Chemical Energetics

You will be able to

• Draw and interpret reaction pathway diagrams for exothermic and endothermic reactions, showing ΔH and the activation energy Eₐ.
• State that an exothermic reaction has a negative ΔH and transfers energy to the surroundings, so the temperature rises.
• Explain that a catalyst lowers the activation energy but does not change ΔH.

You will be able to

• Calculate ΔH from bond energies: energy to break bonds in the reactants minus energy released making bonds in the products.
• Explain that bond breaking is endothermic and bond making is exothermic.
• Deduce whether a reaction is exothermic or endothermic from the balance of bond breaking and bond making.

You will be able to

• Determine the temperature change ΔT from a temperature–time graph and classify a reaction as exothermic or endothermic.
• Describe how the amount of reactant affects the temperature change, and why it stops increasing when the other reactant runs out.
• Explain why a lid and an insulated cup reduce heat loss and give a more accurate temperature change.