Thermochemistry (a branch of thermodynamics) is the study of the movement of heat energy (called enthalpy) in chemistry. Changes in enthalpy and entropy (a measure of the number of ways of arranging energy in a substance) can be used to accurately predict the outcome of chemical reactions.
A chemical reaction involves the breaking of bonds in reactants and the forming of bonds in products. If the energy required to break bonds in reactants is less than the energy released when bonds are formed in products, the reaction is exothermic.
This section involves more complex energy cycles than those you came across in Section 5.2. Born Haber cycles can be used to determine the lattice enthalpies and enthalpies of solution of ionic compounds.
According to the second law of thermodynamics, a process is spontaneous if it causes the total entropy of universe to increase. Entropy is a measure of the number of ways in which the energy in a system can be arranged.
It is not always possible to measure the enthalpy change of a reaction directly. Energy cycles offer an indirect route, using the known enthalpy changes of other reactions.
All physical and chemical processes are accompanied by an energy change. This section explains how the enthalpy change of a chemical reaction can be determined from the effect that it has on the temperature of the surroundings.