If there is no change in pressure or no change in volume of the system, then the heat of the reaction will be equal to the change in internal energy of the system.
According to the first law of thermodynamics,
The total change in internal energy of the system is equal to the sum of the heat added to the system and the work done by the system.
So, we can write,
ΔH = ΔQ + W
Where,
ΔH is the heat of the reaction,
ΔQ is the heat evolved or heat given,
W is the work done by the system,
We know work by the gas is,
W = VΔP or PΔV
Where V is the volume of the gas and P is pressure of the gas.
So, for the heat of the system to be equal to that of the hear evolved in the system.
W = 0
For W to be zero,
Either, ΔV or ΔP has to be zero,
So, we can conclude that if change in the pressure of the system of the change in volume of the system is zero.
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