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.