|Thermochemistry is the branch of chemistry concerned with the heat evolved or absorbed in a chemical process. Thermochemistry approaches chemical change through the First Law of Thermodynamics, but finds it more convenient to define a state function, the enthalpy, which is the sum of a system's internal energy and the product of its pressure and volume (H = E + PV).|
The enthalpy is an extremely useful construct. We understand from the First Law of Thermodynamics that the heat flow occuring in a process such as mixing, phase change, or chemical reaction must be equal to the combination of internal energy change for the system and thermodynamic work. With the enthalpy change, though, we can describe the heat flow as a change in a single state function, the enthalpy, as long as we model the change over constant pressure. Because enthalpy change is not path dependent, but dependent on change in thermodynamic state, we can derive the extremely useful principle, Hess's Law of Heat Summation, which states that the heat exchange accompanying a transformation is the same whether the process occurs in one or several steps. A large part of skill in Thermochemistry involves imagining pathways for chemical change that help you understand the enthalpy change of a process which may or may not occur by that path.
The sequence of thermochemical reasoning is one of the primary conceptual arcs underlying the understanding of chemistry, so while there may only be a couple of questions that lead to an 'enthalpy change' as an answer, your thermochemical understanding will be in operation throughout the exam. When you learn to see the substances involved in a chemical reaction as a thermodynamic system, you can see any chemical process as a transformation through which internal energy may change, thermodynamic work may be performed, and heat may be evolved or absorbed. Over the next few modules, after we begin making the connections between Thermochemistry and Chemical Thermodynamics, I hope you will concentrate now to have a deeper understanding of spontaneity and chemical equilibrium later.