Thermodynamics is the study of energy transformations. Thermochemistry is the study of the thermal energy changes that accompany chemical and physical changes. Heat is one form of energy. Heat is the energy transferred between objects or systems, which may be measured by temperature changes. The total energy is a property of a substance or system; it cannot be measured directly.

Any process occurring within a system that produces or releases heat into the surroundings is called exothermic, and its value is given a negative sign by convention. Any process that absorbs heat from the surroundings into the system is called endothermic, and its value has a positive sign. Note that an exothermic reaction is not necessarily a spontaneous reaction, and its negative sign should not be interpreted as such. This type of interpretation and criterion is reserved for AG, the Gibbs free energy change, defined in equation 2.34 and discussed thereafter.

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