Enthalpy Changes: Exothermic & Endothermic

Active learning works for enthalpy changes because students need to touch the concept, not just see it. They must feel the warming of a calorimeter or puzzle through Hess’s Law calculations to grasp that energy is conserved in measurable ways.

• Year 12
• Chemistry
• 3 activities
• 25 min–50 min

Learning Objectives

1. Compare the energy changes associated with exothermic and endothermic reactions using graphical representations.
2. Explain the definitions and conditions for standard enthalpy changes of formation and combustion.
3. Calculate enthalpy changes for reactions by analyzing bond breaking and bond making processes.
4. Differentiate between exothermic and endothermic processes based on energy profile diagrams and enthalpy change values.

Inquiry Circle

⏱ 50 min·Small Groups

Inquiry Circle: Fuel Efficiency Challenge

Groups use simple calorimetry to measure the enthalpy of combustion for different alcohols. They must then compare their results and discuss the sources of error, such as heat loss to the surroundings.

Prepare & details

Differentiate between exothermic and endothermic reactions using energy profile diagrams.

Facilitation Tip: During the Fuel Efficiency Challenge, circulate with an infrared thermometer and ask groups to predict which fuel will cause the largest temperature rise before they begin.

Setup: Groups at tables with access to source materials

Materials: Source material collection, Inquiry cycle worksheet, Question generation protocol, Findings presentation template

• Analyze
• Evaluate
• Create
• Self-Management
• Self-Awareness

Think-Pair-Share

⏱ 25 min·Pairs

Think-Pair-Share: Hess's Law Puzzles

Students are given a set of 'energy puzzle pieces' (enthalpy changes for various steps). They must work in pairs to arrange them into a Hess's Law cycle to find the enthalpy of a target reaction.

Prepare & details

Explain the concept of standard enthalpy change of formation and combustion.

Facilitation Tip: In Hess’s Law Puzzles, require pairs to justify each step with bond energies before sharing with the class to build accountability.

Setup: Standard classroom seating; students turn to a neighbor

Materials: Discussion prompt (projected or printed), Optional: recording sheet for pairs

• Understand
• Apply
• Analyze
• Self-Awareness
• Relationship Skills

Stations Rotation

⏱ 30 min·Small Groups

Stations Rotation: Enthalpy Definitions

Stations feature different chemical equations. Students must identify if the equation represents an enthalpy of formation, combustion, or neutralisation, and explain the 'standard conditions' required for each.

Prepare & details

Analyze the energy changes involved in bond breaking and bond making.

Facilitation Tip: At the Enthalpy Definitions stations, have students rotate with a single shared notebook where each group must summarize key words in their own language before moving on.

Setup: Tables/desks arranged in 4-6 distinct stations around room

Materials: Station instruction cards, Different materials per station, Rotation timer

• Remember
• Understand
• Apply
• Analyze
• Self-Management
• Relationship Skills

Teaching This Topic

Teach enthalpy changes by starting with the physical experience of heat transfer before introducing abstract diagrams. Use calorimetry data to confront the temperature/heat misconception early, then layer in Hess’s Law as a problem-solving tool rather than a memorized rule. Research shows students grasp energy conservation better when they balance equations with real energy values they have measured themselves.

What to Expect

By the end of these activities, students will confidently distinguish between exothermic and endothermic processes, explain temperature vs. heat with real data, and apply Hess’s Law to calculate enthalpy changes they cannot measure directly.

Watch Out for These Misconceptions

Common Misconception

During the Fuel Efficiency Challenge, watch for students who assume the fuel that burns fastest releases the most energy. Redirect by asking them to compare temperature changes per gram of fuel burned rather than total temperature rise.

What to Teach Instead

During the Station Rotation on Enthalpy Definitions, correct the idea that exothermic reactions always feel hot immediately by having students calculate expected temperature changes for 50 mL vs. 200 mL of water using the same energy release.

Assessment Ideas

Exit Ticket

After the Fuel Efficiency Challenge, provide students with two incomplete energy profile diagrams. Ask them to complete the diagrams for an exothermic and an endothermic reaction and label which represents which type.

Quick Check

During Hess’s Law Puzzles, listen for students to explain why bond breaking requires energy input and bond making releases energy when they justify their puzzle solutions.

Discussion Prompt

After the Station Rotation on Enthalpy Definitions, pose the question: 'Why is it important to consider bond breaking as an endothermic process and bond making as an exothermic process when calculating the overall enthalpy change of a reaction?' Use student responses to assess their understanding of energy conservation in reactions.

Extensions & Scaffolding

• Challenge early finishers to design a calorimetry experiment for an unknown metal and predict its specific heat capacity using class data.
• For students who struggle, provide pre-labeled calorimeter diagrams with missing variables so they focus on the relationship between mass, temperature change, and energy released.
• Deeper exploration: Ask students to research how real-world engineers use enthalpy changes to design safe battery systems for electric vehicles, connecting classroom calculations to industry practice.

Key Vocabulary

Enthalpy Change (ΔH)	The heat energy absorbed or released during a chemical reaction at constant pressure. A negative ΔH indicates an exothermic reaction, while a positive ΔH indicates an endothermic reaction.
Exothermic Reaction	A reaction that releases energy, usually in the form of heat, into the surroundings. The enthalpy of the products is lower than that of the reactants.
Endothermic Reaction	A reaction that absorbs energy, usually in the form of heat, from the surroundings. The enthalpy of the products is higher than that of the reactants.
Energy Profile Diagram	A graph that shows the change in energy during a chemical reaction. It plots reaction progress against enthalpy, illustrating activation energy and the overall enthalpy change.
Standard Enthalpy of Formation (ΔHf°)	The enthalpy change when one mole of a compound is formed from its constituent elements in their standard states under standard conditions (298 K and 1 atm).
Standard Enthalpy of Combustion (ΔHc°)	The enthalpy change when one mole of a substance reacts completely with oxygen under standard conditions.