How to teach enthalpy cycles post-16

It is crucial for students to understand thermodynamics so they can grasp how energy changes in chemical reactions affect the real world and products at their fingertips. Sometimes literally, with everyday applications in hand warmers and first aid instant cold packs. Concepts such as Hess’s law, enthalpy changes and the conservation of energy lay the foundations for advanced studies. By mastering these topics, students can better understand processes such as the synthesis of chemicals, the efficiency of industrial reactions and the environmental impact of chemical processes.

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Use a hand warmer to demonstrate an exothermic reaction as a hook to thermodynamics and energy changes

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Highlight known examples of endothermic reactions too, such as instant cold packs in first aid kits

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Use this guide as a framework to teach these concepts effectively, address common misconceptions and offer practical classroom activities. Although the guide is aimed at foundation concepts, I have included resources suited to more advanced topics, such as Born–Haber cycles and entropy.

What students need to know

To master thermodynamics in chemistry, students need a solid understanding of the following key concepts:

• Conservation of energy: energy cannot be created or destroyed, only transformed. This principle underpins enthalpy cycles and Hess’s law.
• Enthalpy as a state function: enthalpy depends only on the initial and final states of a system, not the path taken. This concept is crucial for understanding why Hess’s law works.
• Hess’s law: the enthalpy change of a reaction is the same regardless of the route taken, providing the start and end conditions are the same.
• Standard enthalpy changes of reaction (ΔH^⦵_r), formation (ΔH^⦵_f) and combustion (ΔH^⦵_c): we measure these under standard conditions (298 K, 100 kPa, 1 mol dm^-3). When students understand these definitions, they can better construct accurate enthalpy cycles.
• Sign convention: the correct interpretation of exothermic (negative enthalpy change) and endothermic (positive enthalpy change) reactions is essential for accurate calculations.

Common misconceptions

Exam reports often highlight errors in constructing and interpreting enthalpy cycles, perhaps because students have an incomplete understanding of the definitions. In formation reactions one mole of a compound is created from its elements in their standard states, while in combustion reactions one mole of a substance is burned in oxygen. When students know the specifics of these reactions they can calculate enthalpy changes in a systematic way, construct enthalpy cycles and apply Hess’s law.

Students often struggle with the concept that energy conservation means the total energy change of a reaction is independent of the path taken. When students do not understand enthalpy as a state function it can lead to confusion when they apply Hess’s law.

Ideas for your classroom

Provide clear, step-by-step instructions and scaffolded support when teaching enthalpy. Gradually reduce support as students become more confident, ensuring a solid understanding of each step before moving on.

Explain that a Hess cycle (figure 1) is a diagram that shows alternative routes between reactants and products which you can use to calculate unknown enthalpy changes using known enthalpy changes. Being able to construct and analyse these cycles is a practical application of Hess’s law and is essential for solving many thermodynamic problems in chemistry.

Step-by-step guide to constructing a Hess cycle:

• Write the balanced chemical equation, including state symbols, for the reaction.
• Identify known enthalpy changes (e.g. formation or combustion).
• Draw the cycle, showing alternative routes from common reactants to common products.
• Use Hess’s law to set up equations relating the enthalpy changes around the cycle.

Use visual aids, interactive simulations and hands-on experiments to make abstract concepts tangible. Start with practical work to observe an example of Hess’s law in practice, followed by an instructed cycle and compare the values obtained. Use other examples to promote discussion and problem solving.

Use the downloadable resource to introduce Hess’s law with a simple experiment. Get students to collect experimental data to measure the enthalpy change of hydration of copper(II) sulfate, which you cannot measure directly. When measuring enthalpy changes, students often forget that the thermometer measures the temperature of the surroundings. Remind students that an increase in thermometer reading during an experiment indicates an exothermic reaction, as the system releases energy to the surroundings.

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