Thermochemistry: Study of the heat released or absorbed by chemical reactions.

Rearrangements of atoms that occur during chemical reactions involve both bond breaking and bond formation. Bond breaking results in absorption of heat from the surroundings and bond formation in release of heat to the surroundings.

Heat: The transfer of energy as a result of a temperature difference between a system and its surroundings. The quantity of energy transferred from the surroundings to the system is given the symbol q. The system means the substance, or reactants and products of a reaction, and the surroundings everything else. When energy is transferred from the system to the surrroundings q is negative. Chemists frequently make measurements at constant (atmospheric) pressure. Energy transferred at constant pressure is given the symbol q_p.

A chemical equation can be made more meaningful and informative by showing the states of the reactants and products and the quantity of heat released or absorbed.
example:

implies that:
when 2 mole of gaseous dihydrogen reacts with one mole of gaseous dioxygen forming two mole of liquid water 570 kilojoule of energy is released as heat from the reacting system to its surroundings.
The Δ means change, r stands for reaction and H is the symbol for enthalpy.
Δ_rH is the enthalpy change of the reaction.

Enthalpy change: ΔH, the change in energy of a system which undergoes a change at constant pressure.
ΔHq_p.

Exothermic reaction: A reaction that releases energy to the surroundings.
ΔH < 0
Endothermic reaction: A reaction that absorbs energy from the surroundings.
ΔH > 0

Δ_rH is negative for an exothermic reaction (the energy of the system, the reactants and products, is less after the reaction than before) and positive for an endothermic reaction (the energy of the system is greater than before).
Note that the per mole (mol⁻¹) in the units of Δ_rH refers to the stoichiometric coefficients in the equation as amounts in moles.
Thus, if the equation for the burning of hydrogen were written as

It is most important to understand the difference between the enthalpy change for a system and that of a reaction. The magnitude of former depends on the amount present in the system and has units of energy, e.g. kJ, while the amount of the latter is defined by the chemical equation and has common units of kJ mol⁻¹.

Latent heat: An old term for the enthalpy change of a change in state.
example:

Ice has to be heated for it to melt, so ΔH is positive.
This is called the enthalpy of fusion (or melting) and given the symbol Δ_fusH(H₂O).
The symbols for enthalpy of sublimation and vaporisation are Δ_subH and Δ_vapH respectively.

Note:  The convention used here is to place the explanatory subscript between the Δ and the H, as recommended by IUPAC, the International Union of Pure and Applied Chemistry. Until recently the subscript was placed after the H and r was omitted for enthalpy changes of chemical reactions.

Calorimetry: The measurement of enthalpy change using a calorimeter.

Calorimeter: An apparatus for measuring the heat absorbed or released in a chemical reaction.

Heat capacity: symbol C, the ratio of the heat supplied to the temperature rise produced
i.e. the energy required to raise the temperature of a substance or system by one degree (^oC or K) when no chemical or phase changes occur.

Heat capacity at constant pressure: Symbol, C_p, the heat capacity when the change occurs at constant pressure, the most common condition.

Specific heat capacity: Symbol, c_p, the heat capacity of unit mass of a substance;
common units, J K⁻¹ g⁻¹
Physicists use J K⁻¹ kg⁻¹

Molar heat capacity: Symbol, C_p,m, the heat capacity of one mole of a substance;
common units, J K⁻¹ mol⁻¹.