Lesson overview
Bond energies: Can you predict the energy change from bonds?
Take ball-and-stick models apart bond by bond and rebuild the products while an energy board behind them steps up for every bond broken and down for every bond made. Do it for three reactions, find where the board ends, and work out ΔH from bond energies: energy in − energy out.
Suggested sequence
- Lesson 1 · Breaking and making bonds. Bond breaking is endothermic, bond making is exothermic, and ΔH comes from the difference. Read together (10–15 min), show or explore the experiment (10 min), practise with self-checking (15–20 min).
- Lesson 2 · Calculating ΔH from bond energies. Supplement: calculating ΔH from bond energies, and finding an unknown bond energy. Read together and discuss (15–20 min), practise with self-checking (20–25 min).
- End of module · final check. 5 new questions, 20–30 min, without hints or notes; pupils check their answers once they have answered them all.
- One to two weeks later · return questions. 3 questions, 15 min, lesson closed.
Learning goals
Part 1: Breaking and making bonds
- Supplement: state that bond breaking is an endothermic process and bond making is an exothermic process, and explain the enthalpy change of a reaction in terms of bond breaking and bond making (5.1.7); place the reactants, products, ΔH and Ea on a reaction pathway diagram from information given (5.1.6)
Part 2: Calculating ΔH from bond energies
- Supplement: calculate the enthalpy change of a reaction using bond energies (5.1.8)
Misconceptions to listen for
- “Breaking bonds releases energy.” — Breaking a bond always takes energy in: the atoms are pulled apart against the attraction of the shared pair of electrons. Energy is given out when new bonds are made.
- “Breaking a bond and making a bond both give energy out.” — If both gave energy out, a molecule could be broken and remade again and again to give out energy for ever. Breaking takes energy in; making gives the same amount out.
- “Breaking a bond and making a bond both take energy in.” — Atoms coming together are pulled into the bond by the shared pair, so making a bond gives energy out. Only breaking takes energy in.
- “Making a bond gives out less energy than breaking it takes in.” — Making a bond gives out exactly the same amount of energy as breaking the same bond takes in: 338 kJ/mol for C–Cl either way.
- “The number in front of a formula can be ignored when you count bonds.” — Count the bonds in one molecule, then multiply by the number in front of the formula: every bond in the molecule is counted that many times.
- “Joining two atoms takes energy in, because work is needed to push them together.” — The two atoms are pulled into the bond by the attraction of the shared pair to both nuclei, so energy is given out as the bond forms.
- “ΔH = energy out − energy in.” — That flips the sign. ΔH = energy taken in breaking bonds − energy given out making bonds, so an exothermic reaction comes out negative.
- “A fuel gives out energy because its bonds break.” — Breaking the bonds in the fuel takes energy in. The energy comes out when the new bonds in the products form, and those give out more than the breaking took in.
- “A positive ΔH means the reaction gives out energy.” — A positive ΔH means the chemicals gain energy: more is taken in breaking bonds than is given out making bonds. The reaction is endothermic.
- “The top of the hump is at the level of the separate atoms, 2A + 2B.” — The staircase breaks every bond first only to add up the energy. Ea is 130 kJ/mol, so the hump is 370 kJ/mol below 2A + 2B.
Scope
Bond breaking and bond making (5.1 Supplement 7–8): bond breaking as endothermic and bond making as exothermic, the enthalpy change of a reaction from the energy in and the energy out, and calculating ΔH from bond energies. Exothermic and endothermic reactions, reaction pathway diagrams and activation energy are in the previous module; Hess’s law and energy cycles are not part of 0620.