Overview
This lecture covers the main concepts of chemical energetics, focusing on exothermic and endothermic reactions, reaction pathway diagrams, activation energy, enthalpy change, and calculating enthalpy using bond energies.
Exothermic and Endothermic Reactions
- Chemical reactions are classified as exothermic or endothermic based on heat exchange with surroundings.
- Exothermic reactions transfer heat energy to the surroundings, increasing the temperature.
- Endothermic reactions absorb heat energy from the surroundings, decreasing the temperature.
- Exothermic reactions release heat, endothermic reactions absorb heat.
Reaction Pathway Diagrams and Activation Energy
- Reaction pathway (energy level) diagrams show energies of reactants and products.
- Exothermic: reactants have more energy than products; energy difference is released as heat.
- Endothermic: products have more energy than reactants; energy is absorbed from surroundings.
- Activation energy is the minimum energy required to start a reaction, shown as a “bump” in the diagram.
Enthalpy Change and Calculations
- Enthalpy change (ΔH) measures heat transfer during a reaction.
- ΔH is negative for exothermic reactions, positive for endothermic reactions.
- To calculate ΔH using bond energies:
- Write the balanced equation.
- Find the total bond energies for all bonds broken (energy in).
- Find the total bond energies for all bonds formed (energy out).
- ΔH = energy in − energy out.
Bond Making and Breaking
- Bond breaking is endothermic (requires energy), bond making is exothermic (releases energy).
- If energy needed to break bonds < energy released when making bonds: exothermic (ΔH negative).
- If energy needed to break bonds > energy released: endothermic (ΔH positive).
Example Calculation Using Bond Energies
- For methane reacting with chlorine to form methyl chloride and hydrogen chloride:
- Only one C-H bond is broken per methane molecule.
- Energy in = bond energies of 1 C-H + 1 Cl-Cl = 654 kJ/mol.
- Energy out = bond energies of 1 C-Cl + 1 H-Cl = 769 kJ/mol.
- ΔH = 654 − 769 = −115 kJ/mol (exothermic reaction).
Key Terms & Definitions
- Exothermic Reaction — releases heat to surroundings.
- Endothermic Reaction — absorbs heat from surroundings.
- Activation Energy (Ea) — minimum energy required for a reaction to occur.
- Enthalpy Change (ΔH) — heat energy change during a reaction.
- Bond Energy — energy required to break one mole of a specific bond.
Action Items / Next Steps
- Review reaction pathway diagrams and be able to label energy levels and activation energy.
- Practice enthalpy change calculations using different sets of bond energies.
- Prepare for exam questions on identifying exothermic and endothermic reactions.