GCSE Chemistry - Energy Changes

Exothermic reaction

The energy decreases and the temperature rises because heat energy is given out by the reaction to the surroundings. More energy is given out in forming new bonds than was needed to break the old ones.

Endothermic reaction

The energy increases and the temperature decreases, since heat energy is taken in by the reaction from the surroundings. Less energy is given out in forming new bonds than was needed to break the old ones.

Uses of endothermic changes

Cold packs, Thermal decomposition, Electrolysis, Citric acid with sodium hydrogen carbonate, Some sports injury packs

Uses of exothermic changes

Hand warmers, Self heating cans, Neutralisation, Combustion, Many oxidation reactions

Endothermic change often used in cold packs

Dissolving a salt in water

Solid used in the base of self-heating coffee cans

Calcium oxide

Why is it essential that the coffee stays out of contact with the solid in the base of the self-heating can?

Highly corrosive and toxic

What do reaction profiles show?

The relative energies of the reactants and products

Exothermic reaction

Endothermic reaction

Activation energy

The minimum amount of energy that particles must have to react

Why is bond breaking an endothermic process?

It requires energy being put in

Units for bond energies

kJ/mol

Which way do the electrons flow in a chemical cell?

To the less reactive metal

How does a hydrogen fuel cell work?

• Oxygen and the hydrogen fuel is taken in

• The electrolyte is alkaline (for example potassium hydroxide solution)

• It reacts with the oxygen and the hydrogen to produce an electric current

• The hydrogen and the oxygen react to produce the waste product of water

• Water and heat is carried out of the cell