First Law of Thermodynamics

www.physics.sfsu.edu/~lwilliam/ 111/thermo/sld006.htm

Stated simply: the total energy of the universe does not change. This does not mean that the form of the energy cannot change. Indeed, chemical energies of a molecule can be converted to thermal, electrical or mechanical energies.
The internal energy of a system can change only by work or heat exchanges. From this the change in the free energy of a system can be shown by the following equation:

DE= q - w Eqn. 1

When q is negative heat has flowed from the system and when q is positive heat has been absorbed by the system. Conversely when w is negative work has been done on the system by the surrounding and when positive, work has been done by the system on the surroundings.
In a reaction carried out at constant volume no work will be done on or by the system, only heat will be transferred from the system to the surroundings. The end result is that:

DE = q Eqn. 2

When the same reaction is performed at constant pressure the reaction vessel will do work on the surroundings. In this case:

DE = q - w Eqn. 3

where  w = PDV Eqn. 4

When the initial and final temperatures are essentially equal (e.g. in the case of biological systems):

DV = Dn[RT/P] Eqn. 5

therefore,  w = DnRT Eqn. 6

by rearrangement of equation 3 and incorporation of the statements in equations 4-6, one can calculate the amount of heat released under constant pressure:

q = DE + w = DE + PDV = DE + DnRT Eqn. 7

In equation 7 Dn is the change in moles of gas per mole of substance oxidized (or reacted), R is the gas constant and T is absolute temperature.

Four Gas Processes