Topic 5 - Energy Changes

What is conservation of energy

When chemical reactions occur, energy is conserved:

• The amount of energy in the universe at the beginning is the same as at the end.

• This means if a reaction transfers energy to the surroundings, the product molecules must have less energy than the reactants, by the amount transferred

Exothermic reactions

• An exothermic reaction is one that transfers energy to the surroundings, so the temperature of the surroundings increases.

• Product molecules must have less energy than the reactants by the amount transferred.

Examples of exothermic reactions include:

• combustion

• many oxidisation reactions

• neutralisation

Everyday examples of exothermic reactions include:

• self-heating cans (eg, for coffee)

• hand warmers

Endothermic reactions

• An endothermic reaction is one that takes in energy from the surroundings, so the temperature of the surroundings decreases

• Product molecules must have more energy than reactants

Examples of endothermic reactions include:

• thermal decomposition

• the reaction of citric acid and sodium hydrogencarbonate

• Some sports injury packs are based on endothermic reactions

Exothermic and Endothermic energy profile diagrams

What do energy profiles show

• Energy profiles can be used to show the relative energies of reactants and products, the activation energy, and the overall energy change of a reaction

• You can tell which reaction is exothermic or endothermic by looking at whether the reactants or products have more energy (higher up the energy scale = higher energy)

Exothermic:

• heat is released to the surroundings, so products have less energy than reactants

• it’s exothermic because we finished with less energy than we started with

Endothermic:

• heat taken in from the surroundings, so products have more energy than reactants

• it’s endothermic because we finished with more energy than we started with

Energy change of reactants - what happens during a chemical reaction

• Energy must be supplied to break bonds in the reactants

• Energy is released when bonds in the products are formed

• The energy needed to break bonds and the energy released when bonds are formed can both be calculated from bond energies