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The enthalpy change is calculated from the difference in enthalpy between the enthalpy of the products (Hp) and the reactants (Hr).
ΔH = Hp – Hr
The calculation is product minus reaction, which means that enthalpy changes can be positive or negative. If the enthalpy change for one mole of substance is calculated, then the enthalpy change is expressed in kJ.mol-1. Enthalpy change can be calculated from the potential energy diagram.
- In exothermic reactions, chemical potential energy is changed into heat energy, and released. As energy is released the enthalpy of the products (Hp) must be lower than the enthalpy of the reactants (Hr). ΔH is negative for an exothermic reaction.
- In endothermic reactions, heat energy is taken in from the surroundings and changed into chemical potential energy. As energy is gained in this reaction, the enthalpy of the products (Hp) must be greater than the enthalpy of the reactants (Hr). ΔH is positive for an endothermic reaction.
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