• Quick retrieval from earlier learning (Reaction profiles: energy change and activation energy). Students answer on mini whiteboards.
• Show answers together and note who needs support in the main task.

1. On a reaction profile, the reactants are at 80 kJ, the peak is at 120 kJ and the products are at 40 kJ. What is the overall energy change, in kJ? (Use a minus sign if energy is released.) → -40 kJPrompt: energy change = energy of products − energy of reactants
2. Reactants: 140 kJ. Peak of the profile: 190 kJ. Products: 110 kJ. What is the activation energy, in kJ? → 50 kJPrompt: Activation energy is the minimum energy needed to start the reaction.
3. Why does a mixture of methane and air not burn until a spark is added? (Methane is endothermic / The spark removes carbon dioxide / The spark provides the activation energy / The spark adds oxygen) → The spark provides the activation energyPrompt: Reactions need a minimum energy to start.

• Drop an antacid tablet in water (or show a clip) and ask whether a new substance is being made.
• Share the objective: Calculate energy changes from bond energies. Read the success criteria together.

• Work through the example out loud, saying why you do each step, not just what you do.
• Example: CH₄ + 2O₂ → CO₂ + 2H₂O. Bond energies (kJ/mol): C–H 413, O=O 498, C=O 805, O–H 464. Calculate the energy change, in kJ/mol.
• Step 1: broken: 4 × 413 + 2 × 498 = 2648
• Step 2: made: 2 × 805 + 4 × 464 = 3466
• Step 3: 2648 − 3466 = −818
• Step 4: So the answer is: −818 kJ/mol
• Answer: -818 kJ/mol
• Pause after each step and ask a student to say what comes next.

• Do these together. Students try each step on whiteboards before you reveal it.
• Use the hints as prompts rather than giving the answer.

1. CH₄ + 2O₂ → CO₂ + 2H₂O. Bond energies (kJ/mol): C–H 413, O=O 498, C=O 805, O–H 464. Calculate the energy change, in kJ/mol. → -818 kJ/molPrompt: energy change = energy to break bonds − energy released making bonds
2. In a reaction, 950 kJ is needed to break bonds and 1350 kJ is released when new bonds form. Is the reaction exothermic or endothermic? (exothermic / endothermic) → exothermicPrompt: Breaking bonds takes in energy; making bonds releases it.

• Students work on the student sheet. Circulate and check the first two answers of every student early.
• Students who finish: write a question of their own for a partner, with the answer.

The 6 questions are on the student sheet below, with answers in the key.

• Two questions on their own. Sort answers into got it / nearly / not yet to plan the next lesson.

1. In an exothermic reaction, how do the energies of bonds broken and bonds made compare? (more energy is released making bonds than is taken in breaking bonds / they are always equal / no bonds are made / breaking bonds releases more energy) → more energy is released making bonds than is taken in breaking bondsPrompt: The overall change is negative.
2. In a reaction, 1200 kJ is needed to break bonds and 1600 kJ is released when new bonds form. Is the reaction exothermic or endothermic? (exothermic / endothermic) → exothermicPrompt: Breaking bonds takes in energy; making bonds releases it.

Free online practice with hints and printable worksheets for this skill: eurekami.com/science/grade-11.