GCSE Chemistry: Energy & Rates — Deep Revision Workbook

Inside GCSE Chemistry: Energy & Rates — Deep Revision Workbook

sort exothermic and endothermic reactions and their uses (hand warmers, sports packs), read reaction profiles and calculate energy changes from bond energies (energy in − energy out), work out mean rates and read tangents on rate graphs, explain the five rate factors and catalysts with collision theory, apply Le Chatelier to a dynamic equilibrium, and finish with a written exam-style 6-marker. About 45 minutes.

Learning objectives

• energy changes and everyday uses — exothermic reactions (energy out, surroundings warm) versus endothermic reactions (energy in, surroundings cool), with combustion, thermal decomposition, self-heating hand warmers and sports cold packs;
• reaction profiles and bond energies — the activation-energy hump, the position of products, and two bond-energy calculations (energy in to break bonds − energy out to make bonds) [higher];
• measuring and graphing rate — gas volume and mass loss, the mean-rate formula (quantity ÷ time), and using tangents and gradients to read a rate graph [higher];
• collision theory, catalysts and enzymes — temperature, concentration, pressure, surface area and catalysts, how a catalyst lowers the activation energy, and enzymes as biological catalysts;
• reversible reactions, dynamic equilibrium and Le Chatelier — the ⇌ symbol, closed systems, and the effect of temperature, pressure and concentration on the position of equilibrium [higher].

Activities in this book

1. Section 1 — Exothermic, Endothermic & Everyday Uses

   exothermic reactions transfer energy out to the surroundings, so the temperature rises — combustion, neutralisation, oxidation and self-heating hand warmers are exothermic. Endothermic reactions take energy in, so the temperature falls — thermal decomposition and the reaction in a sports cold pack are endothermic.

2. Section 2 — Reaction Profiles & Bond Energies

   a reaction profile plots energy against progress of reaction, with the activation-energy hump between reactants and products. Using bond energies: overall energy change = energy in to break bonds − energy out to make bonds. Breaking bonds is endothermic (energy in); making bonds is exothermic (energy out). A negative overall value means the reaction is exothermic.

3. Section 3 — Measuring & Graphing Rate

   the mean rate of a reaction = quantity of reactant used or product formed ÷ time taken, measured as gas volume (cm³/s) or mass lost (g/s). On a rate graph the gradient is the rate: draw a tangent to find the rate at one instant. The line is steepest at the start and levels off as reactants run out.

4. Section 4 — Collision Theory, Catalysts & Equilibrium

   collision theory says particles react only when they collide with enough energy (the activation energy). Rate rises with higher temperature, higher concentration or pressure, larger surface area and a catalyst. A catalyst provides a route with a lower activation energy and is not used up; enzymes are biological catalysts. A reversible reaction (⇌) reaches dynamic equilibrium; Le Chatelier: raising the temperature shifts it in the endothermic direction, raising the pressure shifts it towards fewer gas molecules.

5. Section 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 Sections 1–4, and link cause to effect — for example, "at a higher concentration there are more particles in the same volume, so collisions are more frequent, so more successful collisions happen 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 uses — hand warmers and cold packs — before 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 both calculations step by step.
• Practise mean rate as a division with units (cm³/s or g/s), then link the graph gradient and a drawn tangent to the rate at an instant.
• Explain rate with collisions. Tie every factor — temperature, concentration, pressure, surface area, catalyst — back to frequency of collisions and activation energy, then extend to enzymes, reversible reactions and Le Chatelier.