Written RevisionQuestions Multiple ChoiceRevision Questions Summary Notes (Lesmahagow High School

A reaction profile is a diagram showing the change in chemical potential energy, referred to as the energy pathway, as a chemical reaction proceeds from reactants to product.

Enthalpy is the term used in Chemistry for the chemical potential energy of a substance (there is a slight difference between the two measures, but for Higher this difference can be ignored). It is given the symbol H. It is not possible to directly measure the enthalpy of a chemical, however during a chemical reaction, the enthalpy change can be measured in kilojoules (kJ). The enthalpy change is given the symbol ΔH.

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 National 5, you encountered the concept of Exothermic Reactions and Endothermic Reactions.

In exothermic reactions, chemical potential energy is changed into heat energy, and released, causing the temperature to rise. As energy is released in this reaction, the enthalpy of the products (Hp) must be lower than the enthalpy of the reactants (Hr). ΔH must therefore have a negative value for an exothermic reaction.

Runaway reactions such as those causing the disasters in Bhopal and Seveso occur when the rate at which a chemical reaction releases energy exceeds the capabilities of the plant to remove heat.

In endothermic reactions, heat energy is taken in from the surroundings and changed into chemical potential energy, causing the temperature to fall. As energy is gained in this reaction, the enthalpy of the products (Hp) must be greater than the enthalpy of the reactants (Hr). ΔH must therefore have a positive value for an endothermic reaction.

With endothermic reactions requiring heating to continue, any endothermic process incurs energy costs. This must be considered when assessing the economic viability of the chemical process concerned.

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