Energy Changes in Chemical Reactions

Energy Changes in Chemical Reactions

Middle School

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Learning Objectives

• Differentiate between reactions that absorb energy and those that release energy.

• Explain that energy is needed to break bonds and released when bonds form.

• Apply the law of conservation of energy to track energy in chemical systems.

• Analyze how concentration and temperature affect the speed of a reaction.

• Use energy changes to help design a device that heats or cools.

Key Vocabulary

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Thermal Energy

This is the total energy that comes from the movement of all the tiny particles in an object.

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System and Surroundings

A system is the specific chemical reaction being studied, while the surroundings are everything else around it.

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Exothermic Reaction

This is a type of chemical reaction that releases energy, usually in the form of heat or light.

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Endothermic Reaction

This is a chemical reaction that absorbs energy from its surroundings, often making the area feel colder.

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Conservation of Energy

The Law of Conservation of Energy states that energy is never created or destroyed, only transferred between objects.

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Chemical Potential Energy

This is the energy stored within the chemical bonds of a substance, waiting to be released during a reaction.

Key Vocabulary

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Concentration

Concentration is the amount of a substance that has been dissolved in a given volume.

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Photosynthesis

Photosynthesis is the process where plants convert light energy into chemical energy to make their food.

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Chemiluminescence

Chemiluminescence is a chemical reaction that releases energy in the form of light instead of heat.

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Energy and Chemical Bonds

Breaking Bonds

• Chemical reactions start by breaking the bonds that hold reactant atoms together.

• This process requires an input of energy from the surroundings to pull atoms apart.

• Energy stored in chemical bonds is known as chemical potential energy.

Forming Bonds

• The atoms then rearrange to form new bonds, creating the final products.

• This process releases energy into the environment, often in the form of heat or light.

• The strength of the bonds determines how much energy is released.

Multiple Choice

What is chemical potential energy?

• Energy that is stored in the bonds holding atoms together.

• Energy that is released when atoms move apart.

• Energy that is required to create new atoms.

• Energy that is found only in the form of heat or light.

Multiple Choice

How does the flow of energy differ between breaking and forming chemical bonds?

• Breaking bonds requires an input of energy, while forming bonds releases energy.

• Breaking bonds releases energy, while forming bonds requires an input of energy.

• Both breaking and forming bonds require an input of energy from the surroundings.

• Both breaking and forming bonds release energy into the environment as heat or light.

Multiple Choice

If a chemical reaction releases a large amount of light and heat, what can be concluded about the process?

• The bonds formed in the products were very strong.

• The bonds broken in the reactants were very weak.

• The reaction did not involve breaking any chemical bonds.

• The atoms did not rearrange to form new products.

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Exothermic vs. Endothermic Reactions

Exothermic Reactions

Endothermic Reactions

Multiple Choice

What is the main characteristic of an exothermic reaction?

• It releases energy into the surroundings, making them feel hot.

• It absorbs energy from the surroundings, making them feel cold.

• It creates energy from nothing, making the surroundings hot.

• It destroys energy, making the surroundings feel cold.

Multiple Choice

Why does an endothermic reaction cause its surroundings to feel colder?

• More energy is needed to break reactant bonds than is released when forming product bonds.

• More energy is released when forming product bonds than is needed to break reactant bonds.

• The reaction destroys the bonds of the reactants, releasing cold energy.

• The reaction absorbs heat and converts it into chemical bonds.

Multiple Choice

A chemical process is represented by the general formula: Reactants -> Products + Energy. What can be concluded about this process?

• The reaction is exothermic and will cause the surroundings to feel hotter.

• The reaction is endothermic and will cause the surroundings to feel colder.

• The reaction is exothermic and will cause the surroundings to feel colder.

• The reaction is endothermic and will cause the surroundings to feel hotter.

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Factors Affecting Reaction Rate

Concentration

• Increasing reactant concentration means more particles are in the same amount of space.

• This closeness leads to more frequent collisions between the particles, which helps speed up the reaction.

• More collisions result in a faster reaction, so the reaction rate increases with higher concentration.

Temperature

• Increasing the temperature gives particles more kinetic energy, making them move around much more quickly.

• This faster movement leads to more frequent collisions between the particles, which helps speed up the reaction.

• These collisions are also more energetic, making it more likely for a chemical reaction to occur.

Multiple Choice

What is the primary effect of increasing either the concentration or the temperature on a chemical reaction?

• It increases the rate of the reaction.

• It decreases the rate of the reaction.

• It stops the reaction from happening.

• It changes the reactants into different substances.

Multiple Choice

What is the common reason why increasing reactant concentration and increasing temperature both speed up a reaction?

• They both cause the reactant particles to collide more frequently.

• They both decrease the kinetic energy of the particles.

• They both cause the particles to become larger in size.

• They both require the addition of a new substance.

Multiple Choice

If a scientist increases the temperature of a reaction, what two changes will lead to a faster reaction rate?

• The particles will collide more often and with more energy.

• The particles will stop moving completely.

• The number of particles in the container will decrease.

• The particles will only collide with more energy, not more often.

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The Law of Conservation of Energy

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

• In a chemical reaction, energy moves between the system and its surroundings.

• For example, chemical potential energy can be converted into thermal energy and released.

• The total amount of energy remains constant throughout this entire energy transfer.

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Multiple Choice

Which statement best describes the Law of Conservation of Energy?

• Energy can be created from nothing, but it cannot be destroyed.

• Energy can be destroyed and removed from a system permanently.

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

• The total amount of energy in a system is always decreasing over time.

Multiple Choice

When a chemical reaction releases heat, what kind of energy transfer is occurring?

• Thermal energy is converted into chemical potential energy.

• New energy is created in the form of heat.

• Chemical potential energy is converted into thermal energy.

• Energy from the surroundings is destroyed during the reaction.

Multiple Choice

If a chemical reaction in a beaker causes the beaker to feel warm, what is the best explanation for what happened to the total energy?

• The total energy in the system has increased because new heat energy was created.

• The total energy in the system has decreased because chemical energy was used up.

• The total energy remains constant because it was only transferred from the system to its surroundings.

• The total energy becomes zero once the chemical reaction is complete.

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Designing with Chemical Reactions

Hand Warmers

• ​Hand warmers utilize exothermic reactions that release thermal energy, making the pack feel warm to the touch.

• ​​A controlled reaction, such as iron oxidation, is used to generate a specific, safe amount of heat.

• ​Design criteria include total temperature change, duration of heat, overall cost, and ensuring the device is safe.

Instant Cold Packs

• ​Cold packs use an endothermic process that absorbs thermal energy from the surroundings, making the pack feel cold.

• ​​This is often achieved by dissolving ammonium nitrate in water, which absorbs a large amount of heat energy.

• ​They are designed to absorb enough heat to become effective for applications like treating sports injuries safely.

Multiple Choice

What is the main purpose of the chemical reactions used in hand warmers and cold packs?

• To create a new substance from scratch.

• To produce a controlled change in temperature.

• To make the products change color.

• To generate electricity for small devices.

Multiple Choice

What is the fundamental difference between the process occurring in a hand warmer and a cold pack?

• Exothermic reactions release heat, while endothermic reactions absorb heat.

• Exothermic reactions happen instantly, while endothermic reactions are slow.

• Exothermic reactions use iron, while endothermic reactions use water.

• Exothermic reactions are safe, while endothermic reactions are dangerous.

Multiple Choice

A design team is tasked with creating a new instant cold pack that is more effective for sports injuries. Which design choice would best achieve this goal?

• Use a thicker plastic for the outer pouch.

• Add more water to the inner pouch.

• Choose a chemical that absorbs more thermal energy when it dissolves.

• Make the overall pack smaller and more compact.

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Energy Changes in Real-Life Reactions

Endothermic Reaction: Photosynthesis

Exothermic Reaction: Chemiluminescence

• Chemiluminescence is an exothermic reaction where energy is released primarily as light instead of heat.

• This production of “cold light” occurs when chemicals are mixed and react with each other.

• A light stick is a perfect example, glowing brightly without any external power source after being activated.

Multiple Choice

What is the primary function of an endothermic reaction like photosynthesis?

• It converts light energy into chemical energy.

• It releases energy primarily as light.

• It mixes chemicals to produce cold light.

• It glows brightly without an external power source.

Multiple Choice

Why does a light stick glow after being activated?

• The reaction absorbs energy from the surrounding air.

• The reaction releases energy in the form of light.

• The reaction uses light energy to create sugar.

• The reaction requires an external power source to glow.

Multiple Choice

If a chemical reaction causes its container to feel warm to the touch, what is the most likely explanation?

• An exothermic reaction is occurring, releasing energy as heat.

• An endothermic reaction is occurring, absorbing energy from the beaker.

• Photosynthesis is taking place within the beaker.

• The reaction is creating cold light through chemiluminescence.

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Common Misconceptions

Misconception	Correction
Reactions only absorb or release energy, not both.	Reactions both absorb energy to break bonds and release energy to form new ones.
Cold is a substance released from a cold pack.	Cold is the absence of heat. Cold packs absorb thermal energy from their surroundings.
Energy is created in hot packs and destroyed in cold packs.	Energy is only transferred, never created or destroyed. It just changes form.

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Summary

• Breaking chemical bonds requires energy, while forming new chemical bonds releases energy.

• Exothermic reactions release energy; endothermic reactions absorb energy from the surroundings.

• Higher concentration or temperature increases the rate of a chemical reaction.

• The Law of Conservation of Energy states that energy is never created or destroyed.

• Hand warmers and cold packs are practical applications of these energy principles.

• Photosynthesis is an endothermic process, and chemiluminescence is an exothermic process.

Poll

On a scale of 1-4, how confident are you about the concepts covered in today's review?

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