GCSE Chemistry: Energy Changes & Rates — Revision Workbook

Inside GCSE Chemistry: Energy Changes & Rates — Revision Workbook

sort exothermic reactions from endothermic ones with everyday examples, read a reaction profile and find the activation energy, calculate an overall energy change from bond energies (energy in − energy out), work out a mean rate of reaction as quantity ÷ time, use collision theory to explain the four rate factors and how a catalyst works, and describe a reversible reaction reaching dynamic equilibrium. About 30 minutes.

Learning objectives

• energy changes — exothermic reactions (energy given out, surroundings warm) versus endothermic reactions (energy taken in, surroundings cool), with everyday examples such as combustion, neutralisation and cold packs;
• reaction profiles and activation energy — reading the energy diagram, the activation-energy hump, and the position of products for an exothermic change, plus bond-energy calculations (energy in to break bonds − energy out to make bonds) [higher];
• rate of reaction — measuring gas volume or mass loss, the mean-rate formula (quantity ÷ time), and using tangents and gradients to read a rate graph;
• collision theory and rate factors — temperature, concentration/pressure, surface area and catalysts, and how a catalyst lowers the activation energy;
• reversible reactions and dynamic equilibrium — the ⇌ symbol, closed systems and a first look at Le Chatelier’s principle [higher].

Activities in this book

1. Task 1 — Exothermic & Endothermic Reactions

   an exothermic reaction transfers energy to the surroundings, so the temperature of the surroundings goes up — combustion, neutralisation and most oxidation reactions are exothermic. An endothermic reaction takes energy in from the surroundings, so the temperature falls — thermal decomposition and the reaction in a sports cold pack are endothermic.

2. Task 2 — Reaction Profiles, Activation Energy & Bond Energies

   a reaction profile plots energy against progress of reaction. The activation energy is the minimum energy colliding particles need to react — the height of the hump. For an exothermic reaction the products sit lower than the reactants. Using bond energies: overall energy change = energy to break bonds (in) − energy released making bonds (out). A negative answer means exothermic.

3. Task 3 — Measuring the Rate of Reaction

   the mean rate of a reaction = quantity of reactant used or product formed ÷ time taken. You can measure the volume of gas made (cm³/s) or the mass lost (g/s). On a rate graph the gradient gives the rate: draw a tangent to find the rate at one instant, and the steeper the line, the faster the reaction.

4. Task 4 — Collision Theory, Rate Factors & Equilibrium

   collision theory says particles react only when they collide with enough energy (the activation energy). Rate goes up with higher temperature, higher concentration or pressure, larger surface area, and a catalyst. A catalyst speeds up a reaction by providing a route with a lower activation energy and is not used up. A reversible reaction (shown by ⇌) in a closed system reaches dynamic equilibrium.

5. Task 5 — Exam-Style 6-Marker

   this is a longer written question, like the 6-mark answers on the exam. Plan your points, use the keywords from Tasks 1–4, and link cause to effect — for example, "more particles have energy greater than the activation energy, so more collisions are successful each second, so the rate increases."

Suggestions for teaching

• Fix the direction of energy first. Drill exothermic (out, warmer) against endothermic (in, cooler) with concrete examples before touching profiles or calculations.
• Draw the profile. Model an exothermic and an endothermic profile, labelling reactants, products and the activation-energy hump, so the bond-energy sign makes sense.
• Show the bond-energy method. Always lay it out as energy in (bonds broken) minus energy out (bonds made); a negative answer means exothermic. Work H₂ + Cl₂ → 2HCl together.
• Practise mean rate as a division. Rehearse quantity ÷ time with units (cm³/s or g/s), then link the graph gradient and tangent to rate at an instant.
• Explain rate with collisions. Tie every factor — temperature, concentration, surface area, catalyst — back to frequency of collisions and the activation energy, then extend to reversible reactions and equilibrium.