Science · 6th Grade · Molecules in Motion · Weeks 1-9

Energy in Chemical Reactions

Students investigate how some chemical reactions release heat (exothermic) while others absorb it (endothermic).

• Common Core State Standards
• MS-PS1-5

About This Topic

Not all chemical reactions look or feel the same: some produce heat, some feel cold, and some generate light or sound. Aligned with MS-PS1-5, this topic introduces students to the energy dimension of chemical reactions. Exothermic reactions release energy, usually as heat, to the surroundings and feel warm or hot. Endothermic reactions absorb energy from the surroundings and feel cold. Both involve the same atomic rearrangement process, but they differ in the balance between energy required to break existing bonds and energy released when new bonds form.

Hand warmers, combustion, and rusting are all exothermic. Photosynthesis, cold packs, and dissolving ammonium nitrate in water are endothermic. Students encounter these in daily life without recognizing the chemistry involved. Naming the energy patterns helps students see chemistry as something that explains familiar experiences.

At 6th grade, students do not need to work with bond energies formally or calculate energy values. The goal is a conceptual framework: reactions either release or absorb energy, and this comes from changes in how atoms are bonded. Active learning approaches that let students directly feel the temperature change during a reaction make the abstract energy dimension physically real.

Key Questions

1. Differentiate between exothermic and endothermic reactions with examples.
2. Predict whether a reaction will feel hot or cold based on its energy profile.
3. Analyze the role of energy in breaking and forming chemical bonds.

Learning Objectives

• Classify chemical reactions as exothermic or endothermic based on observable temperature changes.
• Explain the energy transfer occurring during exothermic and endothermic reactions using bond breaking and forming as a model.
• Predict whether a given chemical process, such as a hand warmer activating or an instant cold pack forming, is exothermic or endothermic.
• Analyze provided scenarios to identify examples of exothermic and endothermic processes in everyday life.

Before You Start

Introduction to Chemical Reactions

Why: Students need a basic understanding of what a chemical reaction is, involving the rearrangement of atoms and formation of new substances, before exploring the energy changes involved.

Heat and Temperature

Why: Students must be able to distinguish between heat and temperature and understand that heat is a form of energy transfer to grasp the concepts of reactions releasing or absorbing heat.

Key Vocabulary

Exothermic Reaction	A chemical reaction that releases energy, usually in the form of heat, into its surroundings. These reactions often feel warm or hot to the touch.
Endothermic Reaction	A chemical reaction that absorbs energy, usually in the form of heat, from its surroundings. These reactions often feel cold to the touch.
Chemical Bonds	The forces that hold atoms together in molecules. Energy is required to break these bonds, and energy is released when new bonds are formed.
Energy Profile	The overall balance of energy absorbed to break bonds versus energy released when new bonds form in a chemical reaction, determining if it is exothermic or endothermic.

Watch Out for These Misconceptions

Common Misconception

Students often think all chemical reactions produce heat, since the dramatic reactions they typically see in class (fire, bubbling) tend to be exothermic.

What to Teach Instead

Endothermic reactions in which the container becomes noticeably cold are a reliable correction. Having students hold the container during the reaction is a low-tech but effective way to challenge this assumption through direct sensory experience.

Common Misconception

Many students believe that if a reaction produces gas, it must be releasing energy.

What to Teach Instead

The baking soda and vinegar reaction produces carbon dioxide gas but actually absorbs a small amount of heat. Gas production and heat release are independent indicators. Peer discussion comparing this reaction to combustion, which also produces CO2 but is strongly exothermic, helps students separate the two ideas.

Active Learning Ideas

Inquiry Circle: Hot and Cold Reactions

Groups perform two reactions side by side: dissolving calcium chloride in water (exothermic, the chemistry behind hand warmers) and dissolving ammonium nitrate in water (endothermic, the chemistry behind cold packs). They measure temperature changes, classify each reaction, and explain the direction of energy flow.

40 min·Small Groups

Think-Pair-Share: Energy Profile Diagrams

Students examine two unlabeled energy diagrams: one showing reactants at a higher energy level than products (exothermic) and one showing the reverse (endothermic). They discuss with a partner which diagram represents each type and must explain their reasoning using the diagram before the class shares out.

20 min·Pairs

Gallery Walk: Exothermic or Endothermic?

Image stations feature everyday reactions: hand warmers, cold packs, burning candles, photosynthesis, baking bread, and rusting metal. Students rotate, classify each as exothermic or endothermic, and note the specific evidence of energy release or absorption that led to their classification.

30 min·Small Groups

Stations Rotation: Bond Energy Exploration

Using rubber bands as physical 'bonds,' students explore a model where breaking bonds requires effort (energy input) and forming new bonds releases energy. They compare the effort required to break reactant bonds versus the release from forming product bonds, building an intuition for why some reactions are exothermic and others are endothermic.

45 min·Small Groups

Real-World Connections

• Emergency medical technicians use instant cold packs, which are a common example of an endothermic reaction, to reduce swelling and pain in patients with injuries.
• Campers and hikers rely on chemical hand warmers, a practical application of exothermic reactions, to stay warm in cold weather by releasing heat when exposed to air.
• Chemists in food science research develop new preservation methods, some of which involve controlling exothermic reactions like oxidation to extend shelf life.

Assessment Ideas

Quick Check

Provide students with two sealed bags, one containing baking soda and vinegar (exothermic) and another containing ammonium nitrate and water (endothermic). Ask students to feel each bag and record their observations, then classify each reaction as exothermic or endothermic with a brief explanation.

Exit Ticket

On an index card, ask students to draw a simple diagram representing either an exothermic or endothermic reaction. They should label the diagram to show energy being released or absorbed and write one sentence explaining their choice.

Discussion Prompt

Pose the question: 'Imagine you are designing a new type of reusable hand warmer. Would you focus on creating an exothermic or endothermic process, and why? What are the advantages and disadvantages of your choice?' Facilitate a class discussion on their reasoning.

Frequently Asked Questions

What is the difference between exothermic and endothermic reactions?

Exothermic reactions release energy, usually as heat, to the surroundings, so the reaction container feels warm. Endothermic reactions absorb energy from the surroundings, so the container feels cold. The key question is whether the reaction gives heat to its environment or takes heat from it.

Where does the energy in an exothermic reaction come from?

It comes from the chemical bonds between atoms. Forming new bonds releases energy. If the total energy released in forming the product bonds is greater than the energy needed to break the original reactant bonds, the excess energy is released as heat, making the reaction exothermic.

How can active learning help students understand exothermic and endothermic reactions?

Temperature is something students can feel directly. Having students hold the outside of a reaction container while a reaction proceeds turns the abstract idea 'energy is released or absorbed' into a direct sensory experience. Comparing two simultaneous reactions side by side, one hot and one cold, also reinforces the contrast and helps students build their classification skills with immediate, concrete feedback.

Is photosynthesis exothermic or endothermic?

Photosynthesis is endothermic. The plant absorbs light energy from the sun and uses it to build glucose molecules. The energy is stored in the chemical bonds of glucose rather than released to the surroundings, so photosynthesis requires a continuous input of energy to proceed.