Exothermic and Endothermic Reactions: Feeling the Heat in Chemistry

Introduction: Chemistry You Can Feel

Some parts of chemistry feel rather abstract. Atoms are too small to see, bonds are invisible, and energy changes can sound like something hidden inside a textbook.

But exothermic and endothermic reactions are different.

These are chemical reactions you can often feel.

A test tube becomes warm. A beaker turns cold. A reaction fizzes, bubbles, changes colour, or seems to quietly steal heat from the room. For GCSE students, this is one of the first times chemistry becomes physically noticeable. For A Level students, the same idea becomes far more quantitative, as they begin to calculate enthalpy changes, bond energies and energy profiles.

The basic question is simple:

Does the reaction give out heat, or does it take heat in?

The chemistry behind that question is fascinating.

What Is an Exothermic Reaction?

An exothermic reaction is a reaction that transfers energy to the surroundings, usually as heat.

That means the surroundings get warmer.

In a school laboratory, this might be seen as a rise in temperature on a thermometer or temperature probe. In everyday life, exothermic reactions are everywhere.

Examples include:

burning fuels
respiration in living cells
neutralisation between an acid and an alkali
some displacement reactions
hand warmers
setting concrete
combustion in a gas boiler or car engine

One of the classic GCSE examples is the reaction between an acid and an alkali.

For example:

hydrochloric acid + sodium hydroxide → sodium chloride + water

When these react, heat is released. The temperature of the solution increases, and students can record this change.

At first, this can seem almost magical. Two clear liquids are mixed together and suddenly the temperature rises. Nothing dramatic may be visible, but energy has been transferred.

That is often one of the most important lessons in chemistry: not all important changes are obvious to the eye.

What Is an Endothermic Reaction?

An endothermic reaction takes in energy from the surroundings.

This means the surroundings become colder.

Students often remember endothermic reactions because the temperature drop can be surprisingly large. A beaker can feel cold to the touch, and in some demonstrations condensation may form on the outside.

Examples include:

thermal decomposition reactions
some reactions between acids and carbonates
dissolving certain salts in water
instant cold packs used for sports injuries
photosynthesis

A simple classroom example is dissolving ammonium nitrate in water. The process absorbs heat from the surroundings, causing the temperature to fall.

This is the same general idea behind some instant cold packs. When the chemicals inside the pack mix, heat is absorbed, making the pack cold enough to help reduce swelling after an injury.

In other words, endothermic chemistry is not just a school experiment. It has real uses.

The GCSE Practical: Measuring Temperature Change

At GCSE, students usually investigate temperature changes using simple equipment:

polystyrene cup
thermometer or temperature probe
measuring cylinder
acid and alkali, or other reacting chemicals
lid to reduce heat loss
stopwatch
stirring rod

A polystyrene cup is often used because it is a good insulator. It helps reduce heat transfer between the reaction mixture and the room.

A typical method might be:

Measure a known volume of acid into a polystyrene cup.
Record the starting temperature.
Add a measured volume of alkali.
Stir gently.
Record the highest temperature reached.
Calculate the temperature change.

If the temperature rises, the reaction is exothermic.

If the temperature falls, the reaction is endothermic.

This sounds straightforward, but it teaches several important scientific skills.

Students must measure accurately, control variables, repeat readings and think about sources of error. They also learn that practical science is rarely as neat as a textbook diagram.

Practical Example: Acid and Alkali Neutralisation

Suppose a student mixes hydrochloric acid with sodium hydroxide solution.

Starting temperature: 20°C
Highest temperature: 27°C

Temperature change:

27 − 20 = 7°C

The temperature has increased, so the reaction is exothermic.

A good GCSE answer might say:

The temperature increased by 7°C, showing that heat energy was transferred from the reaction mixture to the surroundings. Therefore, the neutralisation reaction was exothermic.

That final sentence matters. Students should not just say “it got hotter”. They need to connect the observation to the energy transfer.

This is often where marks are won or lost.

Practical Example: Endothermic Temperature Drop

Now imagine a student dissolves a salt in water.

Starting temperature: 21°C
Lowest temperature: 15°C

Temperature change:

15 − 21 = −6°C

The temperature has decreased, so the process is endothermic.

A strong answer might say:

The temperature fell by 6°C, showing that energy was taken in from the surroundings. Therefore, the process was endothermic.

The negative temperature change is important. It shows that the energy transfer has gone in the opposite direction.

Why Do Some Reactions Give Out Heat?

Chemical reactions involve breaking bonds and making new bonds.

This is the key idea.

Breaking bonds requires energy.

Making bonds releases energy.

Whether a reaction is exothermic or endothermic depends on the balance between these two processes.

In an exothermic reaction:

less energy is needed to break bonds
more energy is released when new bonds form
overall, energy is released to the surroundings

In an endothermic reaction:

more energy is needed to break bonds
less energy is released when new bonds form
overall, energy is taken in from the surroundings

This is one of the most important ideas for students to understand. Heat is not simply “stored inside chemicals” and then released like steam from a kettle. Energy changes happen because chemical bonds are being broken and formed.

Reaction Profile Diagrams

GCSE students are also expected to represent these reactions using energy profile diagrams.

These diagrams show the energy of the reactants and products during a reaction.

For an exothermic reaction, the products have less energy than the reactants. The energy difference is released to the surroundings.

The diagram usually shows:

reactants higher up
products lower down
an arrow showing energy released
an activation energy hump

For an endothermic reaction, the products have more energy than the reactants. Energy must be taken in from the surroundings.

The diagram usually shows:

reactants lower down
products higher up
an arrow showing energy taken in
an activation energy hump

These diagrams are extremely useful because they help students see the energy change clearly.

They also introduce another important idea: activation energy.

Activation Energy: The Push Needed to Start

Even exothermic reactions usually need some energy to begin. This is called the activation energy.

A match will not light itself just because burning is exothermic. It needs the initial energy from friction. A fuel will not burn unless it is ignited. Hydrogen and oxygen can release a lot of energy when they react, but they need a spark.

Activation energy is like pushing a ball over a hill. Once it gets over the top, it can roll down the other side.

This is why reaction profile diagrams show a hump. The reactants must first gain enough energy to reach the top of the energy barrier before the reaction can proceed.

Where Students Often Get Confused

Students often make several common mistakes with this topic.

The first is thinking that “hot” automatically means dangerous and “cold” means safe. This is not always true. Some endothermic reactions involve harmful chemicals, and some exothermic reactions may be mild.

The second mistake is mixing up system and surroundings. In school chemistry, we usually measure the surroundings, such as the solution in the cup. If the thermometer goes up, heat has been transferred to the surroundings.

The third mistake is forgetting that bond breaking always takes in energy. Students sometimes write that energy is released when bonds break, but this is incorrect. Energy is released when bonds are made.

The fourth mistake is drawing energy profile diagrams the wrong way round. For exothermic reactions, products go lower than reactants. For endothermic reactions, products go higher.

A simple memory aid is:

Exo = exit. Energy exits the reaction.

Taking It Further at A Level

At A Level, this topic becomes more mathematical.

Students move from simply saying “the temperature went up” to calculating the actual energy change.

They use the equation:

q = mcΔT

where:

q is the heat energy transferred in joules
m is the mass of the solution in grams
c is the specific heat capacity
ΔT is the temperature change

For aqueous solutions, students often use:

c = 4.18 J g⁻¹ °C⁻¹

This allows students to calculate how much energy has been transferred during a reaction.

They may then calculate enthalpy change per mole, usually in kJ mol⁻¹.

This is where the GCSE practical becomes the foundation for much more advanced chemistry.

A Level Example: Calculating Energy Change

Suppose 50 cm³ of acid reacts with 50 cm³ of alkali.

Total volume = 100 cm³

Assuming the density is 1 g cm⁻³, the mass is approximately:

100 g

Temperature rise = 6°C

Using:

q = mcΔT

q = 100 × 4.18 × 6

q = 2508 J

This is:

2.508 kJ

Because the temperature has risen, the reaction is exothermic. The enthalpy change would be negative when expressed for the reaction.

This is another major difference between GCSE and A Level.

At GCSE, students may say:

The reaction is exothermic because the temperature increased.

At A Level, students may need to calculate:

The enthalpy change is negative because heat energy is released to the surroundings.

The same idea is still there, but the level of detail has increased.

Why Enthalpy Changes Are Negative or Positive

This can confuse students at first.

For an exothermic reaction, the reaction releases energy. The system loses energy, so the enthalpy change is negative.

For an endothermic reaction, the system gains energy. The enthalpy change is positive.

So:

exothermic reaction: ΔH is negative
endothermic reaction: ΔH is positive

This is why careful language matters. The thermometer measures the temperature change of the surroundings, but the enthalpy change refers to the system.

That distinction becomes very important at A Level.

Why Practical Results Are Never Perfect

In school experiments, the calculated value often differs from the accepted value.

This does not mean the experiment has failed.

It means real experiments have limitations.

Possible sources of error include:

heat lost to the air
heat absorbed by the cup or thermometer
incomplete reaction
inaccurate volume measurements
temperature not recorded at exactly the highest or lowest point
assuming the density is exactly 1 g cm⁻³
assuming the specific heat capacity is the same as water

This is an excellent opportunity to teach students that science is not just about getting “the right answer”. It is about understanding the method, improving the design and evaluating the evidence.

A better experiment might use a lid, insulation, a digital temperature probe, repeated readings and a graph to extrapolate the temperature change more accurately.

Everyday Chemistry: Hot Packs, Cold Packs and Fuels

One reason this topic is so useful is that it connects directly to everyday life.

Hand warmers use exothermic processes to release heat slowly. Some use the oxidation of iron, while reusable gel hand warmers often involve crystallisation.

Cold packs use endothermic processes to absorb heat. They are useful for sports injuries because they quickly become cold when the chemicals inside mix.

Burning fuels is exothermic. That includes methane in gas boilers, petrol in car engines and hydrogen in fuel cells.

Photosynthesis is endothermic overall because plants take in energy from sunlight to convert carbon dioxide and water into glucose and oxygen.

So the topic is not just a laboratory exercise. It links chemistry to medicine, sport, energy, biology and climate science.

Personal Reflection: When Chemistry Becomes Real

One of the best things about teaching this topic is that students can experience it directly.

There is something powerful about watching a student hold a polystyrene cup, look at the thermometer and realise that chemistry has changed the temperature without a flame, heater or battery.

For some students, that moment matters. It turns chemistry from a list of equations into a physical process they can observe, measure and explain.

At GCSE, the aim is to recognise and describe the energy change.

At A Level, the challenge is to calculate it accurately and understand what it means in terms of enthalpy and bond energies.

Both levels are connected. The simple school practical is the first step towards a much deeper understanding of chemical energetics.

Conclusion: Chemistry Is an Energy Story

Exothermic and endothermic reactions are not just definitions to memorise.

They are part of the bigger story of chemistry.

Every chemical reaction involves energy. Bonds break, bonds form, heat may be released, or heat may be absorbed. A simple temperature change in a cup can reveal what is happening at the molecular level.

For GCSE students, this topic builds practical skills, graph skills and scientific explanations.

For A Level students, it becomes the foundation for enthalpy calculations, bond energy questions and thermodynamics.

The key idea is beautifully simple:

Exothermic reactions give energy out. Endothermic reactions take energy in.

But behind that simple idea lies one of the most important principles in chemistry: chemical change and energy change are inseparable.

05 June 2026