5.5 Energy changes

Unit Summary

This unit aligns with the AQA GCSE Chemistry: Trilogy specification and develops a coherent understanding of the energy changes that accompany chemical reactions. It builds upon prior knowledge of bonding, structure, and chemical reactivity to explain how the breaking and forming of bonds involve energy transfer between substances and their surroundings.

Students will learn to distinguish between exothermic and endothermic reactions, understanding that exothermic reactions release energy to the surroundings—often as heat or light—while endothermic reactions absorb energy. Through both theoretical study and practical investigation, they will explore how these energy transfers can be observed as heating or cooling effects, and how such effects are applied in everyday and industrial contexts, such as self-heating cans, instant cold packs, and combustion reactions.

The unit emphasises the role of bond energies in determining the overall energy change of a reaction. Students will interpret energy level diagrams (reaction profiles), identifying activation energy and explaining how catalysts affect it. They will calculate energy changes using bond energy data, reinforcing their quantitative and analytical skills.

Substantive knowledge developed in this unit includes:

The distinction between exothermic and endothermic reactions in terms of energy transfer.

The concept of conservation of energy in chemical reactions.

The interpretation and construction of energy level diagrams.

The relationship between bond breaking (endothermic) and bond making (exothermic).

The use of bond energy calculations to predict reaction energetics.

Disciplinary knowledge is strengthened through practical and investigative work. Students will measure temperature changes during reactions, use data to classify reactions as exothermic or endothermic, and draw evidence-based conclusions. These experiences reinforce the importance of careful data collection, controlled experimentation, and linking observations to theoretical explanations.

Assessment for Learning (AfL) strategies are embedded throughout the unit, using retrieval practice to consolidate prior learning about chemical reactions, diagnostic questioning to address misconceptions (e.g., confusing heat and energy), and scaffolded tasks to develop students’ ability to describe and explain energy transfers clearly and quantitatively.

Links to real-world applications are made explicit, demonstrating how understanding energy changes underpins technologies such as energy-efficient fuels, heating and cooling systems, and renewable energy research. These contexts help students appreciate the broader significance of energy transformations in both science and everyday life.

Designed for accessibility and progression, this unit enables students to confidently describe, represent, and quantify energy changes in chemical reactions. By the end of the unit, learners will be able to interpret energy profiles, relate bond energies to reaction energetics, and explain how and why reactions release or absorb energy—laying a strong foundation for further study in physical chemistry and energy science.