Heat changes at constant pressure the enthalpy

Most chemical processes are accompanied by changes in the volume of the system, and therefore involve both heat and work terms. If the process takes place at a constant pressure, then the work is given by PAV and the change in internal energy will be 

Thus the amount of heat that passes between the system and the surroundings is given by 

Hydrogen chloride gas readily dissolves in water, releasing 75.3 kJ/mol of heat in the process. If one mole of HC1 at 298 K and 1 atm pressure occupies 24.5 liters, find the AU for the system when one mole of HC1 dissolves in water under these conditions. 

Solution In this process the volume of liquid remains practically unchanged, so AV= -24.5 L. The work done is 

In other words, if the gaseous HC1 simply dissolved without volume change, the heat released by the process (75.3 kJ) would cause the system s internal energy to diminish by 75.3 kJ. But the volume decrease due to the disappearance of the gas is equivalent to compression of the system by the pressure of the atmosphere the resulting work done on the system acts to increase its internal energy, so the net value of AU is -72.82 kJ instead of -75.3 kJ. 

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