Topic 5: Energy Changes

What is the law of conservation of energy in chemical reactions?

Energy is conserved; the total energy at the start and end of a reaction is the same.

What is an exothermic reaction?

A reaction that transfers energy to the surroundings, increasing temperature.

What is an endothermic reaction?

A reaction that absorbs energy from the surroundings, decreasing temperature.

Give two examples of exothermic reactions.

Combustion and neutralisation.

Give two examples of endothermic reactions.

Thermal decomposition and citric acid + sodium hydrogencarbonate.

What are everyday uses of exothermic reactions?

Self-heating cans and hand warmers.

What are everyday uses of endothermic reactions?

Sports injury packs.

What is activation energy?

The minimum energy needed for particles to react when they collide.

In reaction profiles, how do you tell if a reaction is exothermic?

Products have less energy than reactants.

In reaction profiles, how do you tell if a reaction is endothermic?

Products have more energy than reactants.

What must happen to bonds during a chemical reaction?

• Energy is taken in to break bonds (endothermic)

• Energy is released when bonds form (exothermic)

How is overall energy change calculated?

Energy in to break bonds – Energy out from forming bonds = Overall energy change

What does it mean if more energy is released forming bonds than breaking them?

The reaction is exothermic.

What does it mean if more energy is required to break bonds than is released forming them?

The reaction is endothermic.