Overview
This lecture covers exothermic and endothermic reactions, energy transfer, reaction profiles, activation energy, and calculating energy changes using bond energies for AQA GCSE Chemistry Combined Science.
Exothermic and Endothermic Reactions
- Exothermic reactions transfer energy to the surroundings; temperature increases are detected.
- Examples of exothermic reactions: combustion (burning), most oxidation reactions, and neutralisation.
- Uses include self-heating cans and hand warmers due to the heat released.
- Endothermic reactions absorb energy from the surroundings; temperature decreases are detected.
- Examples of endothermic reactions: thermal decomposition and citric acid with sodium hydrogen carbonate.
- Uses include sports injury packs to reduce swelling.
Activation Energy
- Activation energy is the minimum energy required for reacting particles to collide successfully and cause a reaction.
Reaction Profiles
- Reaction profiles graph energy changes during a reaction, with energy on the y-axis and reaction progress on the x-axis.
- Activation energy is the peak between reactant and product energy levels on the profile.
- Exothermic reaction profiles: products have less energy than reactants; energy is released.
- Endothermic reaction profiles: products have more energy than reactants; energy is absorbed.
- Overall energy change is the difference between reactants' and products' energy levels.
Calculating Energy Changes (Higher Tier Only)
- Energy is supplied to break bonds in reactants and released when making bonds in products.
- Overall energy change = energy supplied to break bonds − energy released when making bonds.
- A negative value means exothermic; a positive value means endothermic.
- Example: Hydrogen and oxygen to water: total energy supplied (breaking bonds) minus total released (making bonds) gives negative value (exothermic).
- Example: Methane and chlorine reaction calculated similarly, showing exothermic process.
Key Terms & Definitions
- Exothermic reaction — reaction that releases energy to the surroundings.
- Endothermic reaction — reaction that absorbs energy from the surroundings.
- Activation energy — minimum energy needed for particles to react.
- Reaction profile — diagram showing energy changes during a chemical reaction.
- Overall energy change — energy difference between reactants and products.
- Bond energy — energy required to break or form a bond, measured in kJ/mol.
Action Items / Next Steps
- Review and practice bond energy calculations.
- Go over key points on exothermic and endothermic reactions.
- Refer to diagrams of reaction profiles for labeling practice.