The change in internal energy  can be brought about in two ways:

(i) Either by allowing the heat to flow into the system or out of the system.

(ii) By doing work on the system or the work done by the system.

Using the symbol  to represent heat transferred to system and using work done by the system  we can represent the internal energy change of a system,  as

       (First law of thermodynamics)

If the reaction is carried out in a closed container with constant volume, so that

Hence,                   

On the other hand, if a reaction is carried out in open vessel that keeps the pressure constant and allows the volume of the system to change freely. In such case, and .

Hence,   

Also        

As reactions carried out at constant pressure are so common, the heat change for such process is given a special symbol  called the enthalpy change of the reaction. The enthalpy  of the system is the name given to the quantity .

In which of the following cases  and  are not equal to each other?
a)	The reaction involves no gaseous reactant and product
b)	The number of moles of gaseous reactants and gaseous product is not equal to each other
c)	The number of moles of gaseous reactants and gaseous products is equal to each other
d)	The process is carried out in closed vessel

Question ID - 103891 | SaraNextGen Top Answer

The change in internal energy  can be brought about in two ways:

(i) Either by allowing the heat to flow into the system or out of the system.

(ii) By doing work on the system or the work done by the system.

Using the symbol  to represent heat transferred to system and using work done by the system  we can represent the internal energy change of a system,  as

       (First law of thermodynamics)

If the reaction is carried out in a closed container with constant volume, so that

Hence,                   

On the other hand, if a reaction is carried out in open vessel that keeps the pressure constant and allows the volume of the system to change freely. In such case, and .

Hence,   

Also        

As reactions carried out at constant pressure are so common, the heat change for such process is given a special symbol  called the enthalpy change of the reaction. The enthalpy  of the system is the name given to the quantity .

In which of the following cases  and  are not equal to each other?
a)	The reaction involves no gaseous reactant and product
b)	The number of moles of gaseous reactants and gaseous product is not equal to each other
c)	The number of moles of gaseous reactants and gaseous products is equal to each other
d)	The process is carried out in closed vessel