Energy Changes: Bond Breaking and Making

Active learning transforms abstract bond-energy concepts into observable, measurable events. Students feel the temperature drop of an endothermic reaction or see the energy diagram take shape, turning energy transfers from a textbook idea into a tactile experience.

• Year 10
• Chemistry
• 4 activities
• 20 min–45 min

Learning Objectives

1. Explain the energy changes associated with breaking and forming chemical bonds.
2. Compare the energy absorbed during bond breaking to the energy released during bond formation.
3. Classify chemical reactions as exothermic or endothermic based on the relative energy changes of bond breaking and making.
4. Analyze qualitative energy diagrams to represent exothermic and endothermic reactions.

Concept Mapping

⏱ 45 min·Small Groups

Demo Stations: Temperature Changes

Prepare stations with safe salts: calcium chloride for exothermic dissolution and ammonium nitrate for endothermic. Students in groups add water, measure temperature changes with probes or thermometers, and record data in tables. Conclude with class discussion linking observations to bond processes.

Prepare & details

Explain that energy is absorbed to break bonds and released when new bonds are formed.

Facilitation Tip: During Demo Stations, circulate with a wireless temperature probe so every pair sees the live graph instead of waiting for a thermometer reading.

Setup: Tables with large paper, or wall space

Materials: Concept cards or sticky notes, Large paper, Markers, Example concept map

• Understand
• Analyze
• Create
• Self-Awareness
• Self-Management

Concept Mapping

⏱ 30 min·Pairs

Pairs Sort: Bond Balance Cards

Distribute cards showing bonds broken and formed with relative energy values. Pairs compare totals to classify reactions as exothermic or endothermic, then justify with sketches. Pairs share one example with the class for peer feedback.

Prepare & details

Relate the overall energy change of a reaction to the relative strengths of bonds broken and formed.

Setup: Tables with large paper, or wall space

Materials: Concept cards or sticky notes, Large paper, Markers, Example concept map

• Understand
• Analyze
• Create
• Self-Awareness
• Self-Management

Concept Mapping

⏱ 35 min·Small Groups

Small Groups: Energy Diagram Builds

Provide reaction equations and sticky notes for bond energies. Groups construct energy profile diagrams on large paper, labelling breaking and making steps. Present to class and predict reaction type based on net change.

Prepare & details

Predict whether a reaction will be exothermic or endothermic based on a qualitative comparison of bond energy changes.

Setup: Tables with large paper, or wall space

Materials: Concept cards or sticky notes, Large paper, Markers, Example concept map

• Understand
• Analyze
• Create
• Self-Awareness
• Self-Management

Concept Mapping

⏱ 20 min·Whole Class

Whole Class: Prediction Relay

Display reactions on board one by one. Students take turns predicting exothermic or endothermic based on bond comparisons, explaining to class. Tally accuracy and revisit misconceptions at end.

Prepare & details

Explain that energy is absorbed to break bonds and released when new bonds are formed.

Setup: Tables with large paper, or wall space

Materials: Concept cards or sticky notes, Large paper, Markers, Example concept map

• Understand
• Analyze
• Create
• Self-Awareness
• Self-Management

Teaching This Topic

Start with simple, safe reactions such as dissolving ammonium chloride in water. Students can hold the beaker and feel the cooling effect while you narrate the bond-breaking story. Avoid early reliance on bond-energy tables; let them build qualitative understanding first. Research shows that anchoring macroscopic observations to atomic-level stories keeps misconceptions from taking hold.

What to Expect

By the end, students should confidently point to broken and formed bonds in a reaction and explain whether the surrounding temperature rises or falls because of the bond-energy balance. They will use the language of exothermic and endothermic correctly and defend their reasoning with data from their own experiments.

Watch Out for These Misconceptions

Common Misconception

During Demo Stations, watch for students who say breaking bonds releases energy after they see a temperature drop.

What to Teach Instead

Prompt them to link the cooling to bond breaking absorbing heat by asking, 'Why does the beaker feel cold if the reaction is absorbing energy?'

Common Misconception

During Pairs Sort: Bond Balance Cards, listen for students who assume any strong-bond product guarantees an exothermic reaction.

What to Teach Instead

Have them total the bond energies on both sides of each card before deciding, so they see that the weaker bonds broken may outweigh the stronger ones formed.

Common Misconception

During Whole Class: Prediction Relay, some students may insist familiar combustion reactions are endothermic.

What to Teach Instead

Ask them to sketch the bonds in CO2 and H2O formed during methane combustion and compare their bond strengths to the weaker bonds broken in CH4 and O2.

Assessment Ideas

Quick Check

After Pairs Sort: Bond Balance Cards, present students with a simple reaction such as the decomposition of hydrogen peroxide. Ask them to identify bonds broken and formed and predict whether the reaction is exothermic or endothermic, explaining their reasoning based on the card totals.

Discussion Prompt

After Small Groups: Energy Diagram Builds, facilitate a class discussion where students explain how they decided whether the reaction was exothermic or endothermic based on their diagrams.

Exit Ticket

During Demo Stations, provide students with two scenarios: Reaction A releases 500 kJ/mol and absorbs 200 kJ/mol. Reaction B releases 300 kJ/mol and absorbs 400 kJ/mol. Ask them to calculate the net energy change for each reaction and classify each as exothermic or endothermic.

Extensions & Scaffolding

• Challenge: Give students two unfamiliar reactions and ask them to predict which is more exothermic based only on bond types shown in data tables.
• Scaffolding: Provide a partially completed energy diagram template for students to fill in during the Small Groups activity.
• Deeper: Introduce bond energy values (kJ/mol) and have students calculate approximate ΔH for the Demo Station reactions using the data.

Key Vocabulary

Bond Breaking	The process of separating atoms that are chemically bonded together. This process requires energy input from the surroundings.
Bond Making	The process of forming new chemical bonds between atoms. This process releases energy into the surroundings.
Exothermic Reaction	A reaction where more energy is released during bond making than is absorbed during bond breaking, resulting in a net release of energy to the surroundings.
Endothermic Reaction	A reaction where more energy is absorbed during bond breaking than is released during bond making, resulting in a net absorption of energy from the surroundings.