Overview
This lecture covers chemical energetics, distinguishing between exothermic and endothermic reactions, the use of energy level diagrams, bond energies, and how to calculate enthalpy change for chemical reactions.
Exothermic and Endothermic Reactions
- Chemical reactions can be exothermic (release heat) or endothermic (absorb heat).
- Exothermic reactions transfer heat to the surroundings, increasing the temperature.
- Endothermic reactions absorb heat from the surroundings, lowering the temperature.
Energy Level (Reaction Pathway) Diagrams
- Reaction pathway diagrams show the energy of reactants and products.
- In exothermic reactions, reactants have more energy than products; energy difference is released as heat.
- In endothermic reactions, products have higher energy than reactants; energy is absorbed from surroundings.
- The initial “bump” in the diagram is the activation energy needed to start the reaction.
Enthalpy Change and Bond Energies
- Activation energy (Eₐ) is the minimum energy required for reactants to react.
- Enthalpy change (ΔH) is the heat energy transferred during a reaction.
- ΔH is negative for exothermic reactions and positive for endothermic reactions.
- To calculate ΔH using bond energies:
- Write a balanced equation.
- Add total bond energies for bonds broken (reactants) = energy in.
- Add total bond energies for bonds formed (products) = energy out.
- ΔH = energy in − energy out.
- Bond breaking is endothermic (energy absorbed); bond making is exothermic (energy released).
Worked Example: Methane and Chlorine Reaction
- CH₄ + Cl₂ → CH₃Cl + HCl.
- Only one CH bond and one Cl–Cl bond are broken; one C–Cl bond and one H–Cl bond are formed.
- Energy in: 654 kJ/mol; energy out: 769 kJ/mol; ΔH = 654 - 769 = -115 kJ/mol.
- Negative ΔH means the reaction is exothermic.
Key Terms & Definitions
- Exothermic reaction — releases heat to surroundings, raising temperature.
- Endothermic reaction — absorbs heat from surroundings, lowering temperature.
- Activation energy (Eₐ) — minimum energy needed to start a reaction.
- Enthalpy change (ΔH) — heat energy change in a reaction.
- Bond energy — energy required to break one mole of a bond in kJ/mol.
Action Items / Next Steps
- Practice calculating enthalpy change using bond energies with other reactions.
- Review energy level diagrams and memorize the differences between exothermic and endothermic reactions.