Page No 117: - Chapter 4 - Chemical Kinetics - Exercise Solutions - Ncert Solutions class 12 - Chemistry

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Question 4.1:

From the rate expression for the following reactions, determine their order of reaction and the dimensions of the rate constants.

(i) 3 NO(g) → N₂O (g) Rate = k[NO]²

(ii) H₂O₂ (aq) + 3 I⁻ (aq) + 2 H⁺ → 2 H₂O (l) +   Rate = k[H₂O₂][I⁻]

(iii) CH₃CHO(g) → CH₄(g) + CO(g) Rate = k [CH₃CHO]^3/2

(iv) C₂H₅Cl(g) → C₂H₄(g) + HCl(g) Rate = k [C₂H₅Cl]

Answer:

(i) Given rate = k [NO]²

Therefore, order of the reaction = 2

Dimension of 

(ii) Given rate = k [H₂O₂] [I⁻]

Therefore, order of the reaction = 2

Dimension of 

(iii) Given rate = k [CH₃CHO]^3/2

Therefore, order of reaction = 

Dimension of 

(iv) Given rate = k [C₂H₅Cl]

Therefore, order of the reaction = 1

Dimension of 

Question 4.2:

For the reaction:

2A + B → A₂B

the rate = k[A][B]² with k = 2.0 × 10⁻⁶ mol⁻² L² s⁻¹. Calculate the initial rate of the reaction when [A] = 0.1 mol L⁻¹, [B] = 0.2 mol L⁻¹. Calculate the rate of reaction after [A] is reduced to 0.06 mol L⁻¹.

Answer:

The initial rate of the reaction is

Rate = k [A][B]²

= (2.0 × 10⁻⁶ mol⁻² L² s⁻¹) (0.1 mol L⁻¹) (0.2 mol L⁻¹)²

= 8.0 × 10⁻⁹ mol⁻² L² s⁻¹

When [A] is reduced from 0.1 mol L⁻¹ to 0.06 mol⁻¹, the concentration of A reacted = (0.1 − 0.06) mol L⁻¹ = 0.04 mol L⁻¹

Therefore, concentration of B reacted  = 0.02 mol L⁻¹

Then, concentration of B available, [B] = (0.2 − 0.02) mol L⁻¹

= 0.18 mol L⁻¹

After [A] is reduced to 0.06 mol L⁻¹, the rate of the reaction is given by,

Rate = k [A][B]²

= (2.0 × 10⁻⁶ mol⁻² L² s⁻¹) (0.06 mol L⁻¹) (0.18 mol L⁻¹)²

= 3.89 mol L⁻¹ s⁻¹

Question 4.3:

The decomposition of NH₃ on platinum surface is zero order reaction. What are the rates of production of N₂ and H₂ if k = 2.5 × 10⁻⁴ mol⁻¹ L s⁻¹?

Answer:

The decomposition of NH₃ on platinum surface is represented by the following equation.

Therefore,

However, it is given that the reaction is of zero order.

Therefore,

Therefore, the rate of production of N₂ is

And, the rate of production of H₂ is

= 7.5 × 10⁻⁴ mol L⁻¹ s⁻¹

Question 4.4:

The decomposition of dimethyl ether leads to the formation of CH₄, H₂ and CO and the reaction rate is given by

Rate = k [CH₃OCH₃]^3/2

The rate of reaction is followed by increase in pressure in a closed vessel, so the rate can also be expressed in terms of the partial pressure of dimethyl ether, i.e.,

If the pressure is measured in bar andtime in minutes, then what are the units of rate and rate constants?

Answer:

If pressure is measured in bar and time in minutes, then

Unit of rate = bar min⁻¹

Therefore, unit of rate constants

Question 4.5:

Mention the factors that affect the rate of a chemical reaction.

Answer:

The factors that affect the rate of a reaction are as follows.

(i) Concentration of reactants (pressure in case of gases)

(ii) Temperature

(iii) Presence of a catalyst