T}=0$
(b) $\Delta_p=0$
(c) $\mathrm{q}=0$
(d) $\mathrm{w}=0$
Answer:
(c) $q=0$

Question 62.
The enthalpies of all elements in their standard states are
(a) unity
(b) zero
(c) $<0$
(d) different for each element
Answer:
(b) zero
Question 63.
$\Delta U^{\ominus}$ of combustion of methane is $-\mathrm{X} \mathrm{kJ} \mathrm{mol}^{-1}$. The value of $\Delta \mathrm{He}$ is
(a) $=\Delta U^{\ominus}$
(b) $>\Delta U^{\ominus}$
(c) $<\Delta U^{\ominus}$
(d) 0
Answer:
(c) $<\Delta \mathrm{U} U^{\ominus}$
Question 64.
The enthalpy of combustion of methane, graphite and dihydrogen at $298 \mathrm{~K}$ are $-890.3 \mathrm{~kJ} \mathrm{~mol}^{-1}$ $-393.5 \mathrm{~kJ} \mathrm{~mol}^{-1}$ and $285.8 \mathrm{~kJ} \mathrm{~mol}^{-1}$ respectively. Enthalpy of formation of $\mathrm{CH}_4$ will be
(a) $-74.8 \mathrm{~kJ} \mathrm{~mol}^{-1}$
(b) $-52.27 \mathrm{k} \mathrm{mol}^{-1}$
(c) $74.8 \mathrm{~kJ} \mathrm{~mol}^{-1}$
(d) $+52.26 \mathrm{~kJ} \mathrm{~mol}^{-1}$
Answer:
(a) $-74.8 \mathrm{~kJ} \mathrm{~mol}^{-1}$
Solution:
$
\begin{aligned}
& \mathrm{CH}_4+3 \mathrm{O}_4 \rightarrow \mathrm{CO}+2 \mathrm{H}_2 \mathrm{O} \Delta \mathrm{H}_1=-890.3 \mathrm{~kJ} \mathrm{~mol}^{-1} \\
& \mathrm{C}+\mathrm{O}_2 \rightarrow \mathrm{CO}_2=-393.5 \mathrm{~kJ} \mathrm{~mol}^{-1} \ldots \ldots \ldots .(2)
\end{aligned}
$

$
\mathrm{H}_5+1 / 2 \mathrm{O}_2 \rightarrow \mathrm{H}_2 \mathrm{O}=-285.8 \mathrm{~kJ} \mathrm{~mol}^{-1}
$
Equation (1) is reversed.
Equation (3) $\times 2$

$\begin{aligned}
& \Delta \mathrm{H}_1=+890.3 \mathrm{~kJ} \mathrm{~mol}^{-1} \\
& \Delta \mathrm{H}_2=-393.5 \mathrm{~kJ} \mathrm{~mol}^{-1} \text { [Eq. (2) as such] } \\
& \Delta \mathrm{H}_3=-571.6 \mathrm{~kJ} \mathrm{~mol}^{-1} \text { [Eq. (3) } \times 2 \text { ] } \\
& \Delta \mathrm{H}_{\mathrm{f}} \text { of methane }=\Delta \mathrm{H}_1+\Delta \mathrm{H}_2+\Delta \mathrm{H}_3
\end{aligned}$

$
\Delta \mathrm{H}_{\mathrm{f}}=+890.3-965.1=-74.8 \mathrm{KJ} \mathrm{mol}^{-1}
$
Question 65 .
A reaction, $\mathrm{A}+\mathrm{B} \rightarrow \mathrm{C}+\mathrm{D}+\mathrm{q}$ is found to have a positive entropy change. The reaction will be
(a) possible at high temperature
(b) possible only at low temperature
(c) not possible at any temperature
(d) possible at any temperature
Answer:
(d) possible at any temperature
Question 66.
Consider the following statements.
(i) Thermodynamics is independent of atomic and molecular structure.
(ii) It includes whether a particular reaction is feasible or not under a given set of temperature and concentration of reactants and products.
(iii) It can determine the rate at which the reaction take place.
Which of the above statements is/are not correct'?
(a) (i) only
(b) (ii) only
(c) (iii) only
(d) (i) and (ii) only
Answer:
(c) (iii) only
Question 67.
A fundamental goal of thermodynamics is the
(a) prediction of spontaneity of the reaction.
(b) determination of rate of the chemical reaction.
(c) evaluation of the microscopic properties.
(d) both (b) and (c)
Answer:
(a) prediction of spontaneity of the reaction.

Question 68 .
The first law of thermodynamics states that
(a) $\Delta \mathrm{U}=\mathrm{q}-\mathrm{w}$
(b) $\Delta \mathrm{U}=\mathrm{q}+\mathrm{w}$
(c) $\Delta \mathrm{U}+\mathrm{q}=\mathrm{w}$
(d) $\Delta \mathrm{U}=\mathrm{w}-\mathrm{q}$
Answer:
(b) $\Delta \mathrm{U}=\mathrm{q}+\mathrm{w}$
Question 69.
Anything which separates the system from its surroundings is called
(a) Boundary
(b) Partition
(c) Universe
(d) Outer layer
Answer:
(a) Boundary
Question 70.
Hot water in a thermos flask is an example of
(a) closed system
(b) open system
(c) isolated system
(d) isochoric system
Answer:
(c) isolated system
Question 71.
Which one of the following is an example for closed system?
(a) Hot water contained in a thermos flask
(b) A gas contained in a cylinder fitted with a piston
(c) All living things
(d) Hot water contained in a open beaker
Answer:
(b) A gas contained in a cylinder fitted with a piston
Question 72.
Statement-I: All living things are open systems.
Statement-II: Because they continuously exchange matter and energy with the surroundings
(a) Statement-I and II arc correct and Statement-II is the correct explanation of Statement-I
(b) Statement-I and II are correct but Statement-II is not the correct explanation of Statement-I
(c) Statement-I is correct and Statement-II is wrong.
(d) Statement-I is wrong but Statement-II is correct.
Answer:

(a) Statement-I and II arc correct and Statement-II is the correct explanation of Statement-I

Question 73.
Which one of the following is an extensive property?
(a) Molar volume
(b) Density
(c) Molaritv
(d) Entropy
Answer:
(d) Entropy
Question 74.
Which one of the following is ail intensive property?
(a) Specific heat capacity
(b) Mass
(c) Enthalpy
(d) Heat capacity
Answer:
(a) Specific heat capacity
Question 75.
Which one of the following is not an extensive property?
(a) Mole
(b) Energy
(c) Molar mass
(d) Free energy
Answer:
(c) Molar mass
Question 76.
Which one of the following is not an intensive property?
(a) Density
(b) Molaritv
(c) Molality
(d) Mole
Answer:
(d) Mole
Question 77.
Which one of the following depend on the mass of the system?
(a) Density
(b) Mole fraction
(c) Mass
(d) Molar mass
Answer:
(c) Mass

Question 78.
Which one of the following is independent to the mass of the system?
(a) Volume
(b) Enthalpy
(c) Entropy
(d) Density
Answer:
(d) Densily
Question 79.
The process in which there is no exchange of heat between the system and surrounding is called
(a) Adiabatic process
(b) Isothermal process
(c) Isobaric process
(d) Isochoric process
Answer:
(a) Adiabatic process.
Question 80.
For an adiabatic process
(a) $\mathrm{dU}=0$
(b) $\mathrm{dT}=0$
(c) $q=0$
(d) $q=w$
Answer:
(c) $q=0$

Question 81 .
For an isothermal process
(a) $\mathrm{dH}=0$
(b) $\mathrm{dP}=0$
(c) $\mathrm{dT}=0$
(d) $\mathrm{dV}=0$
Answer:
(c) $\mathrm{dT}=0$
Question 82 .
The process in which the volume of the system remains constant is called
(a) Adiabatic process
(b) Isothermal process
(c) Isobaric process
(d) Isochoric process
Answer:
(d) Isochoric process
Question 83.
For an isochoric process
(a) $q=0$
(b) $\mathrm{dp}=\mathrm{O}$
(c) $d v=O$
(d) $\Delta \mathrm{U}=0$
Answer:
(c) $d v=O$
Question 84 .
Combustion of fuel in a bomb calorimeter is an example of
(a) adiabatic process
(b) isochoric process
(c) isobaric process
(d) isothemal process

Answer:
(b) isochoric process
Question 85.
Which one of the following is not a path function?
(a) Work
(b) Heat
(c) Pressure
(d) Either (a) or (b)
Answer:
(c) Pressure
Question 86.
Which one of the following is a path function?
(a) Pressure
(b) Volume
(c) Temperature
(d) Heat
Answer:
(d) Heat
Question 87.
Which one of the following is a state function?
(a) Internal energy
(b) Enthalpy
(c) Free energy
(d) All the above
Answer:
(d) All the above
Question 88.
Statement-I: Internal energy of a system is an extensive property.
Statement-II: Internal energy depends on the amount of the substances present in the system.
(a) Statement-I and II are correct and Statement-II is the correct explanation of Statement-I.
(b) Statement-I and II arc correct hut Statement-II is not the correct explanation of Statement-I.

(c) Statement-I is correct but Statement-II is wrong.
(d) Statement-I is wrong but Statement-II is correct.
Answer:
(a) Statement-I and II are correct and Statement-II is the correct explanation of Statement-I.
Question 89.
The SI unit of heat is
(a) Joule
(b) Kelvin
(c) $\mathrm{Kg}$
(d) $\mathrm{Kg} \mathrm{mol}^{-1}$
Answer:
(a) Joule
Question 90.
Which one of the following is the quantity of heat required to raise the temperature of $\mathrm{I} g \mathrm{~m}$ of water by $1{ }^{\circ} \mathrm{C}$ ?
(a) 1 Joule
(b) 1 Calorie
(c) 1 Kelvin
(d) 1 Kilo joule
Answer:
(b) 1 Calorie
Question 91.
Which one of the following is equal to 1 Joule?
(a) $\mathrm{Nm}^{-1}$
(b) $\mathrm{Nm}^2$
(c) $\mathrm{Nm}$
(d) $\mathrm{Kg} \mathrm{ms}^{-2}$
Answer:
(c) $\mathrm{Nm}$
Question 92 .
Consider the following statements.
(i) Work is a state function.
(ii) Work brings a temporary effect in the surroundings.
(iii) Work appears only at the boundary of the system.
Which of the above statements is/are not correct?
(a) (iii) only
(b) (i) and (ii)
(c) (i) and (iii)
(d) (ii) and (iii)

Answer:
(b) (i) and (ii)
Question 93.
Which of the following represents the gravitational work?
(a) $Q_V$
(b) F.x
(c) $\mathrm{mgh}$
(d) $-\mathrm{P} \Delta \mathrm{V}$
Answer:
(c) $\mathrm{mgh}$
Question 94 .
Match the List-I and List-II using the correct code given below the list.

Answer:
(a) $4 \quad 3 \quad 1 \quad 2$
Question 95.
Match the List-I and List-II using the correct code given below the list.

Answer:

(b) 3 4 1 2
Question 96.
Enthalpy is defined as
(a) $\mathrm{q}+\mathrm{w}$
(b) $\mathrm{q}-\mathrm{P} \Delta \mathrm{V}$
(c) $\mathrm{U}+\mathrm{PV}$
(d) $w$
Answer:
(c) $\mathrm{U}+\mathrm{PV}$
Question 97.
Which one of the following always he negative?
(a) Enthalpy of combustion
(b) Enthalpy of fusion
(c) Enthalpy of vapourisation
(d) Enthalpy of sublimation
Answer:
(a) Enthalpy of combustion
Question 98.
For an ideal gas
(a) $\mathrm{C}_{\mathrm{p}}-\mathrm{C}_{\mathrm{V}}=\mathrm{O}$
(b) $\mathrm{C}_{\mathrm{p}}-\mathrm{C}_{\mathrm{V}}=\mathrm{R}$
(c) $\mathrm{C}_{\mathrm{V}}-\mathrm{C}_{\mathrm{p}}=\mathrm{R}$
(d) $\mathrm{C}_{\mathrm{V}}-\mathrm{C}_{\mathrm{p}}>\mathrm{R}$
Answer:
(b) $\mathrm{C}_{\mathrm{p}}-\mathrm{C}_{\mathrm{V}}=\mathrm{R}$

(c) $U+P V$
(d) $\mathrm{w}$
Answer:
(c) $\mathrm{U}+\mathrm{PV}$
Question 97.
Which one of the following always he negative?
(a) Enthalpy of combustion
(b) Enthalpy of fusion
(c) Enthalpy of vapourisation
(d) Enthalpy of sublimation
Answer:
(a) Enthalpy of combustion
Question 98.
For an ideal gas ...........
(a) $\mathrm{C}_{\mathrm{p}}-\mathrm{C}_{\mathrm{V}}=\mathrm{O}$
(b) $\mathrm{C}_{\mathrm{p}}-\mathrm{C}_{\mathrm{V}}=\mathrm{R}$
(c) $\mathrm{C}_{\mathrm{V}}-\mathrm{C}_{\mathrm{p}}=\mathrm{R}$
(d) $\mathrm{C}_{\mathrm{V}}-\mathrm{C}_{\mathrm{p}}>\mathrm{R}$
Answer:
(b) $\mathrm{C}_{\mathrm{p}}-\mathrm{C}_{\mathrm{V}}=\mathrm{R}$
Question 99.
The standard substance used in the enthalpy of combustion of a substance in bomb calorimeter is
(a) methane
(b) acetic acid
(c) propane
(d) benzoic acid
Answer:
(d) benzoic acid
Question 100 .
The standard value of enthalpy of combustion of benzoic acid is
(a) $-3227 \mathrm{~kJ} \mathrm{~mol}^{-1}$
(b) $+3227 \mathrm{~kJ} \mathrm{~mol}^{-1}$
(c) -32.27 Ici $\mathrm{mol}^{-1}$
(d) $+32.27 \mathrm{~kJ} \mathrm{~mol}^{-1}$
Answer:
(a) $-3227 \mathrm{~kJ} \mathrm{~mol}^{-1}$

Question 101.
The heat of neutralization of a strong acid and strong base is around
(a) $+57.32 \mathrm{~kJ}$
(b) $-57.32 \mathrm{~kJ}$
(c) $-3227 \mathrm{~kJ} \mathrm{~mol}^{-1}$
(d) $+3227 \mathrm{~kJ} \mathrm{~mol}^{-1}$
Answer:
(b) $-57.32 \mathrm{~kJ}$
Question 102.
Which of the following is not a spontaneous process?
(a) All water fall runs down hill.
(b) A lump of sugar dissolves in cup of coffee.
(c) Heat flow from hotter object to colder one.
(d) A water flow from a well to upper reservoir.
Answer:
(d) A water flow from a well to upper reservoir.
Question 103.
Which one of the following is an endothermic process?
(a) $\mathrm{CH}_4+2 \mathrm{O}_2 \rightarrow \mathrm{CO}_2+2 \mathrm{H}_2 \mathrm{O}$
(b) $\mathrm{H}^{+}+\mathrm{OH}^{-} \rightarrow \mathrm{H}_2 \mathrm{O}$
(c) $\mathrm{NH}_4 \mathrm{NO}_3 \stackrel{\mathrm{H}_2 \mathrm{O}}{\longrightarrow} \mathrm{NH}_4^{+}+\mathrm{NO}_3^{-}$
(d) $\mathrm{C}+\mathrm{O}_2 \rightarrow \mathrm{CO}_2$
Answer:
(c) $\mathrm{NH}_4 \mathrm{NO}_3 \stackrel{\mathrm{H}_2 \mathrm{O}}{\longrightarrow} \mathrm{NH}_4^{+}+\mathrm{NO}_3^{-}$
Question 104.
The SI unit of entropy is
(a) $\mathrm{Nm}$
(b) $\mathrm{Cal} \mathrm{mol}^{-1}$
(c) $\mathrm{KJ} \mathrm{mol}^{-1}$
(d) $\mathrm{JK}^{-1}$
Answer:
(d) $\mathrm{JK}^{-1}$

Question 105.
In which of the following entropy decreases?
(a) melting of ice
(b) evaporation of water
(c) crystallization of sugar
(d) dissolution of salt
Answer:
(c) crystallization of sugar
Question 106.
Gibbs's free energy is defined as
(a) $\mathrm{G}=\mathrm{H}+\mathrm{TS}$
(b) $\mathrm{G}=\mathrm{H} \times \mathrm{TS}$
(c) $\mathrm{G}=\mathrm{H}-\mathrm{TS}$
(d) $\mathrm{G}=\mathrm{H} / \mathrm{TS}$
Answer:
(c) $\mathrm{G}=\mathrm{H}-\mathrm{TS}$
Question 107.
Match the List-I and List-II using the correct code given below the list.

Answer:

(b) 2 4 1 3
Question 108.
Van't Hoff equation is ............
(u) $\Delta \mathrm{G}^{\circ}=\mathrm{AH}^{\circ}-\mathrm{T} \Delta \mathrm{S}^{\circ}$
(b) $\mathrm{G}=\mathrm{H}-\mathrm{TS}$
(c) $\Delta \mathrm{G}^{\circ}=-2.303 \mathrm{RT} \log \mathrm{K}_{\text {eq }}$
(d) $\Delta \mathrm{S}=\Delta \mathrm{H} / \mathrm{T}$
Answer:
(c) $\Delta \mathrm{G}^{\circ}=-2.303 \mathrm{RT} \log \mathrm{K}_{\mathrm{eq}}$
Question 109.
Mathematically, the third law of thermodynamics is expressed as $\lim _{\mathrm{T} \rightarrow 0} \mathrm{~S}=0$
(b) $\lim _{T \rightarrow 0} \mathrm{~S}=1$
(c) $\mathrm{AU}=\mathrm{q}+\mathrm{w}$
(d) $\mathrm{G}=\mathrm{H}-\mathrm{TS}$
Answer:
$\lim _{\mathrm{T} \rightarrow 0} \mathrm{~S}=0$
II. Answer briefly ( 2 or 3 marks)
Question 1.
What is the aim of the study of chemical thermodynamics?

Answer:

The main aim of the study of chemical thermodynamics is to learn
- Transformation of energy from one form into another form.
- Utilization of various forms of energies.
- Change in the properties of system produced by chemical or physical effects.
Question 2.
What are the scope of thermodynamics?
Answer:
The scope of thermodynamics:
- To derive feasibility of a given process.
- It also helps in predicting how far a physical (or) chemical change can proceed. until the equilibrium conditions are established.
Question 3.
What are the limitations of the thermodnamics?
Answer:
- Thermodynamics suggests feasibility of reaction but fails to suggest rate of reaction. It is concerned only with the initial and the final states of the system. It is not concerned with the path by which the change occurs.
- It does not reveal the mechanism of a process.
Question 4.
Define
1. System
2. Surroundings.
Answer:
1. System:
A system is defined as any portion of matter under thermodynamic consideration. which is separated from the rest of the universe by real or imaginary boundaries. e.g.. Water taken in a beaker, balloon filled with air, seed, plant, flower and bird.

2. Surroundings:
Everything in the universe that is not the part of system and can interact with system is called as surroundings.
Question 5 .
What is meant by isolated system? Give example.
Answer:
- A system which can exchange neither matter nor energy with its surroundings is called an isolated system.
- Here boundary is scaled and insulated.
- Hot water contained in a thermos flask, is an example for an isolated system.
Question 6.
Explain a closed system with an example.
Answer:
- A system which can exchange only energy but not matter with its surroundings is called a closed system.
- Here the boundary is sealed but not insulated.
- Hot water contained in a closed beaker is an example for a closed system.
- In this system heat is transferred to the surroundings but no water vapour can escape from this system.
- A gas contained in a cylinder fitted with a piston constitutes a closed system.
Question 7.
What is meant by open system? Give example.
Answer:
- A system which can exchange both matter and energy with its surroundings is called an open system.
- Hot water contained in an open beaker is an example for open system.
- In this system, both water vapour and heat is transferred to the surroundings through the imaginary boundary.
- All living things are open systems because they continuously exchange matter and energy with the surroundings.
Question 8.
What are extensive properties?
Answer:
- The property that is depend on the mass or size of the system is called an extensive property.
- e.g., Volume, number of moles, mass and internal energy.

Question 9.
What is reversible process? Give an example.
Answer:
The process in which the system and surroundings can be restored to the initial state from the final state without producing any changes in the thermodynamics properties of the Universe is called a reversible process. e.g.,
$
\mathrm{H}_2+\mathrm{I}_2 \rightleftharpoons 2 \mathrm{HI}
$
Question 10.
What is an irreversible process? Give an example.
Answer:
- The process in which the system and surroundings cannot be restored to the initial state from the final state is called a reversible process.
- e.g.. All the processes occurring in nature are irreversible processes.
Question 11.
Define cyclic process. Give example.
Answer:
When a system returns to its original state after completing a series of changes, then it is said that a cycle is completed. This process is known as a cyclic process. For a cyclic process $\mathrm{dU}=$ $0, \mathrm{dH}=0, \mathrm{dP}=0, \mathrm{dV}=0$ and $\mathrm{dT}=0$.
Question 12.
What is meant by internal energy?
Answer:
- Internal energy (U) of a system is equal to the energy possessed by all its constituents namely atoms, ions and molecules.
- The energy of a molecule is equal to the sum of its translational energy $\left(\mathrm{U}_{\mathrm{v}}\right)$, vibrational energy $\left(\mathrm{U}_{\mathrm{b}}\right.$ ), rotational energy $\left(\mathrm{U}_{\mathrm{e}}\right)$, bond energy ( $\mathrm{jh}$ ' electronic energy and energy due to molecular interactions $\left(\mathrm{U}_{\mathrm{i}}\right)$.

 $\mathrm{U}=\mathrm{U}_{\mathrm{t}}+\mathrm{U}_{\mathrm{v}}+\mathrm{U}_{\mathrm{r}}+\mathrm{U}_{\mathrm{b}}+\mathrm{U}_{\mathrm{e}}+\mathrm{U}_{\mathrm{i}}$
- The total energy of all the molecules of the system is called internal energy.
Question 13.
Define Heat. Give its unit.
Answer:
- Heat (q) is regarded as energy in transit across the boundary separating a system from its surroundings.
- Heat changes result in temperature differences between system and surroundings.
- Heat is a path function.
- Units of heat: SI unit of heat is the joule (J).
Question 14.
Write a note about the sign convention of heat.
Answer:
- The symbol of heat is q.
- if the heat flows into the system from the surroundings, the energy of a system increases. Hence it is taken to be positive $(+q)$.
- if heat flows out of the system into the surroundings. energy of the system decreases. Hence it is taken to be negative $(-\mathrm{q})$.
Question 15.
What is meant by work? Give its unit.
Answer:
- Work is defined as the force (F) multiplied by the displacement (x).
- $-\mathrm{w}=$ F. $\mathrm{x}$
- Minus (-) sign indicates the work done by the system
- Unit of work: The SI unit of work is Joule (J).
Question 16.
Explain about gravitational work. Give its unit.

Answer:
1. When an object is raised to a certain height against the gravitational field, gravitational work is done on the object.
2. For example, if an object of mass ' $m$ ' is raised through a height ' $h$ ' against acceleration due to gravity ' $\mathrm{g}$ ', then the gravitational work carried out is ' $\mathrm{mgh}$ '.
$
\begin{aligned}
& \mathrm{w}=\mathrm{m} \cdot \mathrm{g} \cdot \mathrm{h} \\
& \mathrm{w}=\mathrm{Kg} \cdot \mathrm{ms}^{-2} \cdot \mathrm{m} \\
& \mathrm{w}=\mathrm{Kg} \mathrm{m} \mathrm{m}^2 \mathrm{~s}^{-2} \\
& \mathrm{w}=\text { Joule }
\end{aligned}
$
Question 17.
Define electrical work. Give its unit.
Answer:
- When a charged body moves from one potential region to another electrical work is done.
- If the electrical work done is $\mathrm{QV}$. Where $\mathrm{V}$ is the potential difference and $\mathrm{Q}$ is the quantity of electricity.
$
\begin{aligned}
& \mathrm{w}=\mathrm{QV} \\
& \mathrm{w}=\text { Coulomb } . \text { Volts } \\
& \mathrm{w}=\text { Joule }
\end{aligned}
$
Question 18.
Write a note about mechanical work. Give its unit.
Answer:
1. Mechanical work is defined as force multiplied by the displacement through which the force acts.
2. Whenever a force $(\mathrm{F})$ acts on an object and the object undergoes a displacement $(\mathrm{x})$ in the direction of the force, then the mechanical work is said to be done.
3. Mathematically $w=F \cdot x$
$
\begin{aligned}
& \mathrm{w}=\mathrm{F} \cdot \mathrm{x}=\mathrm{N} \cdot \mathrm{m} \\
& \mathrm{w}=\text { Joule }
\end{aligned}
$

Question 19.
Define Zeroth law of thermodynamics (or) Law of thermal equilibrium.
Answer:
Zeroth law of thermodynamics states that 'If two systems at different temperatures are separately in thermal equilibrium with a third one, then they tend to be in thermal equilibrium with themselves'.
Question 20.
Define enthalpy of a system. Give its unit.
Answer:
Enthalpy is a thermodynamic property of a system. Enthalpy $(\mathrm{H})$ is defined as sum of the internal energy $(\mathrm{U})$ of a system and the product of pressure and volume of the system. $\mathrm{H}=\mathrm{U}+\mathrm{PV}$
Unit of enthalpy: $\mathrm{KJ} \mathrm{mol}^{-1}$.
Question 21.
Define standard heat of formation.
Answer:
The standard heat of formation of a compound is defined as "The change in enthalpy that takes place when one mole of a compound is formed from its elements, all substances being in their standard states ( $298 \mathrm{~K}$ and 1 atm pressure).
Question 22.
Define specific heat capacity of a system.
Answer:
Specific heat capacity of a system is defined as the heat absorbed by one gram of a substance in raising its temperature by one Kelvin at a specified temperature.
$\mathrm{Cm}=\frac{q}{\left(\mathrm{~T}_2-\mathrm{T}_1\right)}$
Where $\mathrm{C}$ specific heat capacity, $\mathrm{q}=$ amount otheat absorbed. $\mathrm{m}=$ mass and $\mathrm{T}_1, \mathrm{~T}_2$ temperatures.
Question 23.
Derive the value of molar heat capacity as constant volume.
Answer:
According to the first law of themrndynamics, $\mathrm{dq}=\mathrm{dU}+\mathrm{PdV}$
Dividing both sides by dT, we have
$
\frac{d q}{d T}=\frac{(d U+P d V)}{d T}
$
At constant volume $\mathrm{dV}=0$, then
$
\begin{aligned}
& \frac{d q}{d T}=\left(\frac{d U}{d T}\right)_{\mathrm{V}} \\
& \mathrm{C}_{\mathrm{V}}=\left(\frac{d U}{d T}\right)_{\mathrm{V}}
\end{aligned}
$

Thus the heat capacity ai constant volume $\left(\mathrm{C}_{\mathrm{V}}\right)$ is defined as the rate of change of internal energy with respect to temperature at constant volume.
Question 24.
Derive the value of molar heat capacity at constant pressure.
Answer:
We know.
$
\mathrm{H}=\mathrm{U}+\mathrm{PV}
$
Differentiating the above equation with respect to temperature at constant pressure we get, $\left(\frac{d H}{d T}\right)_{\mathrm{P}}=?\left(\frac{d U}{d T}\right)_{\mathrm{P}}+\mathrm{P}\left(\frac{d V}{d T}\right)_{\mathrm{P}}$
We know,
$
\mathrm{C}=\frac{d q}{d T}
$
But at constant pressure $\mathrm{dq}=\mathrm{dH}$
Hence we get,
$
\mathrm{C}_{\mathrm{P}}=\left(\frac{d H}{d T}\right)_{\mathrm{P}}
$
Thus heat capacity at constant pressure $\left(C_P\right)$ is defined as the rate of change of enthalpy with respect to temperature at constant pressure.
Question 25.
Prove that for an ideal gas, $\mathrm{C}_{\mathrm{P}}$ is greater than $\mathrm{C}_{\mathrm{V}}$.
Answer:
1. It is clear that two heat capacities are not equal and $\mathrm{C}_{\mathrm{P}}$ is greater than $\mathrm{C}_{\mathrm{V}}$ by a factor which is related to the work done.
2. At a constant pressure, a part of heat absorbed by the system is used up in increasing the internal energy of the system and the other for doing work by the system.
3. At constant volume, the whole of heat absorbed is utilized in increasing the temperature of the system as there is no work done by the system. Thus $C_P$ is greater than $C_V$.
$
\mathrm{C}_{\mathrm{P}}=\frac{d H}{d T} ; \mathrm{C}_{\mathrm{V}}=\frac{d U}{d T}
$
4. By definition, $\mathrm{H}=\mathrm{U}+\mathrm{PV}$ for 1 mole of an ideal gas.
$
\mathrm{H}=\mathrm{U}+\mathrm{RT}
$
By differentiating this equation with respect to temperature $T$. we get,
$
\begin{aligned}
& \frac{d H}{d T}=\frac{d U}{d T}+\mathrm{R} \\
& \mathrm{C}_{\mathrm{P}}=\mathrm{C}_{\mathrm{V}}+\mathrm{R} \\
& \mathrm{C}_{\mathrm{P}}-\mathrm{C}_{\mathrm{V}}=\mathrm{R}
\end{aligned}
$
Thus for an ideal gas, $C_P$ is greater than $C_V$ by the gas constant $R$.

Question 26.
What are the applications of Bomb Calorimeter?
Answer:
- Bomb calorimeter is used to determine the amount of heat released in combustion reaction.
- It is used to determine the calorific value of food.
- Bomb calorimeter is used in many industries such as metabolic study. food processing and explosive testing.
Question 27.
Define heat of solution.
Answer:
The heat of solution is defined as "the change in enthalpy of the system when one mole of a substance is dissolved in a specified quantity of solvent at a given temperature".
Question 28.
Define molar heat of fusion.
Answer:
The molar heat of fusion is defined as "the change in enthalpy when one mole of a solid substance is converted into the liquid state at its melting points'.
Question 29.
What is meant by molar heat of vapourisation?
Answer:
The molar heat of vapourisation is defined as the change in enthalpy when one mole of liquid is converted into vapour or gaseous state at its boiling point.
Question 30.
1. What is sublimation?
2. Define molar heat of sublimation.

Answer:
1. Sublimation is a process when a solid changes directly into gaseous state without changing into liquid state.
2. Molar heat of sublimation is defined as the change in enthalpy when one mole of a solid is directly converted into the gaseous state at its sublimation temperature.
Question 31.
Define heat of transition?
Answer:
The heat of transition is defined as the change in enthalpy when one mole of an element changes from one allotropic form to another.
Question 32.
How do you measure the enthalpy of formation of carbon monoxide?
Answer:
1. Hess's law can be applied to calculate the enthalpy of formation of carbon monoxide. It is
very difficult to control the oxidation of graphite to give pure $\mathrm{CO}$. However, enthalpy for the oxidation of graphite to $\mathrm{CO}_2$ can be easily measured and enthalpy of oxidation of $\mathrm{CO}$ to $\mathrm{CO}_2$ is also measurable.
2. The application of Hess's law enables us to estimate the enthalpy of formation of $\mathrm{CO}$. $\mathrm{C}+\mathrm{O}_2 \rightarrow \mathrm{CO}_2 \Delta \mathrm{H}^{\circ}=393.5 \mathrm{~kJ}$
$
\mathrm{CO}+1 / 2 \mathrm{O}_2 \rightarrow \mathrm{CO}_2 \Delta \mathrm{H}^{\circ}=-283 \mathrm{~kJ}
$
on inverting equation (2), we get
$
\mathrm{CO}_2 \rightarrow \mathrm{CO}+1 / 2 \mathrm{O}_2 \Delta \mathrm{H}^{\circ}=+283 \mathrm{~kJ}
$
on adding equations (2) and (3), we get
$
\mathrm{C}+1 / 2 \mathrm{O}_2 \rightarrow \mathrm{CO} \Delta \mathrm{H}^{\circ}=3935+283=110.5 \mathrm{~kJ}
$
Question 33 .
What are the important features of lattice enthalpy?
Answer:

1. Higher lattice energy shows greater electrostatic attraction and therefore a stronger bond in the solid.
2. The lattice enthalpy is greater for ions of higher charge and smaller radii.
Question 34 .
Why there is a need for second law of thermodynamics? Give its importance.
Answer:
- Thermodynamics first law tells that there is an exact equivalence between various forms of energy and that heat gained is equal to heat loss.
- Practically it is not possible to convert the heat energy into an equivalent amount of work.
- To explain this, another law is needed which is known as second law of thermodynamics.
- The second law of thermodynamics helps us to predict whether the reaction is feasible or not and also tell the direction of the flow of heat.
- It also tells that energy cannot be completely converted into equivalent work.
Question 35.
Write the entropy statement of second law of thermodynamics.
Answer:
Whenever a spontaneous process takes place, it is accompanied by an increase in the total entropy of the Universe.
$\Delta \mathrm{S}_{\text {universe }}>\Delta \mathrm{S}_{\text {system }}+\Delta \mathrm{S}_{\text {surroundings }}$
Question 36.
Write the Clausius statement of second law of thermodynamics.
Answer:
Clausius statement: Heat flows spontaneously from hot objects to cold objects and to get it flow in the opposite direction, we have to spend some work.

Question 37.
What are spontaneous reaction? Give three examples for spontaneous reaction.
Answer:
reaction that does occur under the given set of conditions is called a spontaneous reaction.
Example:
- A waterfall runs downhill, but never up, spontaneously.
- Heat flows from hotter object to a colder one.
- Ageing process.
Question 38.
Define standard entropy of formation.
Answer:
Standard entropy of formation is defined as "the entropy of formation of 1 mole of a compound from the elements under standard conditions". It is denoted as $\Delta \mathrm{S}_{\mathrm{f}}{ }^{\circ}$. We can calculate the value of entropy of a given compound from the values of $\mathrm{S}^{\circ}$ of elements. $\Delta \mathrm{S}_{\mathrm{f}}{ }^{\circ}=\sum \Delta \mathrm{S}_{\text {products }}{ }^{\circ}-\sum \Delta \mathrm{S}_{\text {reactants }}{ }^{\circ}$
Question 39.
What is entropy of fusion?
Answer:
When one mole of the solid melts at its melting point reversibly the heat absorbed is called molar heat of fusion. The entropy change is given by
$
\Delta \mathrm{S}_{\mathrm{f}}=\frac{\Delta \mathrm{H}_f}{\mathrm{~T}_f}
$
Where $\Delta \mathrm{H}_{\mathrm{f}}=$ molar heat of fusion, $\mathrm{T}_{\mathrm{f}}$ is melting point.
Question 40.
What is entropy of Vapourisation?
Answer:
When one mole of liquid is boiled at its boiling point reversibly, the heat absorbed is called as molar heat of vaporization. The entropy change is given by
$
\Delta \mathrm{S}_{\mathrm{V}}=\frac{\Delta \mathrm{H}_v}{\mathrm{~T}_b}
$
where $\Delta \mathrm{H}_{\mathrm{V}}$ is molar heat of vapourisation. $\mathrm{T}_{\mathrm{b}}$ is boiling point.
Question 41.
Define entropy of transition.
Answer:
When one mole of a solid changes reversibly from one allotropic form to another at its transition temperature. The entropy change is given
$
\Delta \mathrm{S}_{\mathrm{t}}=\frac{\Delta \mathrm{H}_t}{\mathrm{~T}_t}
$
Where $\Delta \mathrm{H}_{\mathrm{t}}=$ molar heat of transition and $\mathrm{T}_{\mathrm{t}}=$ transition temperature.

Question 42.
Explain the following:
1. Out of diamond and graphite, which has greater entropy? Why?
2. From thermodynamic point of view, in which system the animals and plants belong?
Answer:
1. Graphite has greater entropy, because it is loosely packed.
2. Animals and plants belong to open system.
Question 43.
What is the condition spontaneity in terms of free energy change?
Answer:
1. If $\Delta \mathrm{G}$ is negative, process is spontaneous.
2. if $\Delta \mathrm{G}$ is positive, process is non-spontaneous.
3. if $\Delta \mathrm{G}=0$, the process is in equilibrium.
Question 44.
Why standard entropy of an elementary substance is not zero whereas standard enthalpy of formation is taken as zero?
Answer:
A substance has a perfectly ordered arrangement only at absolute zero. Hence, entropy is zero only at absolute zero. Enthalpy of formation is the heat change involved in the formation of one mole of the substance from its elements. An element formed from its constituents means no heat change.
Question 45.
The equilibrium constant for a reaction is one or more if $\Delta \mathrm{G}^{\circ}$ for it is less than zero. Explain.
Answer:
$G^{\ominus}=-\mathrm{RT}$ in $\mathrm{K}$, thus if $G^{\ominus}$ is less than zero Le., it is negative, then In $\mathrm{K}$ will be positive and hence $\mathrm{K}$ will be greater than one.

Question 46.
Many thermo dynamically feasible reactions do not occur under ordinary conditions. Why?
Answer:
Under ordinary conditions, the average energy of the reactants may be less than threshold energy. They require some activation energy to initiate the reaction.
Question 47.
Predict in which of the following, entropy increases or decreases.
1. A liquid crystallizes into a solid
2. Temperature of a crystallized solid is raised from $0 \mathrm{~K}$ to $115 \mathrm{~K}$
3. $2 \mathrm{NaHCO}_{3(\mathrm{~s})} \rightarrow \mathrm{NaCO}_2 \mathrm{CO}_{3(\mathrm{~s})}+\mathrm{CO}_{2(\mathrm{~s})}+\mathrm{H}_2 \mathrm{O}_{(\mathrm{s})}$
4. $\mathrm{H}_{2(\mathrm{~s})} \rightarrow 2 \mathrm{H}_{(\mathrm{g})}$
Answer:
1. After freezing, the molecules attain an ordered state and therefore, entropy decreases.
2. At $\mathrm{O} \mathrm{K}$ the constituent particles are in static form therefore, entropy is minimum. If the temperature is raised to $115 \mathrm{~K}$ particles begin to move and entropy increases.
3. Reactant, $\mathrm{NaHCO}_3$ is solid. Thus, its entropy is less in comparison to product which has high entropy.
4. Here, one molecule gives two atoms. Thus, number of particles increases and this leads to more disordered form.
5 - Mark Questions
Question 1.
Explain how heat absorbed at constant pressure is measured using coffee cup calorimeter with neat diagrani.
Answer:

1. Measurement of heat change at constant pressure can be done in a coffee cup calorimeter.
2. We know that $\Delta H=q_p$ (at constant $\mathrm{P}$ ) and therefore, heat absorbed or evolved, $q_p$ at constant pressure is also called the heat of reaction or enthalpy of reaction, $\Delta \mathrm{H}_{\mathrm{r}}$
3. in an exoihermic reaction, heat is evolved, and system loses heat to the surroundings. Therefore, $\mathrm{q}_{\mathrm{p}}$ will be negative and $\Delta \mathrm{H}_{\mathrm{r}}$ will also be negative.
4. Similarly in an endothermic reaction, heat is absorbed, $\mathrm{q}_{\mathrm{p}}$ is positive and $\Delta \mathrm{H}_{\mathrm{r}}$ will also be positive.
Question 2.
List the characteristics of entropy.
Characteristics of entropy:
Answer:
1. Entropy is a thermodynamic state function that is a measure of the randomness or disorderliness of the system.
2. In general, the entropy of gaseous system is greater than liquids and greater than solids. The symbol of entropy is $\mathrm{S}$.
3. Entropy is defined as for a reversible change taking place at a constant temperature $(T)$, the change in entropy $(\Delta S)$ of the system is equal to heat energy absorbed or evolved $(q)$ by the system divided by the constant temperature $(\mathrm{T})$.
$
\Delta S_{\mathrm{sys}}=\frac{q_{\mathrm{rev}}}{T}
$
4. If heat is absorbed, then AS is positive and there will be increase in entropy. If heat is evolved, $\Delta S$ is negative and there is a decrease in entropy.
5. The change in entropy of a process represented by $\mathrm{AS}$ and is given by the equation, $\Delta \mathrm{H}_{\text {sys }}=\mathrm{S}_{\mathrm{f}}-\mathrm{S}_{\mathrm{i}}$
6. If $\mathrm{S}_{\mathrm{f}}>\mathrm{S}_{\mathrm{i}}, \Delta \mathrm{S}$ is positive, the reaction is spontaneous and reversible. If $\mathrm{S}_{\mathrm{f}}<\mathrm{S}_{\mathrm{i}}, \Delta \mathrm{S}$ is negative, the reaction is non-spontaneous and irreversible.
7. Unit of entropy: SI unit of entropy is J K'.
Question 3.
Explain about the characteristics of work.
Characteristics of work:

Answer:
1. Work is defined as the force (F) multiplied by the displacement (x).
$
-\mathrm{w}=\mathrm{F} \cdot \mathrm{x}
$
The - ve sign is introduced to indicate that the work has been done by the system by spending a part of its internal energy.
2. Work is a path function.
3 . Work appears only at the boundary of the system.
4. Work appears during the change in the state of the system.
5 . Work brings a permanent effect in the surroundings.
6. Units of work: The SI unit of work is the joule (J) or Kilojoule (KJ).
7. If work done by the system, the energy of the system decreases, hence by convention work is taken to be negative (- $\mathrm{w})$.
8. If work done by the system, the energy of the system increases, hence by convention work is taken to be positive $(+w)$.
Question 4.
Derive the relationship between work for a reversible reaction and the charge in volume during comoression and expansion.
Answer:
1. During expansion, work is done by the system; since V1 > V1, the sign obtained for work will be negative.

2. During compression, work is done on the system; since $\mathrm{V}<\mathrm{V}$., the sign obtained for work will be positive.
3. If the pressure is not constant, but changes during the process such that it is always infinitesimally greater than the pressure of the gas, then, at each stage of compression, the volume decreases by an infinitesimal amount, $\mathrm{dV}$.
4. We can calculate the work done on the gas by the relation,
$\mathrm{w}=-\int_{V_i}^{V_f} P d V$
5. In a compression process, $P_{e x t}$ the external pressure is always greater than the pressure of the system. i.e., $P_{e x t}=\left(P_{\text {int }}+d p\right)$. In an expansion process, the external pressure is always less than the pressure of the system i.e., $P_{\text {ext }}=\left(P_{\text {int }}-d p\right)$.
6. In general case, we can write, $P_{e x t}=\left(P_{\text {int }} \pm d p\right)$. Such processes are called reversible processes. For a compression process, work can be related to internal pressure of the system under reversible conditions by writing equation.
$
w_{\text {rev }}=-\int_{V_i}^{V_f} P_{e x} d V=-\int_{V_i}^{V_f}\left(P_{\text {int }} \pm d P\right) d V
$
Since $d p . d v$ is very small, we can write,
$
w_{\mathrm{rev}}=-\int_{V_i}^{V_f} P_{\text {int }} \cdot d V
$
For a given system
$\mathrm{P}_{\text {int }} \mathrm{V}=\mathrm{nRT}$
$\mathrm{P}_{\text {int }} \mathrm{V}=\frac{n R T}{V}$
$w_{\mathrm{rev}}=-\int_{V_i}^{V_f} \frac{n R T}{V} d v$
$w_{\mathrm{rev}}=-n R T \int_{V_i}^{V_f} \frac{d V}{V}$
$w_{\mathrm{rev}}=-n R T \ln \left(\frac{V_f}{V_i}\right)$
$w_{\mathrm{rev}}=-2.303 n R T \log \left(\frac{V_f}{V_i}\right)$

7. If $\mathrm{V}_{\mathrm{f}}>\mathrm{V}_{\mathrm{i}}$ (expansion), the sign to work done by the process is negative. If $\mathrm{V}_{\mathrm{f}}<\mathrm{V}_{\mathrm{i}}$ (compression), the sign to work done by the process is positive.
Question 5.
Write the various definition of first law of thermodynamics.
First law of thermodynamics:
Answer:
- The total energy of an isolated system remains constant though it may change from one form to another.
- Whenever energy of a particular type disappears equivalent amount of another type must be produced.
- Total energy of a system and surroundings remains constant.
- Energy can neither be created nor destroyed, but may be converted from one form to another.
- The change in the internal energy of a closed system is equal to the energy that passes through its boundary as heat or work.
- Heat and work are equivalent ways of changing a systems internal energy.
Question 6.
Derive the various mathematical statements of the first law.
Answer:
Mathematical statement of the First law of Thermodynamics is $\Delta \mathrm{U}=\mathrm{q}+\mathrm{w}$
Case 1:
For a cyclic process involving isothermal expansion of an ideal gas
$
\Delta \mathrm{U}=0
$
$
\therefore \mathrm{q}=-\mathrm{w}
$
In other words, during a cyclic process, the amount of heat absorbed by the system is equal to work done by the system.

Case 2:
For an isochoric process (no change in volume) there is no work of expansion.
$
\begin{aligned}
& \Delta \mathrm{V}=0 \\
& \mathrm{w}=0 \\
& \Delta \mathrm{U}=0
\end{aligned}
$
In other words, during isochoric process, the amount of heat supplied to the system is converted to its internal energy.
Case 3:
For an adiabatic process there is no change in heat .i.e., $\mathrm{q} O$. Hence
$
\begin{aligned}
& \mathrm{q}=0 \\
& \Delta \mathrm{U}=\mathrm{w}
\end{aligned}
$
In other words, in an adiabatic process, the decrease in internal energy is exactly equal to the work done by the system on its surroundings.
Case 4 :
For an isobaric process. There is no change in the pressure. P remains constant. Hence
$
\begin{aligned}
& \Delta \mathrm{U}=\mathrm{q}+\mathrm{w} \\
& \Delta \mathrm{U}=\mathrm{q}-\mathrm{P} \Delta \mathrm{V}
\end{aligned}
$
In other words, in an isobaric process a part of heat absorbed by the system is used for PV expansion work and the remaining is added to the internal energy of the system.
Question 7.
What are the characteristics of enthalpy?
Characteristics of enthalpy:
Answer:
1. Enthalpy is a thermodynamic property of a system. Enthalpy $\mathrm{H}$ is defined as sum of the internal energy $(\mathrm{U})$ of a system and the product of pressure and volume of the system. That is, $\mathrm{H}=\mathrm{U}+\mathrm{PV}$
2. Enthalpy is a state function which depends entirely on the state functions $\mathrm{T}, \mathrm{P}$ and $\mathrm{U}$.
3. Enthalpy is usually expressed as the change in enthalpy $(\Delta \mathrm{H})$ for a process between initial and final states.
$
\Delta \mathrm{H}=\Delta \mathrm{U}+\mathrm{P} \Delta \mathrm{V}
$

4. At constant pressure, the heat flow (q) for the process is equal to the change in enthalpy which is defined by the equation. $\Delta \mathrm{H}=\mathrm{q}_{\mathrm{p}}$
5. In an endothermic reaction heat is absorbed by the system from the surroundings that is $\mathrm{q}>0$ (positive). Therefore, at constant Tand $\mathrm{P}$, by the equation above, if $\mathrm{q}$ is positive then $\Delta \mathrm{H}$ is also positive.
6. In an exothermic reaction heat is evolved by the system to the surroundings that is, $q 0$ (negative). If $\mathrm{q}$ is negative, then $\Delta \mathrm{H}$ will also be negative.
7. Unit of enthalpy is $\mathrm{KJ} \mathrm{mol}^{-1}$.
Question 8.
What are thermochemical equation? What are the conventions adopted In writing thermochemical equation?
Answer:
A thermochemical equation is a balanced stoichiometric chemical equation that includes the enthalpy change $(\Delta \mathrm{H})$. Conventions adopted in thermochemical equations:
1. The coefficients in a balanced thermochemical equation refer to number of moles of reactants and products involved in the reaction.
2. The enthalpy change of the reaction $\Delta \mathrm{H}$ has unit $\mathrm{kJ}$.
3. When the chemical reaction is reversed, the value of AR is reversed in sign with the same magnitude.
4. Physical states (gas, liquid, aqueous and solid) of all species is important and must be specified in a thermochemical reaction since AH depends on the phases of reactants and products.
5. if the thermochemical equation is multiplied throughout by a number, the enthalpy change is also be multiplied by the same number value.
6. The negative sign of $\Delta \mathrm{H}$ indicates the reaction to be an exothermic and the positive sign of $\Delta \mathrm{H}$ indicates an endothermic type of reaction.
Question 9.
Calculate the values of $\Delta \mathrm{U}$ and $\Delta \mathrm{H}$ for an ideal gas in terms of $\mathrm{C}_{\mathrm{P}}$ and $\mathrm{C}_{\mathrm{V}}$

Calculation of $\Delta \mathrm{U}$ and $\Delta \mathrm{H}$ :
Answer:
For one mole of an ideal gas, we have
$
\begin{aligned}
& \mathrm{C}_{\mathrm{V}}=\frac{d U}{d T} \\
& \mathrm{dU}=\mathrm{C}_{\mathrm{V}} \cdot \mathrm{dT}
\end{aligned}
$
For a finite change, we have,
$
\Delta \mathrm{U}=\left(\mathrm{U}_2-\mathrm{U}_1\right)=\mathrm{C}_{\mathrm{V}}\left(\mathrm{T}_2-\mathrm{T}_1\right)
$
and for $\mathrm{n}$ moles of an ideal gas we get
$
\Delta \mathrm{U}=\mathrm{nC}_{\mathrm{V}}\left(\mathrm{T}_2-\mathrm{T}_1\right) \ldots \ldots \ldots \ldots . .(1)
$
We know,
$
\begin{aligned}
& \Delta \mathrm{H}=\Delta(\mathrm{U}+\mathrm{PV}) \\
& \Delta \mathrm{H}=\Delta \mathrm{U}+\Delta(\mathrm{PV}) \\
& \Delta \mathrm{H}=\Delta \mathrm{U}+\Delta \mathrm{RT}[: \mathrm{PV}=\mathrm{RT}] \\
& \Delta \mathrm{H}=\Delta \mathrm{U}+\mathrm{R} \Delta \mathrm{T} \\
& \Delta \mathrm{H}=\mathrm{C}_{\mathrm{V}}\left(\mathrm{T}_2-\mathrm{T}_1\right)+\mathrm{R}\left(\mathrm{T}_2-\mathrm{T}_1\right) \\
& \Delta \mathrm{H}=\left(\mathrm{C}_{\mathrm{V}}+\mathrm{R}\right)\left(\mathrm{T}_2-\mathrm{T}_1\right) \\
& \Delta \mathrm{H}=\mathrm{C}_{\mathrm{P}}\left(\mathrm{T}_2-\mathrm{T}_1\right)\left[\therefore \mathrm{C}_{\mathrm{P}}-\mathrm{C}_{\mathrm{V}}=\mathrm{R}\right]
\end{aligned}
$
For $n$ moles of an ideal gas we get $\Delta \mathrm{H}=\mathrm{n} \mathrm{C}_{\mathrm{P}}\left(\mathrm{T}_2-\mathrm{T}_1\right) \ldots \ldots \ldots \ldots \ldots(2)$
Question 10.
Suggest and explain indirect method to calculate lattice enthalpy of magnesium bromide.
Answer:
Born Haber's cycle method:
$
\mathrm{Mg}(\mathrm{s})+\mathrm{Br}_2(1) \rightarrow \mathrm{MgBr}_2 \Delta \mathrm{H}_{\mathrm{f}}{ }^{\circ}
$
Sublimation: $\mathrm{Mg}_{(\mathrm{s})} \rightarrow \mathrm{Mg}_{(\mathrm{g})} \Delta \mathrm{H}_1^{\circ}$
Ionisation: $\mathrm{Mg}_{(\mathrm{g})} \rightarrow \mathrm{Mg}^{2+}(\mathrm{g})+2 \mathrm{e} \Delta \mathrm{H}_2$ 。
Vapourisation: $\mathrm{Br}_2(\mathrm{l}) \rightarrow \mathrm{Br}_{2(\mathrm{~g})} \Delta \mathrm{H}_3^{\circ}$
Dissociation: $\mathrm{Br}_{2(\mathrm{~g})} \rightarrow 2 \mathrm{Br}_{(\mathrm{g})} \Delta \mathrm{H}_4{ }^{\circ} \Delta \mathrm{H}_4{ }^{\circ}$
Electron affinity: $2 \mathrm{Br}_{(\mathrm{g})}+2 \mathrm{e}^{-} \rightarrow 2 \mathrm{Br}^{-}(\mathrm{g}) \Delta \mathrm{H}_5{ }^{\circ}$
Lattice enthalpy: $\mathrm{Mg}^{2+}{ }_{(\mathrm{g})}+2 \mathrm{Br}^{-}(\mathrm{g}) \rightarrow \mathrm{MgBr}_{2(\mathrm{~s})} \Delta \mathrm{H}_6{ }^{\circ}=$ ?
$\Delta \mathrm{H}_{\mathrm{f}}{ }^{\circ}=\Delta \mathrm{H}_1{ }^{\circ}+\Delta \mathrm{H}_1{ }^{\circ}+\Delta \mathrm{H}_2{ }^{\circ}+\Delta \mathrm{H}_3{ }^{\circ}+\Delta \mathrm{H}_4{ }^{\circ}+\Delta \mathrm{H}_5{ }^{\circ}+\Delta \mathrm{H}_6{ }^{\circ}$
$\Delta \mathrm{H}_6{ }^{\circ}=\Delta \mathrm{H}_{\mathrm{f}}{ }^{\circ}-\left(\Delta \mathrm{H}_1{ }^{\circ}+\Delta \mathrm{H}_1{ }^{\circ}+\Delta \mathrm{H}_2{ }^{\circ}+\Delta \mathrm{H}_3{ }^{\circ}+\Delta \mathrm{H}_4{ }^{\circ}+\Delta \mathrm{H}_5{ }^{\circ}\right)$
If we know the values of $\Delta \mathrm{H}_{\mathrm{f}}{ }^{\circ}, \Delta \mathrm{H}_1{ }^{\circ}, \Delta \mathrm{H}_2{ }^{\circ}, \Delta \mathrm{H}_3{ }^{\circ}, \Delta \mathrm{H}_4{ }^{\circ}$ and $\Delta \mathrm{H}_5{ }^{\circ}$, we can calculate the value of $\Delta \mathrm{H}_6{ }^{\circ}$ by indirect method.

Question 11.
Derive the relationship between standard free energy $\left(\Delta \mathrm{G}^{\circ}\right)$ and equilibrium constant $\left(\mathrm{K}_{\mathrm{eq}}\right)$.

Answer:
1. In a reversible process, system is at all times in perfect equilibrium with its surroundings.
2. A reversible chemical reaction can proceed in either direction simultaneously, so that a dynamic equilibrium is set up.
3. This means that the reactions in both the directions should proceed with decrease in free energy, which is impossible.
4. It is possible only $;$ fat equilibrium, the free energy of a system is minimum.
5. Lets consider a general equilibrium reaction, $\mathrm{A}+\mathrm{B} \rightleftharpoons \mathrm{C}+\mathrm{D}$
The free energy change of the above reaction in any state $(\Delta \mathrm{G})$ is related to the standard free energy change of the reaction $\left(\Delta \mathrm{G}^{\circ}\right)$ according to the following equation.
$\Delta \mathrm{G}=\Delta \mathrm{G}^{\circ}+\mathrm{RT}$ in $\mathrm{Q}$.........(1)
where $\mathrm{Q}$ is reaction quotient and is defined as the ratio of concentration of the products to the concentration of the reactants under non-equilibrium condition.
6. When equilibrium is attained, there is no further free energy change i.e. $\Delta \mathrm{G}=0$ and $\mathrm{Q}$ becomes equal to equilibrium constant. Hence the above equation becomes, $\Delta \mathrm{G}^{\circ}=\mathrm{RT}$ in $\mathrm{K}_{\text {eq }}$. (2)
This equation is known as Van't Hoff equation.
$\Delta \mathrm{G}^{\circ}=-2.303 \mathrm{RT} \log \mathrm{K}_{\text {eq }}$
We also know that,
$\Delta \mathrm{G}^{\circ}=\Delta \mathrm{H}^{\circ}-\mathrm{T} \Delta \mathrm{S}^{\circ}=-\mathrm{RT}$ in $\mathrm{K}_{\mathrm{eq}}$