Endothermic Reactions

Chemical reactions always involve changes in energy. In some reactions, energy is transferred from the surroundings into the reacting chemicals.

These reactions are called endothermic reactions.

The word helps us remember its meaning:

Endo = into

Thermic = heat

So, during an endothermic reaction, energy enters the reacting system from the surroundings.

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What Is an Endothermic Reaction?

An endothermic reaction is a chemical reaction that absorbs energy from its surroundings.

The energy is usually transferred as thermal energy.

A simplified representation is:

Reactants + Energy → Products

Because energy is taken from the surroundings, the surroundings often become cooler.

For example, if two substances are mixed and the temperature falls from 24°C to 17°C, this provides evidence that the process is endothermic.

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System and Surroundings

To understand an endothermic reaction, we need to distinguish between the system and the surroundings.

System

The chemicals participating in the reaction.

Surroundings

Everything outside the reacting chemicals, such as:

• the container
• water around the reaction
• the air
• a thermometer

During an endothermic reaction:

Surroundings → Energy → System

The system gains energy while the surroundings lose energy.

As a result, the surroundings often become colder.

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Temperature Changes

Temperature measurements can provide evidence about energy transfer.

Imagine two chemicals are mixed.

Initial temperature:

23°C

Lowest temperature:

15°C

Temperature change:

ΔT = final temperature − initial temperature

ΔT = 15 − 23

ΔT = −8°C

The temperature decreased by 8°C.

This suggests that energy was absorbed from the surroundings.

The process is therefore endothermic.

A useful rule is:

Temperature decreases → usually endothermic

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Measuring an Endothermic Reaction

A simple laboratory experiment can demonstrate an endothermic process.

Students can:

1. measure the initial temperature
2. combine the substances
3. stir the mixture
4. record the lowest temperature reached
5. calculate the temperature change

For example:

Initial temperature = 21°C

Lowest temperature = 12°C

Therefore:

ΔT = 12 − 21 = −9°C

The negative temperature change indicates that the surroundings became colder.

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Examples of Endothermic Processes

Examples commonly studied in chemistry include:

• some reactions between salts and water
• some thermal decomposition reactions
• photosynthesis
• reactions used in some instant cold packs

Some physical changes, such as melting and evaporation, are also endothermic processes because they absorb energy, although they are not chemical reactions.

This distinction is important.

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Instant Cold Packs

Some instant cold packs work using an endothermic dissolving process.

When activated, substances inside the pack mix. Dissolving the chemical requires more energy than is released during the process.

Energy is therefore taken from the surroundings.

The pack becomes colder.

The energy transfer is:

Surroundings → Cold-pack contents

This is why the outside of the pack feels cold.

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Photosynthesis

Photosynthesis is an important biological example of an endothermic reaction.

Plants use energy from sunlight to convert carbon dioxide and water into glucose and oxygen.

The word equation is:

Carbon dioxide + Water → Glucose + Oxygen

Energy from light is required for this reaction.

The balanced chemical equation is:

6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

Light energy is absorbed and ultimately stored as chemical energy in the products.

This demonstrates that endothermic reactions do not always involve something becoming noticeably cold. The key idea is that the reacting system absorbs energy.

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Thermal Decomposition

Some compounds require continuous heating before they will break apart.

These reactions are called thermal decomposition reactions.

For example, calcium carbonate decomposes when strongly heated:

Calcium carbonate → Calcium oxide + Carbon dioxide

CaCO₃ → CaO + CO₂

Energy must be supplied to the reaction.

Therefore, the decomposition is endothermic.

If the heat source is removed, the decomposition does not simply continue releasing its own energy.

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Why Do Endothermic Reactions Absorb Energy?

Chemical reactions involve breaking old bonds and forming new bonds.

Breaking Bonds

Breaking chemical bonds requires energy.

Forming Bonds

Forming new chemical bonds releases energy.

In an endothermic reaction:

Energy required to break bonds > Energy released when new bonds form

The additional energy must come from the surroundings.

Therefore:

Energy enters the reacting system.

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Energy Profile of an Endothermic Reaction

An energy profile diagram shows the energy changes that occur during a reaction.

For an endothermic reaction:

Products have more energy than the reactants.

Energy must therefore have been absorbed during the reaction.

The energy difference between the reactants and products represents the overall energy absorbed.

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Activation Energy

Notice that an energy profile first rises before reaching the products.

Most reactions require some initial energy before they can begin.

This is called the activation energy.

Activation energy is:

the minimum energy required for a chemical reaction to begin

Endothermic and exothermic reactions both require activation energy.

The difference is what happens to energy overall during the complete reaction.

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Enthalpy Change

Chemists often represent the overall energy change of a reaction using ΔH, called the enthalpy change.

For an endothermic reaction:

ΔH > 0

The value is positive because the reacting system has gained energy.

For example:

ΔH = +75 kJ/mol

indicates an endothermic reaction.

For comparison:

Endothermic → positive ΔH

Exothermic → negative ΔH

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Endothermic vs Exothermic

These two types of reactions involve opposite directions of energy transfer.

A useful memory aid is:

ENDOthermic → Energy ENters

EXOthermic → Energy EXits

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Comparing Energy Profiles

The shape of an energy diagram provides another way to distinguish the two types.

Exothermic

Reactants higher → Products lower

Energy is released.

Endothermic

Reactants lower → Products higher

Energy is absorbed.

Therefore, if you are given an unfamiliar energy profile, compare the energy of the reactants and products.

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Interpreting Temperature Data

Consider the following results:

The temperature initially falls from:

24°C → 14°C

This indicates that energy was absorbed from the surroundings.

The process is endothermic.

After reaching the lowest temperature, the mixture begins warming again because thermal energy moves from the wider environment back into the cooler mixture.

The important evidence is the initial temperature decrease.

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Worked Example

A student mixes two substances.

Initial temperature:

26°C

Lowest temperature:

19°C

Step 1 – Calculate ΔT

ΔT = final temperature − initial temperature

ΔT = 19 − 26

ΔT = −7°C

Step 2 – Interpret the result

The temperature decreased by 7°C.

Step 3 – Identify the process

Energy was absorbed from the surroundings.

Therefore, the process was endothermic.

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Identifying an Unknown Reaction

Suppose an unfamiliar reaction has these observations:

• the container becomes cold
• temperature falls from 22°C to 13°C
• energy is absorbed from the surroundings

We can conclude:

The reaction is endothermic.

A strong scientific explanation would be:

The reaction is endothermic because the temperature of the surroundings decreased, providing evidence that energy was transferred from the surroundings into the reacting system.

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Endothermic Physical Changes

The terms endothermic and exothermic can also describe physical processes involving energy transfer.

For example:

Melting

A solid absorbs energy to become a liquid.

Evaporation

A liquid absorbs energy to become a gas.

This explains why sweating can cool the body.

Water in sweat absorbs thermal energy as it evaporates from the skin.

However, remember:

Melting and evaporation are physical changes, not chemical reactions.

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Common Misconception

An endothermic reaction does not simply mean a reaction that is cold.

The important idea is the direction of energy transfer.

A reaction is endothermic when:

Energy moves from the surroundings into the reacting system.

Similarly, an endothermic reaction does not necessarily involve thermal energy. Photosynthesis, for example, absorbs light energy.

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Did You Know?

Some chemical demonstrations can become cold enough to freeze water underneath the reaction container.

This happens because the endothermic process absorbs thermal energy from its surroundings very rapidly.

The cooling is therefore not something the reaction "produces." Instead, it is evidence that thermal energy has been removed from the surroundings.

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Key Terms

Endothermic reaction – A reaction that absorbs energy from the surroundings.

System – The chemicals participating in a reaction.

Surroundings – Everything outside the reacting system.

Temperature change (ΔT) – The difference between final and initial temperature.

Thermal decomposition – Breaking a compound into simpler substances using thermal energy.

Activation energy – The minimum energy required for a reaction to begin.

Energy profile – A diagram showing energy changes during a reaction.

Enthalpy change (ΔH) – The overall energy change associated with a reaction.

Exothermic reaction – A reaction that transfers energy to the surroundings.

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Key Takeaways

• An endothermic reaction absorbs energy from the surroundings.
• Energy moves from the surroundings into the reacting system.
• The surroundings usually become cooler.
• A decrease in temperature can provide evidence of an endothermic process.
• Examples include photosynthesis, some thermal decomposition reactions, and processes used in some instant cold packs.
• Breaking chemical bonds requires energy, while forming bonds releases energy.
• In an endothermic reaction, more energy is required to break bonds than is released when new bonds form.
• The products are at a higher energy level than the reactants.
• Endothermic reactions have a positive ΔH.
• Both endothermic and exothermic reactions require activation energy.
• Endothermic = energy enters the system.
• Exothermic = energy exits the system.
• Endothermic and exothermic describe the direction of energy transfer, not simply whether something feels hot or cold.

Energy Transfer During Reactions

Every chemical reaction involves energy changes.

During a reaction, atoms are rearranged as existing chemical bonds are broken and new bonds are formed. These changes involve transfers of energy between the reacting chemicals and their surroundings.

Energy cannot simply appear or disappear. Instead, it is transferred from one place or energy store to another.

A central idea is:

Energy cannot be created or destroyed — it can only be transferred or transformed.

This is the law of conservation of energy.

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The System and the Surroundings

When studying energy changes, chemists divide everything into two parts:

• the system
• the surroundings

The System

The system is the part we are studying.

In a chemical reaction, this usually means the reacting chemicals.

For example, if hydrochloric acid reacts with sodium hydroxide in a cup, the acid and sodium hydroxide are part of the system.

The Surroundings

The surroundings are everything outside the system.

They may include:

• the reaction container
• water around the reaction
• a thermometer
• the air
• the laboratory
• the person performing the experiment

Energy can move between the system and its surroundings.

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Energy Transfer Between System and Surroundings

There are two main possibilities.

Energy Leaves the System

System → Energy → Surroundings

The surroundings gain energy.

This occurs during an exothermic reaction.

Energy Enters the System

Surroundings → Energy → System

The system gains energy.

This occurs during an endothermic reaction.

The direction of the arrow is extremely important when interpreting energy-transfer diagrams.

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What Is Heat?

Heat refers to energy transferred because of a temperature difference.

Thermal energy naturally transfers from a region of higher temperature to a region of lower temperature.

For example, if a reaction mixture becomes hotter than its surroundings:

Hot reaction mixture → Thermal energy → Cooler surroundings

If a reaction causes the mixture to become colder than its surroundings:

Warmer surroundings → Thermal energy → Colder reaction mixture

Temperature changes can therefore provide evidence about the direction of energy transfer.

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Heat and Temperature Are Not the Same

Heat and temperature are related, but they are not the same thing.

Temperature

Temperature is related to the average kinetic energy of particles.

Heat

Heat describes energy being transferred because of a temperature difference.

For example, when a hot reaction mixture warms a thermometer:

Reaction mixture → Energy transfer → Thermometer

The thermometer's temperature rises because it has received energy.

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Chemical Reactions and Bonds

Chemical reactions rearrange atoms.

This means that chemical bonds in the reactants may be broken and new bonds form in the products.

Two important rules are:

Breaking bonds requires energy.

Forming bonds releases energy.

The overall energy change depends on the balance between these two processes.

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Breaking Bonds

Atoms joined by chemical bonds are attracted to each other.

Energy must be supplied to overcome these attractions and separate the atoms.

Therefore:

Breaking bonds → Energy absorbed

Imagine pulling apart two objects connected by a strong spring. Work must be done to separate them.

Chemical bonds behave differently from springs in many ways, but the analogy helps illustrate why breaking a bond requires energy.

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Forming Bonds

When atoms form new chemical bonds, energy is released.

Therefore:

Forming bonds → Energy released

This gives us the important comparison:

The balance between these determines whether the overall reaction is exothermic or endothermic.

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Exothermic Energy Transfer

In an exothermic reaction, more energy is released when new bonds form than is required to break the original bonds.

For example:

Energy required to break bonds:

300 kJ

Energy released when bonds form:

450 kJ

Overall:

450 − 300 = 150 kJ released

That energy is transferred to the surroundings.

Therefore:

System → 150 kJ → Surroundings

The surroundings usually become warmer.

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Endothermic Energy Transfer

In an endothermic reaction, more energy is required to break bonds than is released when new bonds form.

For example:

Energy required to break bonds:

500 kJ

Energy released when bonds form:

350 kJ

Difference:

500 − 350 = 150 kJ

The reaction must absorb 150 kJ from the surroundings.

Therefore:

Surroundings → 150 kJ → System

The surroundings usually become cooler.

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Conservation of Energy

The law of conservation of energy states that:

Energy cannot be created or destroyed. It can only be transferred or transformed.

This principle applies to every chemical reaction.

Suppose an exothermic reaction transfers 250 kJ of energy to its surroundings.

The system loses:

250 kJ

The surroundings gain:

250 kJ

Energy has not disappeared.

It has simply moved.

Energy lost by system = Energy gained by surroundings

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Conservation in an Endothermic Reaction

The same principle applies to endothermic reactions.

Suppose a reaction absorbs 80 kJ.

The surroundings lose:

80 kJ

The system gains:

80 kJ

Again:

Energy lost by surroundings = Energy gained by system

The total amount of energy remains constant.

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Where Does the Energy Go?

Energy transferred during reactions can appear in several forms.

Thermal Energy

Many reactions transfer energy as heat.

Example:

Combustion

Chemical energy ultimately contributes to heating the surroundings.

Light

Some reactions produce visible light.

Examples include:

• combustion
• fireworks
• glow sticks

Electrical Energy

Chemical reactions inside batteries can produce an electric current through an external circuit.

Sound

Rapid reactions may transfer some energy as sound.

A single chemical reaction may transfer energy through several pathways at the same time.

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Energy Transfer Diagrams

Energy-transfer diagrams show the direction in which energy moves.

For an exothermic reaction:

System → Surroundings

For an endothermic reaction:

Surroundings → System

The arrows show the direction of energy transfer.

When interpreting a diagram, ask:

1. What is the system?
2. What are the surroundings?
3. Which direction does energy move?
4. Does the system gain or lose energy?
5. Is the process exothermic or endothermic?

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Energy Profile Diagrams

An energy profile diagram provides more information about a reaction.

It shows how the energy changes as reactants are converted into products.

The vertical axis represents energy.

The horizontal axis represents the progress of the reaction.

The diagram usually shows:

• reactant energy
• activation energy
• product energy
• overall energy change

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Reading an Exothermic Energy Profile

In an exothermic reaction:

Reactants are higher in energy than products.

For example:

Reactants = 500 kJ

Products = 350 kJ

Difference:

500 − 350 = 150 kJ

The system has lost 150 kJ.

That energy has been transferred to the surroundings.

Therefore, the reaction is exothermic.

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Reading an Endothermic Energy Profile

In an endothermic reaction:

Products are higher in energy than reactants.

For example:

Reactants = 300 kJ

Products = 420 kJ

Difference:

420 − 300 = 120 kJ

The system has gained 120 kJ.

That energy came from the surroundings.

Therefore, the reaction is endothermic.

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Activation Energy

Energy profile diagrams also show activation energy.

Activation energy is the minimum energy required for a reaction to begin.

Even an exothermic reaction usually requires some initial energy.

For example, methane combustion releases large amounts of energy, but methane needs a spark or flame to begin reacting rapidly with oxygen.

The spark provides enough energy for particles to overcome the initial activation-energy barrier.

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Interpreting Temperature Changes

Temperature measurements can provide evidence about energy transfer.

Consider two experiments.

Experiment A

Initial temperature:

20°C

Final temperature:

34°C

The surroundings became warmer.

Energy was transferred:

System → Surroundings

The reaction is exothermic.

Experiment B

Initial temperature:

25°C

Final temperature:

16°C

The surroundings became cooler.

Energy was transferred:

Surroundings → System

The reaction is endothermic.

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Temperature-Time Graphs

Temperature changes can also be represented using graphs.

For an exothermic reaction, temperature generally:

rises → reaches a maximum → gradually falls toward room temperature

For an endothermic reaction, temperature generally:

falls → reaches a minimum → gradually rises toward room temperature

The later return toward room temperature occurs because energy continues to transfer between the reaction mixture and the wider environment.

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Worked Example 1

A chemical reaction occurs inside a cup.

The temperature changes from:

19°C → 32°C

What happened?

The temperature increased.

Direction of energy transfer

System → Surroundings

Type of reaction

Exothermic

Conservation of energy

The energy gained by the surroundings came from the reacting system.

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Worked Example 2

Another reaction causes the temperature to change from:

28°C → 20°C

What happened?

The temperature decreased.

Direction of energy transfer

Surroundings → System

Type of reaction

Endothermic

Conservation of energy

The energy gained by the reacting system came from the surroundings.

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Worked Example 3: Interpreting an Energy Diagram

Suppose an energy profile shows:

Reactants = 250 kJ

Products = 390 kJ

Calculate the overall energy change.

390 − 250 = +140 kJ

The products contain 140 kJ more energy than the reactants.

Therefore, the system must have absorbed 140 kJ.

Energy moved:

Surroundings → System

The reaction is endothermic.

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Putting It All Together

Energy changes during reactions can be understood as a sequence:

Reactants

↓

Energy absorbed to break bonds

↓

Atoms rearrange

↓

New bonds form

↓

Energy released

The balance between the energy absorbed and released determines the overall result.

If more energy is released:

Exothermic

If more energy is absorbed:

Endothermic

Throughout the process:

Total energy is conserved.

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Common Misconception

Chemical reactions do not create energy.

For example, saying:

"Burning fuel creates heat."

is a useful everyday shorthand, but scientifically it is better to say that the chemical reaction transfers energy to the surroundings, largely as thermal energy.

Similarly, an endothermic reaction does not destroy energy.

It absorbs energy from its surroundings.

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Did You Know?

Chemical reactions are responsible for many important energy transfers in everyday life.

They occur in:

• batteries
• engines
• living cells
• hand warmers
• cold packs
• fuels
• food
• industrial chemical processes

Even when the forms and pathways of energy transfer are different, the same fundamental rule always applies:

Energy is conserved.

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Key Terms

System – The part of the universe being studied, usually the reacting chemicals.

Surroundings – Everything outside the system.

Heat – Energy transferred because of a temperature difference.

Exothermic reaction – A reaction that transfers energy to the surroundings.

Endothermic reaction – A reaction that absorbs energy from the surroundings.

Conservation of energy – The principle that energy cannot be created or destroyed.

Activation energy – The minimum energy required for a reaction to begin.

Energy profile – A diagram showing energy changes during a reaction.

Energy transfer – Movement of energy from one place or system to another.

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Key Takeaways

• All chemical reactions involve energy changes and transfers.
• The system usually consists of the reacting chemicals.
• Everything outside the system is the surroundings.
• Energy transferred because of a temperature difference is called heat.
• Breaking chemical bonds requires energy.
• Forming chemical bonds releases energy.
• In an exothermic reaction, energy moves from the system to the surroundings.
• In an endothermic reaction, energy moves from the surroundings to the system.
• Temperature changes can provide evidence about the direction of energy transfer.
• Energy-transfer diagrams use arrows to show the direction energy moves.
• Energy-profile diagrams show the relative energies of the reactants and products.
• Energy cannot be created or destroyed during a chemical reaction.
• Energy lost by one part of the system and surroundings must be gained elsewhere.
• The law of conservation of energy applies to every chemical reaction.

Everyday Examples of Energy Changes

Chemical reactions involving energy changes occur all around us. They happen when we cook food, burn fuels, use batteries, exercise, grow plants, or activate a hand warmer.

Chemical substances contain chemical energy associated with their arrangements and chemical bonds. During a reaction, energy may be transferred between the reacting system and its surroundings.

Some reactions are exothermic, meaning they transfer energy to the surroundings.

Others are endothermic, meaning they absorb energy from the surroundings.

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Energy Changes Are Everywhere

Consider some familiar examples:

The important question is always:

Where does the energy come from, and where does it go?

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Combustion

Combustion occurs when a fuel reacts rapidly with oxygen.

Common examples include:

• natural gas burning on a stove
• petrol burning in an engine
• candles burning
• wood burning
• fuels burning in power stations

Combustion reactions are exothermic.

They transfer chemical energy from the reacting system to the surroundings, mainly as thermal energy, and often as light.

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Combustion of Methane

Methane is the main component of natural gas.

When methane burns:

Methane + Oxygen → Carbon dioxide + Water + Energy

The balanced equation is:

CH₄ + 2O₂ → CO₂ + 2H₂O

Energy is transferred to the surroundings.

This energy can be used for useful purposes such as:

• heating water
• cooking food
• heating buildings
• generating electricity

So the energy pathway can be simplified as:

Chemical energy → Thermal energy + Light

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Combustion in a Car Engine

Cars powered by petrol or diesel also depend on combustion.

Fuel reacts with oxygen inside the engine.

Energy released by the reaction increases the temperature and pressure of gases inside the engine. The expanding gases push pistons, eventually causing the wheels to turn.

A simplified energy pathway is:

Chemical energy in fuel

↓

Thermal energy

↓

Mechanical energy

↓

Movement of the vehicle

Not all the energy becomes useful movement. Large amounts are transferred to the surroundings as thermal energy and sound.

This is why engines become hot.

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Cellular Respiration

Your cells also depend on chemical reactions that release energy.

During cellular respiration, glucose reacts with oxygen.

The overall reaction is:

Glucose + Oxygen → Carbon dioxide + Water + Energy

The balanced equation is:

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + Energy

Respiration is an overall exothermic process.

However, cells do not simply release all of this energy as heat. Much of it is captured in molecules such as ATP, which cells can use to perform biological work.

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Where Does the Energy from Respiration Go?

Energy released during respiration supports many processes.

Cells use energy for:

• muscle contraction
• active transport
• growth
• protein synthesis
• cell division
• maintaining body functions

Some energy is eventually transferred to the surroundings as thermal energy.

The pathway can therefore be represented as:

Chemical energy in glucose

↓

Chemical energy transferred through ATP

↓

Cellular processes + thermal energy

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Why Do We Get Warm During Exercise?

During exercise, muscle cells require more energy.

The rate of cellular respiration increases to help meet this demand.

As chemical energy is transferred through biological processes, some energy ultimately becomes thermal energy.

This contributes to an increase in body temperature.

The body responds through processes such as:

• sweating
• increased blood flow near the skin
• increased breathing

So the warmth you experience during exercise is closely connected to energy transformations occurring inside your cells.

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Photosynthesis

Plants also carry out an important chemical reaction involving energy.

During photosynthesis, plants use light energy to produce glucose from carbon dioxide and water.

Carbon dioxide + Water + Light energy → Glucose + Oxygen

The balanced equation is:

6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

Photosynthesis requires an input of energy.

It is therefore an endothermic process.

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Energy Transformation in Photosynthesis

Plants capture light energy from the Sun.

Through photosynthesis, some of this energy is transformed and stored as chemical energy in organic molecules such as glucose.

The pathway can be simplified as:

Light energy

↓

Photosynthesis

↓

Chemical energy in glucose

Plants can later release usable energy from glucose through cellular respiration.

This creates an important connection:

Sunlight → Photosynthesis → Glucose → Respiration → Cellular work

Much of the chemical energy available to organisms in ecosystems can ultimately be traced back to energy from the Sun.

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Photosynthesis and Respiration

Photosynthesis and respiration are closely connected but involve different overall energy transformations.

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Hand Warmers

Have you ever opened a disposable hand warmer and noticed that it gradually becomes hot?

Many disposable hand warmers contain iron powder.

When the package is opened, oxygen from the air enters and reacts with the iron.

The iron undergoes oxidation.

A simplified representation is:

Iron + Oxygen → Iron oxide + Energy

The reaction is exothermic.

Energy is transferred:

Chemical system → Surroundings

The thermal energy released warms your hands.

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Why Don't Hand Warmers Burn?

Combustion and the oxidation inside many hand warmers both involve reactions with oxygen.

However, the reactions occur at very different rates.

Combustion occurs rapidly and may produce flames.

The oxidation in a hand warmer occurs much more slowly.

Energy is therefore released gradually over several hours instead of very rapidly.

This demonstrates an important idea:

The same general type of energy transfer can occur at very different rates.

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Instant Cold Packs

An instant cold pack produces the opposite sensation.

Many instant cold packs contain water and a substance that dissolves when the pack is activated.

When the substances mix, the overall dissolving process can absorb thermal energy from the surroundings.

The pack becomes colder.

This is an endothermic process.

Energy moves:

Surroundings → System

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Hand Warmers vs Cold Packs

These products provide an excellent comparison of energy transfer.

The useful effect of each product comes directly from the direction of energy transfer.

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Cooking with Chemical Energy

Cooking provides many examples of chemical energy changes.

When natural gas burns on a stove:

Chemical energy → Thermal energy

The thermal energy is then transferred:

Flame → Cooking pan → Food

The combustion reaction is exothermic, but many of the changes occurring in the food itself require an input of energy.

This demonstrates how several energy transfers can occur as part of one everyday activity.

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Batteries

Batteries provide another important application of chemical energy.

Chemical reactions inside a battery create conditions that allow electrons to move through an external circuit.

A simplified energy transformation is:

Chemical energy → Electrical energy

The electrical energy can then be transformed again.

For example, in a flashlight:

Chemical energy

↓

Electrical energy

↓

Light + Thermal energy

In an electric motor:

Chemical energy in battery

↓

Electrical energy

↓

Mechanical energy + Thermal energy

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Glow Sticks

Glow sticks provide an interesting example because much of the energy is released as light rather than heat.

When a glow stick is bent, an inner container breaks and chemicals mix.

A chemical reaction occurs that transfers energy to molecules capable of producing visible light.

This process is called chemiluminescence.

The energy pathway is approximately:

Chemical energy → Light energy

This reminds us that chemical reactions do not always transfer most of their energy as heat.

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Energy Changes in Food

Food contains chemical energy.

Carbohydrates, lipids, and proteins contain molecules that can participate in metabolic reactions.

Cells can release usable energy from food molecules through processes including cellular respiration.

That energy supports:

• movement
• growth
• repair
• active transport
• maintaining body temperature
• building biological molecules

Food is therefore not simply material for building the body. It is also an important energy source.

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Energy Changes in Ecosystems

Energy changes in photosynthesis and respiration connect organisms throughout ecosystems.

Plants capture energy:

Sunlight → Chemical energy in organic molecules

Animals obtain chemical energy by eating plants or other animals.

Both plants and animals carry out respiration:

Chemical energy → Cellular work + thermal energy

Energy therefore flows through ecosystems:

Sun → Producers → Consumers → Surroundings

At each stage, some energy is eventually transferred to the surroundings as thermal energy.

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Real-World Applications

Understanding chemical energy changes allows scientists and engineers to design useful technologies.

Heating

Exothermic reactions can provide thermal energy.

Examples:

• fuels
• hand warmers
• some industrial processes

Cooling

Endothermic processes can provide temporary cooling.

Example:

• instant cold packs

Transportation

Combustion reactions have traditionally provided energy for:

• cars
• aircraft
• ships

Electricity

Chemical reactions in batteries provide electrical energy for:

• phones
• computers
• electric vehicles
• medical devices
• portable electronics

Living Systems

Photosynthesis and respiration provide the foundation for energy transformations throughout much of the living world.

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Identifying Energy Changes

When examining an unfamiliar situation, ask four questions:

1. What is the chemical process?

Is something burning, reacting, dissolving, respiring, or photosynthesising?

2. Where does the energy begin?

For example:

• chemical energy
• light energy
• thermal energy

3. Where does the energy go?

Look for:

• heating
• cooling
• movement
• light
• electricity

4. Is energy entering or leaving the system?

Energy leaves system → Exothermic

Energy enters system → Endothermic

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Worked Example

A student activates two emergency packs.

Pack A

Temperature changes:

21°C → 39°C

The temperature increases.

Energy moves:

System → Surroundings

Therefore:

Pack A involves an exothermic process.

Pack B

Temperature changes:

21°C → 8°C

The temperature decreases.

Energy moves:

Surroundings → System

Therefore:

Pack B involves an endothermic process.

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Putting It All Together

Consider four everyday situations:

A candle burns

Chemical → Thermal + Light

Exothermic

A plant photosynthesises

Light → Chemical

Endothermic overall

A hand warmer activates

Chemical → Thermal

Exothermic

A cold pack activates

Thermal energy from surroundings → Chemical system

Endothermic

All four examples involve energy transformations, but the direction and form of energy transfer are different.

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Common Misconception

Energy is not created by combustion, respiration, batteries, or hand warmers.

Energy is transferred or transformed.

For example, a battery does not create electrical energy from nothing.

Instead:

Chemical energy → Electrical energy → Other energy forms

The total amount of energy is conserved.

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Did You Know?

The chemical energy in many foods can ultimately be traced back to the Sun.

Plants capture light energy through photosynthesis and store some of it as chemical energy in organic molecules. Animals obtain this chemical energy by eating plants or other organisms.

So the energy used by your muscles while running may ultimately have begun as sunlight captured by a plant.

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Key Terms

Chemical energy – Energy associated with chemical substances and their molecular arrangements.

Combustion – A reaction in which a substance reacts rapidly with oxygen and releases energy.

Respiration – Cellular reactions that transfer energy from molecules such as glucose for use by cells.

Photosynthesis – The process by which organisms such as plants use light energy to produce organic molecules.

Exothermic – Describes a process that transfers energy to the surroundings.

Endothermic – Describes a process that absorbs energy from the surroundings.

Oxidation – A chemical process involving loss of electrons; many everyday oxidation processes involve reaction with oxygen.

Chemiluminescence – Production of light as a result of a chemical reaction.

Energy transformation – Conversion of energy from one form to another.

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Key Takeaways

• Chemical reactions involving energy changes occur throughout everyday life.
• Combustion is exothermic and transfers chemical energy to the surroundings.
• Fuels can provide energy for heating, transportation, and electricity generation.
• Cellular respiration transfers energy from glucose for use by cells.
• Photosynthesis transforms light energy into chemical energy stored in organic molecules.
• Hand warmers use exothermic processes to transfer thermal energy to their surroundings.
• Cold packs use endothermic processes that absorb thermal energy from their surroundings.
• Batteries transform chemical energy into electrical energy.
• Glow sticks demonstrate that chemical energy can be transformed into light.
• Energy transformations are important in living organisms, ecosystems, transportation, heating, cooling, and technology.
• Energy is never created or destroyed during these processes—it is transferred or transformed.
• Understanding energy changes allows us to explain how many familiar products and technologies actually work.