Exothermic Reactions

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

These reactions are called exothermic reactions.

The word can help us remember its meaning:

Exo = out

Thermic = heat

So an exothermic reaction transfers energy out to the surroundings, often as thermal energy.

---

What Is an Exothermic Reaction?

An exothermic reaction is a chemical reaction that transfers energy from the reacting system to the surroundings.

The energy may be transferred as:

• thermal energy
• light
• sound
• electrical energy

In many exothermic reactions, the most obvious effect is an increase in the temperature of the surroundings.

We can represent this simply as:

Reactants → Products + Energy

The products contain less chemical energy than the reactants, so the difference is transferred to the surroundings.

---

System and Surroundings

To understand energy changes, it is useful to distinguish between the system and the surroundings.

System

The chemicals taking part in the reaction.

Surroundings

Everything outside the reacting chemicals, such as:

• the container
• water surrounding the reaction
• the air
• a thermometer
• your hand

During an exothermic reaction:

System → Energy → Surroundings

The system loses energy while the surroundings gain energy.

As a result, the surroundings often become warmer.

---

Temperature Changes

Temperature provides an important clue about whether a reaction is exothermic.

Imagine two solutions are mixed.

Before reaction:

Temperature = 22°C

After reaction:

Temperature = 31°C

Temperature change:

ΔT = 31 − 22 = +9°C

The temperature increased.

This provides evidence that energy was transferred from the reaction to the surroundings.

The reaction is therefore exothermic.

A useful rule is:

Temperature increases → usually exothermic

---

Measuring an Exothermic Reaction

A simple experiment can be used to identify an exothermic reaction.

Step 1

Measure the initial temperature of the reactants.

Step 2

Mix the reactants.

Step 3

Measure the highest temperature reached.

Step 4

Calculate the temperature change:

Temperature change = final temperature − initial temperature

For example:

Initial temperature = 20°C

Highest temperature = 35°C

Therefore:

ΔT = 35 − 20 = +15°C

The positive temperature change indicates that the surroundings became warmer.

---

Examples of Exothermic Reactions

Exothermic reactions occur frequently in everyday life and industry.

Common examples include:

• combustion
• respiration
• many neutralisation reactions
• some oxidation reactions
• reactions in disposable hand warmers

---

Combustion

Combustion is one of the easiest exothermic reactions to recognise.

During combustion, a substance reacts rapidly with oxygen and transfers energy to the surroundings.

For example, methane burns in oxygen:

Methane + Oxygen → Carbon dioxide + Water + Energy

Energy is transferred as heat and light.

This is why flames can warm their surroundings.

Examples of combustion include:

• burning natural gas
• burning wood
• burning petrol
• burning candles

---

Respiration

The reactions involved in cellular respiration are overall exothermic.

Cells use glucose and oxygen and release energy:

Glucose + Oxygen → Carbon dioxide + Water + Energy

Some of the released energy is captured chemically by cells for biological processes, while some is eventually transferred as thermal energy.

This contributes to maintaining body temperature in animals.

Respiration demonstrates that exothermic reactions are not limited to flames or laboratory experiments. They also occur continuously inside living organisms.

---

Neutralisation

When an acid reacts with a base, a neutralisation reaction occurs.

A simplified equation is:

Acid + Base → Salt + Water

Many neutralisation reactions are exothermic.

For example, when suitable solutions of hydrochloric acid and sodium hydroxide are mixed, the temperature increases.

The temperature rise provides evidence that energy has been transferred to the surroundings.

---

Hand Warmers

Some disposable hand warmers use exothermic chemical processes.

In many common air-activated hand warmers, iron reacts with oxygen.

The oxidation of iron releases energy gradually as thermal energy.

This is a useful application because the energy released by the reaction is deliberately used to warm the surroundings.

---

Energy Changes During a Reaction

Chemical substances contain chemical energy associated with their arrangements and chemical bonds.

During a chemical reaction:

1. bonds in the reactants must be broken
2. atoms rearrange
3. new bonds form in the products

Breaking bonds requires energy.

Forming bonds releases energy.

For an exothermic reaction:

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

The overall result is that energy is released to the surroundings.

---

Energy Profile of an Exothermic Reaction

An energy profile diagram shows how the energy changes during a chemical reaction.

For an exothermic reaction:

Reactants are at a higher energy level than the products.

The difference between these energy levels represents the overall energy transferred to the surroundings.

Notice that the graph initially rises.

This is because reactions usually require some energy to begin.

This initial energy requirement is called the activation energy.

---

Activation Energy

Even an exothermic reaction may require energy to get started.

The activation energy is the minimum energy required for particles to react successfully.

For example, wood burning is exothermic, but a piece of wood does not normally burst into flames by itself.

It first requires energy from something such as:

• a flame
• a spark
• a sufficiently hot surface

Once combustion begins, the reaction releases enough energy to help sustain further reaction.

---

Exothermic vs Endothermic Reactions

Not all chemical reactions release energy.

An endothermic reaction absorbs energy from the surroundings.

The two can be compared:

A useful memory aid is:

EXOthermic → energy EXits

ENDOthermic → energy ENters

---

Interpreting Temperature Data

Consider this experimental data:

The temperature rises from 21°C to 34°C.

This indicates that the reaction transfers energy to the surroundings.

Therefore, the reaction is exothermic.

After reaching its maximum, the temperature begins to decrease because thermal energy is gradually transferred from the warm reaction mixture to the wider environment.

The important evidence is the initial temperature increase.

---

Worked Example

A student mixes two chemicals in an insulated cup.

Initial temperature:

18°C

Maximum temperature:

27°C

Step 1 – Calculate the temperature change

ΔT = final temperature − initial temperature

ΔT = 27 − 18

ΔT = +9°C

Step 2 – Interpret the result

The temperature increased by 9°C.

Step 3 – Identify the reaction

Energy was transferred from the reaction to the surroundings.

Therefore, the reaction was exothermic.

---

Identifying Exothermic Reactions

When given information about an unfamiliar reaction, look for evidence such as:

• temperature increases
• heat is released
• light is produced
• surroundings become warmer
• products have less energy than reactants
• energy is shown on the product side of a simplified equation

For example:

A reaction causes the temperature of a solution to increase from 19°C to 37°C.

Evidence:

Temperature increased by 18°C.

Conclusion:

The reaction is exothermic because energy was transferred to the surroundings.

---

Common Misconception

An exothermic reaction does not necessarily have to involve fire.

Combustion is exothermic, but many reactions release energy without producing flames.

For example:

• respiration
• acid-base neutralisation
• oxidation inside some hand warmers

can all be exothermic.

The key idea is energy transfer, not whether a flame is visible.

---

Did You Know?

Some exothermic reactions can release energy extremely rapidly, while others release it very slowly.

Combustion may release large amounts of energy in seconds, while oxidation reactions in hand warmers can release thermal energy gradually for several hours.

Both are exothermic because the overall direction of energy transfer is the same:

Reaction → Surroundings

---

Key Terms

Exothermic reaction – A reaction that transfers energy from the reacting system to the surroundings.

System – The chemicals involved in a reaction.

Surroundings – Everything outside the reacting system.

Combustion – A reaction involving a substance reacting with oxygen, usually releasing energy.

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

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

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

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

---

Key Takeaways

• An exothermic reaction transfers energy to the surroundings.
• The surroundings usually become warmer during an exothermic reaction.
• A temperature increase provides evidence that a reaction may be exothermic.
• Common examples include combustion, respiration, neutralisation, and reactions used in hand warmers.
• Chemical reactions involve both breaking and forming bonds.
• Breaking bonds requires energy, while forming bonds releases energy.
• In an exothermic reaction, more energy is released during bond formation than is required for bond breaking.
• On an energy profile, the products are lower in energy than the reactants.
• Exothermic reactions usually have a negative enthalpy change (ΔH).
• Exothermic reactions transfer energy out, while endothermic reactions absorb energy in.
• Exothermic reactions do not necessarily involve flames—the defining feature is the overall transfer of energy to the surroundings.