It is a branch of chemistry that deals with the interrelation of heat with chemical reaction or physical change of state. Thermochemistry is the study of the heat energy which is associated with chemical reactions and/or physical transformations. A reaction may release or absorb energy, and a phase change may do the same, such as in melting and boiling. Thermochemistry focuses on these energy changes, particularly on the system's energy exchange with its surroundings. Thermochemistry is useful in predicting reactant and product quantities throughout the course of a given reaction. In combination with entropy determinations, it is also used to predict whether a reaction is spontaneous or non-spontaneous, favorable or unfavorable. Basic thermochemical ideas include the notion of a system and its surroundings and of closed, open, and isolated systems; the concepts of kinetic energy, potential energy, and internal energy; and the distinction between two types of energy exchanges, heat (q) and work (w). Thermochemistry is the part of thermodynamics that studies the relationship between heat and chemical reactions . Thermochemistry is a very important field of study because it helps to determine if a particular reaction will occur and if it will release or absorb energy as it occurs. Some examples of endothermic reactions are: electrolysis, decomposition and evaporation. The study of these processes, and the factors involved, is known as thermochemistry.

Thermochemistry rests on two generalizations. Stated in modern terms, they are as follows:

• Lavoisier and Laplace's law (1780): The energy change accompanying any transformation is equal and opposite of energy change accompanying the reverse process.
• Hess' law (1840): The energy change accompanying any transformation is the same whether the process occurs in one step or many.

These statements preceded the first law of thermodynamics (1845) and helped in its formulation. Lavoisier, Laplace and Hess also investigated specific heat and latent heat, although it was Joseph Black who made the most important contributions to the development of latent energy changes. The measurement of heat changes is performed using calorimetry, usually an enclosed chamber within which the change to be examined occurs. The temperature of the chamber is monitored either using a thermometer or thermocouple, and the temperature plotted against time to give a graph from which fundamental quantities can be calculated. Modern calorimeters are frequently supplied with automatic devices to provide a quick read-out of information, one example being the differential scanning calorimeter (DSC). A system undergoes a process when one or more of its properties changes. A process relates to the change of state. An isothermal (same-temperature) process occurs when temperature of the system remains constant. An isobaric (same-pressure) process occurs when the pressure of the system remains constant. A process is adiabatic when no heat exchange occurs.

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