Thermochemistry

In thermodynamics and physical chemistry, thermochemistry is the study of energy absorbed or evolved in chemical reactions and any physical transformations, such as melting and boiling. Thermochemistry, generally, is concerned with the energy exchange accompanying transformations, like mixing, chemical reactions, phase transitions,  and including calculations of such quantities as the heat capacity, heat of combustion, heat of formation, enthalpy, and free energy. The measurement of heat changes is performed using calorimetry, generally an enclosed chamber within which the change to be examined occurs. A system can be; an isolated system, when it cannot exchange matter or energy with the surroundings, as with the insulated bomb reactor; a closed system, when it exchange energy but not matter with the surroundings, as with a steam radiator; an open system, when it can exchange both matter and energy with the surroundings, as with a pot of boiling water. This method relates to the change of state. An isothermal process occurs when temperature of the system remains constant. An isobaric process takes place when the pressure of the system remains constant. An adiabatic process takes place when no heat exchange occurs.

Several laws apply when using thermochemical equations:

ΔH is proportional to the quantity of a substance which reacts or is produced by the reaction. Enthalpy is proportional to mass. Thus, if you double the coefficients in an equation, then value of ΔH is multiplied by two.

ΔH for the reaction is equal in magnitude but opposite in sign to ΔH for reverse reaction. This law is commonly applied to phase changes, even if it is true when you reverse any thermochemical reaction.

ΔH is independent of the number of steps involved. This rule is called as Hess's Law. It states that ΔH for a reaction is the same whether it takes place in one step or in a series of steps. Another way to look at it is to remember that ΔH is a state property, so it should be independent of path of a reaction.