Overview
This lecture covers the basics of chemical energetics, focusing on exothermic and endothermic reactions, energy level diagrams, enthalpy changes, bond energies, and calculation methods relevant to IGCSE Chemistry.
Exothermic and Endothermic Reactions
- Exothermic reactions release heat energy to the surroundings, increasing the temperature.
- Endothermic reactions absorb heat energy from the surroundings, decreasing the temperature.
- The direction of heat flow defines whether a reaction is exothermic or endothermic.
Reaction Pathway (Energy Level) Diagrams
- Reaction pathway diagrams show the energy changes from reactants to products.
- In exothermic reactions, the reactants have higher energy than the products; energy difference is released as heat.
- In endothermic reactions, the products have higher energy than the reactants; energy is absorbed from surroundings.
- The initial "bump" in the diagram is the activation energy—minimum energy needed for a reaction to start.
Enthalpy Change and Bond Energies
- Enthalpy change (ΔH) measures the heat energy transferred during a reaction; negative for exothermic, positive for endothermic reactions.
- To calculate ΔH:
- Write the balanced equation.
- Find energy in (total bond energies in reactants).
- Find energy out (total bond energies in products).
- ΔH = energy in – energy out.
- Bond breaking is endothermic (absorbs energy); bond making is exothermic (releases energy).
- If energy needed to break bonds < energy released in making bonds, reaction is exothermic (ΔH negative).
- If energy needed to break bonds > energy released in making bonds, reaction is endothermic (ΔH positive).
Example Calculation
- In the methane and chlorine reaction:
- Energy in: 654 kJ/mol (bonds broken).
- Energy out: 769 kJ/mol (bonds formed).
- ΔH = 654 – 769 = -115 kJ/mol, so the reaction is exothermic.
Key Terms & Definitions
- Exothermic reaction — process that releases heat to the surroundings.
- Endothermic reaction — process that absorbs heat from the surroundings.
- Activation energy (Ea) — minimum energy required for reacting particles to collide and react.
- Enthalpy change (ΔH) — heat energy change during a reaction at constant pressure.
- Bond energy — energy needed to break one mole of a specific bond in gaseous molecules.
Action Items / Next Steps
- Practice calculating enthalpy changes using bond energies for different reactions.
- Review and memorize key terms and definitions.
- Study reaction pathway diagrams and be able to interpret them for both reaction types.