If the reaction of a known amount of substance X, n(X), is carried out in a calorimeter and the temperature change measured, the enthalpy change for that reaction (written with 1 mole of X in the equation) can be calculated as the actual heat transferred is given by

Note the negative sign because if the temperature of the calorimeter rises (ΔT positive) energy has been transfered from the reaction system to the calorimeter (the surroundings) and the reaction is exothermic.

Heat of formation of a substance: Symbol Δ_fH, the heat (enthalpy) change when one mole of that substance is formed from its elements in their standard states.
By convention an element in its most stable form at 100 kPa pressure and 25 ^oC (298 K) is said to be in its standard state and to have an enthalpy of formation, Δ_fH, of zero
Example:

we can also write this equation as:

Heat of combustion of a substance: Symbol Δ_cH, the heat given out when one mole of that substance reacts with dioxygen to give the most oxidised products.
Example:
Δ_cH(CH₄) is the enthalpy change for the reaction

From the appropriate equation above it can be seen that Δ_cH(H₂) = Δ_fH(H₂O).

Hess's law (of constant heat summation): The enthalpy change for a reaction is independent of the way the reaction is carried out (i.e. it depends only on the initial conditions of the reactants and the final conditions of the products). Put another way if a reaction is carried out in a number of steps the enthalpy change is the sum of the enthalpy changes for each individual step.

It follows from Hess's law that for the general reaction

where Σ means "the sum of". This is a very useful expression because the enthalpy change for any reaction can be calculated from tables of heats of formation of the compounds involved.