At first sight the First Law of Thermodynamics may not appear to have so much to do with Chemistry. Chemistry is the study of properties of molecules and their synthsis. Well, heat may be important because reactions can be exothermic and that is likely to be important in the field of energy conversion. However, what about the rest of Chemistry? While it is not necessarily evident in the chemical equations heat and work are part of every chemical reaction. It is important to understand the energetic realationships to predict reactivity. The "work term" is present in every chemical reaction where moles of gas are created or consumed. Oxidation, hydrogenation, generation of CO2

The sign convention

The thermodynamic sign convention is extremely important. We must account for the change in both the system and the surroundings. If we focus on the sustem the sign convention is has a simple interpretation. If the energy of the system increases the sign is positive and if it decreases the sign is negative. For heat flow this is partciularly straightforward. If the system loses heat (i.e. for an exothermic process) the sign is negative. Therefore, if the system gains heat the sign is positive. For work we can consider the work of expansion and comparession. In an expansion we consider that the work is losing energy since it is doing work on the surrounding. Therefore, the work of expansion is negative. These definitions are consistent throughout all of thermodynamics and ultimately it is these definitions that lead to the definition of a negative free energy change as the indication that a process is spontaneous.