Energy Changes

Law of conservation of energy / mass

Energy is conserved and cannot be made or destroyed, only transferred.

→ The amount of energy at the end of a chemical reaction is the same as before the reaction takes place.

Exothermic reaction and examples

A reaction where energy is transferred to the surroundings.

→ surroundings temperature increases

Examples:

• combustion, oxidation reactions and neutralisation (acid + alkali) reaction, hand warmers

Endothermic reaction and examples

A reaction where energy is taken in from the surroundings

→ surroundings temperature decreases

Examples:

• thermal decomposition, photosynthesis, neutralisation, sports injury packs

Activation energy

Minimum amount of energy needed for a reaction to take place.

Reaction profile

Graph which shows the relative energies of reactants and product, as well as activation energy and energy change of the reaction.

Bond energies in chemical reactions

Energy is supplied to break bonds and energy is released when bonds are made.

Bond energies in Exothermic reactions

Energy released from forming bonds is greater than that needed to break the bonds.

Bond energies in Endothermic reactions

Energy released from forming bonds is less than that needed to break the bonds.

Equation for energy change

Energy of reaction = sum of bonds broken - sum of bonds made

Rechargeable cells

Chemical reactions are reversed when an external current is supplied.

Non-rechargeable cells

Reactants are used up, reactions cannot be recharged.

Fuel cell

Fuel cells are supplied by fuel and oxygen to oxidise the fuel to generate electricity.

Overall reaction in hydrogen fuel cell

Cathode: 2 H2 → 4 H+ + 4 e−

Anode: O2 + 4 H+ + 4 e−→ 2 H2O

Overall: 2 H2 + O2 → 2 H2O.

Advantages of hydrogen fuel cells

• no pollutants

• no recharging

Disadvantages of hydrogen fuel cells

• flammable

• Hydrogen difficult to store / transport

• fossil fuel production

• toxic chemicals

• expensive production of hydrogen by electrolysis

Rechargeable cells characteristics

• Fuel doesn’t need to be constantly supplied

• Hard to dispose of → non-biodegradable

• eventually stops working