The enthalpy change of a system is the heat energy change at constant pressure. It is indicated as delta H, where delta means ‘change in’ and H is enthalpy.
When a system gives out heat energy to the surroundings, enthalpy is lost by the system so that the delta H is negative (exothermic).
When a system takes in heat from the surroundings, enthalpy is gained by the system so that delta H is positive (endothermic).
Standard enthalpy of combustion ΔHcO
The standard enthalpy of combustion is defined as the enthalpy change, under standard conditions, when 1 mol of a substance is burned completely in oxygen, with all reactants and products in their standard states.
Enthalpies of combustion are determined experimentally using a calorimeter.
Standard enthalpy of Formation ΔfHO
The standard enthalpy of formation is defined as the enthalpy change under standard conditions, when 1 mol of a compound is formed from its elements with all the reactants and products in their standard states.
By definition for an element the standard enthalphy of formation must be zero.
Enthalpies of formation are usually determined indirectly using Hess’s law.
The heat energy, q, required to change the temperature of a substance by an amount ΔT can be calculated by using this equation: q = m x c x ΔT
Where m is the mass o the substance and c being the specific heat capacity. The units for c are kJ K −1 kg −1. The mass (m) needs to be in kilograms and ΔT in kelvin, q’s