5. Energy changes

Law of conservation of energy

Energy cannot be created or destroyed, only transferred or transformed.

Exothermic reaction

A reaction that releases energy to the surroundings, usually as heat.

Examples of exothermic reactions

Combustion, neutralisation, and respiration.

Everyday uses of exothermic reactions

Self-heating cans and hand warmers.

Endothermic reaction

A reaction that takes in energy from the surroundings.

Examples of endothermic reactions

Thermal decomposition and photosynthesis

Everyday use of endothermic reactions

Cold packs for sports injuries.

Equipment to determine reaction type

A thermometer and an insulated container (e.g., polystyrene cup).

Reaction vessel for measuring temperature changes

A polystyrene cup to reduce heat loss to the surroundings.

Independent variable in metal-acid reaction

The type of metal.

Dependent variable in metal-acid reaction

The temperature change.

Control variables in metal-acid reaction

The volume of acid and the concentration of acid.

Collision Theory

Particles must collide with enough energy and the correct orientation to react.

Activation energy

The minimum amount of energy required for a reaction to occur.

Energy profile for an exothermic reaction

A curve that starts high (reactants), peaks at the activation energy, and ends lower (products).

Activation energy label

The difference in energy between the reactants and the peak of the curve.

Overall energy change label

The difference in energy between the reactants and products.

Energy profile for an endothermic reaction

A curve that starts low (reactants), peaks at the activation energy, and ends higher (products).

Bond breaking energy change

Bond breaking requires energy, so it is endothermic.

Bond production energy change

Bond forming releases energy, so it is exothermic.

Overall energy change calculation

Energy needed to break bonds - energy released when bonds are formed.

Energy changes in exothermic reaction

More energy is released in bond making than is required for bond breaking.

How cells produce electricity

Through a chemical reaction between two different metals in an electrolyte.

Factors affecting cell voltage

The type of metal and the concentration of the electrolyte.

Making a simple cell

Place two different metals in an electrolyte and connect them with wires.

Difference between battery and cell

A battery contains two or more cells connected together.

Advantage of batteries over cells

They provide a greater voltage.

When non-rechargeable cells stop working

When one of the reactants is used up.

Example of a non-rechargeable cell

An alkaline battery.

Recharging rechargeable cells

By reversing the chemical reactions using an external electric current.

Reactivity and cell voltage

Greater differences in reactivity produce a higher voltage.

Fuel cells and potential difference

Hydrogen reacts with oxygen to produce a voltage.

Overall chemical equation in hydrogen fuel cell

2H₂ + O₂ → 2H₂O.

Advantage of hydrogen fuel cells over combustion fuel

They are more efficient and produce no harmful pollutants.

Advantage of hydrogen fuel cells over fossil fuels

They only produce water as a waste product.

Negative environmental consequences of hydrogen fuel cells

Hydrogen production may involve fossil fuels and is energy-intensive.

Half equation at anode in hydrogen fuel cell

H₂ → 2H⁺ + 2e⁻.

Half equation at cathode in hydrogen fuel cell

O₂ + 4H⁺ + 4e⁻ → 2H₂O.