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.