Reaction dynamics is a field within physical chemistry, studying why chemical reactions occur, how to predict their behavior, and how to control them. It is closely related to chemical kinetics, but is concerned with individual chemical events on atomic length scales and over very brief time periods.
The first law of thermodynamics, also known as the law of conservation of energy, deals with the amount of work that can be done by a chemical or physical process and the amount of heat that is absorbed or evolved. Thermochemistry, which deals with the heat produced by chemical reactions and solution processes, is based on the first law. Since the internal energy of a system (U) can be changed a given amount by either heat (q) or work (w), the internal energy of a system can be represented (in Joules) as:
ΔU = q + w
This equation states the postulates of the first law which are
1. The internal energy is a function of the state variables for the system.
2. The change (ΔU) for a process in a closed system may be calculated using the equation above.
On the basis of the first law, tables of enthalpies of formation that may be used to calculate enthalpy changes for reactions that have not yet been studied. Using information on heat capacities of reactants and products makes it possible to calculate the heat of reaction at a temperature where it has not previously been studied.