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Thermochemistry is the application of thermodynamics to the field of chemistry.

In general, thermodynamics deals with the interconversion of various kinds of energy including heat and work, and the corresponding changes in physical properties.

When applied to chemistry, thermodynamics can be used to predict the extent to which material changes proceed, primary examples including chemical reactions (see also chemical equilibrium), phase changes such as boiling or melting, and the formation of solutions from separate components.

Thermochemistry describes several major state functions (i.e., functions that only depend on the where the system is, not on a change in the system) in a reaction. Three are particularly important when a change occurs at constant temperature (T) and pressure (p): Entropy (S), Gibb's free energy (G) and enthalpy (H). Other functions are important under conditions other than constant T, p. In a chemical change at constant T,p, G, H, and S are related as:

            change in G = change in H - T (change in S)
Entropy is a measure of disorder (and is constantly increasing in the universe, or at least reaches a maximum at equilibrium if the system is closed, unable to exchange heat or mass with its surroundings), enthalpy is a measure of the energy of the reaction (temperature is an average of the energy of molecular motion, hence does not depend on the amount of substance, while the total energy quantities do)and Gibbs Free Energy is a measure of non-pV (p=pressure, V=volume) work that must go into a reaction to make it occur (when G is positive) or work that a reaction can do (when negative). It can be shown that the (change of G)/T in a reaction is the change of the total entropy of the reacting system plus its surroundings during a reaction at constant T, p. The change in H, the enthalpy, is equal to the heat when the change occurs at constant pressure; at constant volume, the change in U, the "internal energy", equals the heat. In working out problems in chemical thermodynamics, one must always be careful to specify the conditions. Heat and work, unlike the state functions, depend on the path a system follows in changing between two states, and are thus not state functions; the sum of the heat added to the system from the surroundings, plus the work done on the system by the surroundings, is the internal energy, U. This sum is a state function, although heat and work separately depend on the path the reaction or other change follows between its initial and final state. There are two major Laws of Thermodynamics: The FIRST LAW states that energy is conserved. It is usually expressed as: change in energy = heat + work. There are several statements of the SECOND LAW, one of which, that a closed system has maximum entropy at equilibrium, is given above, and is due to Clausius. A more useful form in many cases is that given by Kelvin: It is impossible to completely convert heat to work in a cyclic process; some of the heat must be lost to a low temperature reservoir. There is also a "Zeroth Law" defining thermal equilibrium, and a Third Law, stating, in one form, that it is not possible to reach the absolute zero of temperature in a finite number of steps. The Third Law has important consequences for the definition of an absolute entropy, which can be taken as zero for a perfectly ordered system at the absolute zero of temperature.