Energy in Chemical Reactions: Exothermic and Endothermic

Active learning works for this topic because students’ physical experience of heat or cold in chemical reactions creates lasting neural pathways. When students feel a temperature change during a hands-on demo, the surprise of cold from a reaction they associate with ‘heat’ anchors the concept more firmly than a lecture ever could.

• 10th Grade
• Chemistry
• 3 activities
• 20 min–35 min

Learning Objectives

1. Classify chemical reactions as exothermic or endothermic based on observed temperature changes.
2. Explain the role of bond breaking and bond forming in determining the overall enthalpy change of a reaction.
3. Analyze the relationship between energy absorbed and energy released in chemical processes.
4. Compare and contrast the energy flow in exothermic and endothermic reactions using particle diagrams.

Experiential Learning

⏱ 30 min·Small Groups

Predict-Observe-Explain: Hot and Cold Pack Demos

Students predict whether dissolving calcium chloride and dissolving ammonium nitrate in water will feel warm or cold. After observing temperature changes with thermometers, they use bond energy reasoning to explain why each behaves as it does, identifying which process (bond breaking or bond forming) dominates in each case.

Prepare & details

Explain where the energy goes when a reaction feels cold.

Facilitation Tip: During the Hot and Cold Pack Demos, circulate with an infrared thermometer to let students see real-time temperature changes projected on a screen.

Setup: Varies; may include outdoor space, lab, or community setting

Materials: Experience setup materials, Reflection journal with prompts, Observation worksheet, Connection-to-content framework

• Apply
• Analyze
• Evaluate
• Self-Awareness
• Self-Management
• Social Awareness

Think-Pair-Share

⏱ 20 min·Pairs

Think-Pair-Share: Where Does the Energy Go?

Ask students where the energy goes during an endothermic reaction if it seems to 'disappear.' Students write individual responses connecting energy conservation to the formation of higher-energy bonds in the products. Pairs compare and then contribute to a class-level energy balance diagram on the board.

Prepare & details

Differentiate between exothermic and endothermic reactions.

Facilitation Tip: In the Think-Pair-Share on energy flow, provide energy accounting diagrams as sentence frames so students practice translating observations into energy notation.

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

Gallery Walk

⏱ 35 min·Small Groups

Gallery Walk: Exothermic and Endothermic in Daily Life

Six stations each show a real-world process: combustion of natural gas, photosynthesis, dissolving NaOH, ice packs, hand warmers, and respiration. Students classify each as exothermic or endothermic and write a one-sentence justification using bond energy language. Groups compare classifications and debate any disagreements.

Prepare & details

Analyze how bond breaking and forming contribute to enthalpy changes.

Facilitation Tip: For the Gallery Walk, assign each poster a unique real-world example so students connect the energy concept to diverse contexts beyond the classroom.

Setup: Wall space or tables arranged around room perimeter

Materials: Large paper/poster boards, Markers, Sticky notes for feedback

• Understand
• Apply
• Analyze
• Create
• Relationship Skills
• Social Awareness

Teaching This Topic

Start with the Hot and Cold Pack Demos to create cognitive conflict, then use the Think-Pair-Share to formalize the vocabulary of exothermic and endothermic. Avoid rushing to the definitions before students experience the phenomena; research shows this delay strengthens long-term retention. Always connect back to bond energy diagrams to prevent students from reducing the topic to simple memorization of examples.

What to Expect

Successful learning looks like students confidently classifying reactions, tracing energy flow with diagrams, and applying the concepts to new contexts without mixing up exothermic and endothermic labels. You will see students using the language of bond energy and thermal energy interchangeably as they explain their reasoning.

Watch Out for These Misconceptions

Common Misconception

During the Hot and Cold Pack Demos, watch for students who assume all reactions release heat because they have only experienced burning or explosions.

What to Teach Instead

After the cold-pack demonstration, have students draw energy flow diagrams showing reactants’ bond energy converting to thermal energy of the surroundings in the exothermic case and thermal energy of the surroundings converting to products’ bond energy in the endothermic case.

Common Misconception

During the Think-Pair-Share on energy flow, watch for students who believe energy is created or destroyed during exothermic reactions.

What to Teach Instead

Use the energy accounting sheets from the Think-Pair-Share to explicitly calculate total energy before and after the reaction, labeling bond energies and thermal energy to reinforce the Law of Conservation of Energy.

Assessment Ideas

Quick Check

After the Gallery Walk, provide a list of chemical processes and ask students to label each as exothermic or endothermic with a one-sentence justification based on heat release or absorption.

Exit Ticket

After the Hot and Cold Pack Demos, present students with a scenario: ‘A reaction mixture feels cold to the touch.’ Ask them to classify the reaction and explain the energy change at the molecular level, referencing bond breaking and forming.

Discussion Prompt

During the Think-Pair-Share, pose the question: ‘If breaking bonds always requires energy and forming bonds always releases energy, how can we predict if a reaction will be exothermic or endothermic?’ Facilitate a class discussion focusing on the net energy change from the balance between bond breaking and forming.

Extensions & Scaffolding

• Challenge: Ask students to design their own cold pack using household chemicals and justify their choice of reactants based on bond energies.
• Scaffolding: Provide a word bank and partially completed energy diagrams for students who struggle to start the Gallery Walk explanations.
• Deeper exploration: Have students research an industrial process (e.g., Haber process, electrolysis) and present the energy trade-offs in bond breaking versus bond forming.

Key Vocabulary

Exothermic Reaction	A chemical reaction that releases energy, usually in the form of heat, causing the surroundings to become warmer.
Endothermic Reaction	A chemical reaction that absorbs energy from its surroundings, usually in the form of heat, causing the surroundings to become cooler.
Enthalpy Change	The total heat content change of a system during a process at constant pressure, often represented by the symbol ΔH.
Bond Energy	The amount of energy required to break one mole of a particular chemical bond, or the energy released when that bond is formed.