Enthalpy change is the heat energy transferred in a reaction at constant pressure (KJ mol-1).
Standard pressure is 100 KPa.
Exothermic reactions give out energy so the enthalpy is negative.
Endothermic reaction absorb energy so the enthalpy is positive.
Reactions are all about making and breaking bonds.
You need energy to break bonds so bond breaking is endothermic. Stronger bonds need more energy to be broken.
Energy is released when bonds are formed so bond forming is exothermic. Stronger bonds release more energy when they are formed.
The enthalpy change is the overall effect of the making and breaking of the bonds.
Enthalpy change in a reaction = Total energy absorbed - Total energy released
Standard enthalpy change of a reaction is the nethalpy change when the reaction occurs in the molar quantities shown in the chemical equation, under standard conditions in their standard states.
Standard enthalpy change of formation is the enthalpy change when 1 mole of a compound is formed from its elements in their standard states under standard conditions.
Standard enthalpy change of combustion is the enthalpy change when 1 mole of a substance is completely burned in oxygen under standard conditions.
Calculating Enthalpy Changes
q = mc∆T
q = heat lost or gained in joules. This is the same as the enthalpy change if the pressure is constant.
m = mass of water in the calorimeter.
c = specific heat capacity or water 4.18 Jg-1 K-1.
∆T = change in temperature of the water solution.
Hess's Law - the total enthalpy change is independent of the route taken.