IGCSE Chemistry Topic 6: Chemical Energetics

Chemical Energetics

Energy Transfer

• Burning fuels (combustion) releases heat energy by reacting fuels with oxygen.
• Hydrogen can be used as a fuel, burnt to produce energy.

Use of Hydrogen as a Fuel

• Hydrogen reacts with oxygen in a fuel cell to generate electricity.
• Oxidization of hydrogen within the fuel cell produces a potential difference.
• The overall reaction in a hydrogen fuel cell involves the oxidation of hydrogen to produce water.

Energetics of a Reaction

Exothermic and Endothermic Reactions

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

• Examples of exothermic reactions include combustion, many oxidation reactions, and neutralization.

• Everyday examples of exothermic reactions include self-heating cans and hand warmers.

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

• Examples of endothermic reactions include thermal decomposition and the reaction of citric acid and sodium hydrogencarbonate.

• Some sports injury packs are based on endothermic reactions.

Calculating Energy of a Reaction

• Calculate the energy of a reaction using bond energies:
  1. Add together all bond energies for reactants (energy in).
  2. Add together all bond energies for products (energy out).
  3. Calculate the energy change: energy in - energy out.
• If energy out > energy in, the energy change will be negative, showing an exothermic reaction.
• If energy out < energy in, the energy change will be positive, showing an endothermic reaction.

Bond Breaking and Bond Forming

• Bond breaking is an endothermic process, requiring energy to break bonds in reactants.
• Bond forming is an exothermic process, releasing energy when bonds are formed in products.

Reaction Profiles

• Reaction profiles can be used to show the relative energies of reactants and products, the activation energy, and the overall energy change of a reaction.
• Arrows on energy level diagrams show the activation energy (energy required to start a reaction - labeled as 'Ea' on diagram).
• Exothermic reactions have products with lower energy than reactants, while endothermic reactions have products with higher energy than reactants.

Energy Level Diagrams

• Energy level diagrams can be used to illustrate exothermic and endothermic reactions.
• Diagrams show the relative energies of reactants and products, the activation energy, and the overall energy change of a reaction.