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WalesChemistry

Unit 2.4 Chemical reactions and energy

6 dot points across 6 inquiry questions. Click any dot point for a focused answer with worked past exam questions where available.

Why is breaking bonds endothermic and making bonds exothermic, and how do we calculate the energy change?

Explain bond breaking and bond making in terms of energy and calculate the energy change of a reaction from bond energies.
A focused answer to WJEC GCSE Chemistry topic 2.4, explaining why bond breaking is endothermic and bond making is exothermic, and showing how to calculate the overall energy change of a reaction from given bond energies.
9 min answer →

How do we show energy changes on a reaction profile, and what makes a reaction exothermic or endothermic?

Describe exothermic and endothermic reactions, draw and interpret reaction profiles, and identify activation energy and overall energy change.
A focused answer to WJEC GCSE Chemistry topic 2.4, covering exothermic and endothermic reactions, drawing and interpreting reaction profile (energy level) diagrams, identifying activation energy and overall energy change, and everyday uses such as hand warmers and cold packs.
9 min answer →

How do temperature, concentration, surface area and catalysts change the rate of a reaction?

Describe and explain the effect of temperature, concentration or pressure, surface area and catalysts on rate using collision theory.
A focused answer to WJEC GCSE Chemistry topic 1.5, explaining how temperature, concentration or pressure, surface area and catalysts each affect the rate of reaction in terms of collision theory, including how catalysts provide a lower activation energy pathway.
9 min answer →

What is the rate of a reaction, and how does collision theory explain it?

Define the rate of reaction, describe how it is measured, and use collision theory to explain how reactions occur.
A focused answer to WJEC GCSE Chemistry topic 1.5, covering what the rate of a reaction means, how rate is measured by following mass or gas volume against time, how to read rate graphs, and how collision theory and activation energy explain reactions.
9 min answer →

What is a reversible reaction, and how does changing conditions shift a dynamic equilibrium?

Describe reversible reactions and dynamic equilibrium, and predict the effect of changing temperature, concentration and pressure using Le Chatelier's principle.
A focused answer to WJEC GCSE Chemistry topic 2.6, covering reversible reactions and the symbol used, dynamic equilibrium in a closed system, and how changing temperature, concentration and pressure shifts the position of equilibrium using Le Chatelier's principle.
9 min answer →

How are ammonia and sulfuric acid made industrially, and why are the conditions chosen as a compromise?

Describe the Haber process and the Contact process, including their conditions, and explain why the conditions are a compromise between yield, rate and cost.
A focused answer to WJEC GCSE Chemistry topic 2.6, covering the Haber process for ammonia and the Contact process for sulfuric acid, their reactions and conditions, the use of the products including fertilisers, and why industrial conditions are a compromise.
9 min answer →