Endothermic and Exothermic Reactions

Endothermic and Exothermic Reactions

Middle School

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Learning Objectives

• Define what endothermic and exothermic reactions are.

• Identify everyday examples of endothermic and exothermic reactions.

• Explain how chemical reactions can either absorb or release energy.

• Understand how the Law of Conservation of Energy applies to chemical changes.

Key Vocabulary

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Endothermic Reaction

A chemical reaction that absorbs energy from its surroundings, usually in the form of heat.

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Exothermic Reaction

A chemical reaction that releases energy into its surroundings, often as heat, light, or sound.

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Conservation of Energy

The principle that energy cannot be created or destroyed, only changed from one form to another.

Energy in Chemical Reactions

• Chemical reactions can absorb energy from their surroundings or release it.

• A new substance or gas bubbles signal an energy transformation has happened.

• ‘Endo-’ means energy enters the reaction, and ‘Exo-’ means energy exits.

Multiple Choice

What is the fundamental difference between an endothermic and an exothermic reaction?

• Only exothermic reactions involve a change in energy.

• Endothermic reactions release energy, while exothermic reactions absorb it.

• Endothermic reactions absorb energy, while exothermic reactions release it.

• Only endothermic reactions create new substances.

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Endothermic Reactions

• Endothermic reactions absorb energy from their surroundings, usually as heat.

• The absorbed energy is used to break the chemical bonds of reactants.

• The surroundings lose heat, causing a decrease in the overall temperature.

• Products are at a higher energy level than the original reactants.

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Multiple Choice

If a chemical reaction causes its surroundings to feel cool, what type of reaction is it?

• A neutral reaction because there is no temperature change.

• An endothermic reaction because it absorbs heat from the surroundings.

• An exothermic reaction because it releases cold.

• A combustion reaction because it involves energy.

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Examples of Endothermic Reactions

• Melting ice cubes absorb energy from the environment to become liquid water.

• Plants absorb light energy to make food from carbon dioxide and water.

• Chemicals in an instant cold pack mix and absorb heat, feeling cold.

• Evaporation, cooking an egg, and baking bread are other examples that absorb heat.

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Multiple Choice

Which of the following is an example of an endothermic process?

• Melting an ice cube

• Burning a candle

• A hand warmer

• Condensation of rain

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What Is an Exothermic Reaction?

• It is a reaction that releases energy into the surroundings.

• ​This energy can be in the form of heat, light, or sound.

• The surroundings get hotter as new chemical bonds are formed.

• Products are at a lower energy level than the reactants.

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Multiple Choice

What is characteristic of an exothermic reaction's energy level diagram?

• The products are at a lower energy level than the reactants.

• The reactants and products are at the same energy level.

• The products are at a higher energy level than the reactants.

• Energy is absorbed to form new bonds.

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Examples of Exothermic Reactions

• Burning wood or candles releases significant amounts of heat and light.

• When liquid water freezes into solid ice, it releases heat energy.

• Chemical hand warmers use the rusting of iron to produce heat.

• Other examples include explosions, self-heating cans, and acid-base neutralization reactions.

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Multiple Choice

Which of the following is an example of an exothermic process?

• Photosynthesis

• Cooking an egg

• Using an instant cold pack

• Combustion of wood

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The Law of Conservation of Energy

• This law states that energy is conserved in all chemical reactions.

• Energy cannot be created or destroyed, only transformed from one form to another.

• Endothermic reactions convert surrounding energy into chemical energy in the products.

• Exothermic reactions convert chemical energy into heat or light for the surroundings.

Multiple Choice

According to the Law of Conservation of Energy, what happens to the energy released in an exothermic reaction?

• It is transformed from chemical energy and released into the surroundings.

• It is transferred to the reactants to break their bonds.

• It is destroyed during the reaction.

• It is created by the formation of new bonds.

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Common Misconceptions about Chemical Reactions

Misconception	Correction
Endothermic reactions "create" cold.	They absorb heat from the surroundings, which makes it feel cold.
Energy is "lost" in an exothermic reaction.	Energy is conserved. It changes form and is released as heat or light.
If a reaction bubbles, it must be exothermic.	Bubbling can be from either endothermic or exothermic reactions.

Multiple Choice

A student mixes two chemicals and observes that the beaker becomes warm. How would you classify this reaction based on the energy of the reactants and products?

• Endothermic, because energy is absorbed from the surroundings.

• Endothermic, because the reactants have less energy than the products.

• Exothermic, because the reactants have more energy than the products.

• Exothermic, because the reaction requires activation energy.

Multiple Choice

Why does an instant cold pack feel cold when activated?

• It freezes the liquid inside, which makes it feel cold.

• It releases cold energy into your hand.

• It undergoes an endothermic reaction, absorbing heat from its surroundings.

• It undergoes an exothermic reaction, releasing heat.

Multiple Choice

Analyze the process of photosynthesis, where plants use sunlight, water, and carbon dioxide to create glucose (food). How does this process relate to the Law of Conservation of Energy?

• It demonstrates the law by transforming light energy into stored chemical energy in glucose.

• It proves that energy can be destroyed as sunlight is used up.

• It violates the law by creating new energy in the form of glucose.

• It is an exothermic reaction that releases energy for the plant to use.

Multiple Choice

Predict the outcome if you dissolve a substance in water and the temperature of the water drops from 25°C to 15°C. What is happening on a molecular level?

• An exothermic reaction is occurring, releasing heat and forming new bonds.

• The water is releasing its heat to the new substance, making it an exothermic reaction.

• A physical change is occurring, but there is no net energy change.

• An endothermic process is occurring, with energy being absorbed to break bonds.

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Summary

• Endothermic reactions absorb energy, making their surroundings feel cold.

• Exothermic reactions release energy, making their surroundings feel hot.

• Energy is used to break chemical bonds and released when new bonds form.

• Energy is always conserved; it just changes from one form to another.

Poll

On a scale of 1-4, how confident are you about distinguishing between endothermic and exothermic reactions?

• 1

• 2

• 3

• 4