• 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 180 kJ and the products are at 100 kJ. What is the overall energy change, in kJ? (Use a minus sign if energy is released.) → 20 kJPrompt: energy change = energy of products − energy of reactants
2. Reactants: 140 kJ. Peak of the profile: 290 kJ. Products: 110 kJ. What is the activation energy, in kJ? → 150 kJPrompt: Activation energy is the minimum energy needed to start the reaction.
3. How does a catalyst change a reaction profile? (It removes the reactants / It raises the products / It lowers the peak (activation energy) but not the overall energy change / It changes an exothermic reaction into an endothermic one) → It lowers the peak (activation energy) but not the overall energy changePrompt: A catalyst provides a different reaction pathway.

• 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. H₂ + Cl₂ → 2HCl. Bond energies (kJ/mol): H–H 436, Cl–Cl 243, H–Cl 432. Calculate the energy change, in kJ/mol. → -185 kJ/molPrompt: energy change = energy to break bonds − energy released making bonds
2. In a reaction, 1350 kJ is needed to break bonds and 1270 kJ is released when new bonds form. Is the reaction exothermic or endothermic? (exothermic / endothermic) → endothermicPrompt: 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. N₂ + 3H₂ → 2NH₃. Bond energies (kJ/mol): N≡N 945, H–H 436, N–H 391. Calculate the energy change, in kJ/mol. → -93 kJ/molPrompt: energy change = energy to break bonds − energy released making bonds
2. In a reaction, 1250 kJ is needed to break bonds and 1500 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.