• Quick retrieval from earlier learning (Reaction profiles: energy change and activation energy; Rates of reaction). 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 50 kJ, the peak is at 110 kJ and the products are at -40 kJ. What is the overall energy change, in kJ? (Use a minus sign if energy is released.) → -90 kJPrompt: energy change = energy of products − energy of reactants
2. A reaction produces 10 cm³ of gas in 20 seconds. What is the mean rate of reaction, in cm³/s? → 0.5 cm³/sPrompt: mean rate = amount of product ÷ time
3. Reactants: 40 kJ. Peak of the profile: 90 kJ. Products: 10 kJ. What is the activation energy, in kJ? → 50 kJPrompt: Activation energy is the minimum energy needed to start the reaction.

• Drop an antacid tablet in water (or show a clip) and ask whether a new substance is being made.
• Share the objective: Predict how an equilibrium shifts (Le Chatelier). Read the success criteria together.

• Work through the example out loud, saying why you do each step, not just what you do.
• Example: All substances are gases in this equilibrium: N₂O₄ ⇌ 2NO₂. The forward reaction is endothermic. The pressure is increased. What happens to the position of equilibrium?
• Step 1: Higher pressure favours the side with fewer gas molecules.
• Step 2: So the answer is: shifts left (more reactants)
• Answer: shifts left (more reactants)
• 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. All substances are gases in this equilibrium: N₂O₄ ⇌ 2NO₂. The forward reaction is endothermic. The pressure is increased. What happens to the position of equilibrium? (shifts right (more products) / shifts left (more reactants) / no change in position) → shifts left (more reactants)Prompt: The system shifts to oppose the change.
2. What effect does a catalyst have on the position of equilibrium? (it stops the reverse reaction / none: it only helps equilibrium be reached faster / it shifts it left / it shifts it right) → none: it only helps equilibrium be reached fasterPrompt: A catalyst speeds up both directions equally.

• 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. All substances are gases in this equilibrium: H₂ + I₂ ⇌ 2HI. The pressure is increased. What happens to the position of equilibrium? (shifts right (more products) / shifts left (more reactants) / no change in position) → no change in positionPrompt: The system shifts to oppose the change.
2. Why does a closed system matter for equilibrium? (Nothing can enter or leave, so both reactions can balance / It keeps the system warm / It adds a catalyst / It makes the reaction irreversible) → Nothing can enter or leave, so both reactions can balancePrompt: What happens if a gaseous product escapes?

Free online practice with hints and printable worksheets for this skill: eurekami.com/science/grade-10.