Endothermic Reactions

In this lesson you will learn about endothermic reactions — reactions that absorb energy from the surroundings. You will explore examples, learn how to draw and interpret reaction profile diagrams for endothermic processes, compare endothermic and exothermic reactions side by side, and study a practical investigation that demonstrates an endothermic change. This topic is part of the Edexcel GCSE Chemistry (1CH0) specification.

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What Is an Endothermic Reaction?

An endothermic reaction is a chemical reaction that takes in (absorbs) energy from the surroundings. The surroundings get cooler and there is a measurable temperature decrease.

In an endothermic reaction:

• The energy needed to break bonds in the reactants is greater than the energy released by forming new bonds in the products.
• The shortfall in energy is taken from the surroundings, usually as heat (thermal energy).
• The products have more energy than the reactants.
• The overall energy change (ΔH) is positive.

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Examples of Endothermic Reactions

Reaction type	Example	Observation
Thermal decomposition	CaCO₃ → CaO + CO₂ (heating limestone)	Requires continuous heating — stops when heat is removed
Citric acid + sodium hydrogen carbonate	C₆H₈O₇ + 3NaHCO₃ → Na₃C₆H₅O₇ + 3H₂O + 3CO₂	Solution gets noticeably cold; fizzing
Photosynthesis	6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂	Plants absorb light energy from the Sun
Dissolving ammonium nitrate in water	NH₄NO₃(s) → NH₄⁺(aq) + NO₃⁻(aq)	Water temperature drops significantly
Electrolysis	Various	Requires continuous electrical energy input

Everyday Uses of Endothermic Reactions

• Instant cold packs — used for treating sports injuries. The pack contains ammonium nitrate and water in separate compartments. When the barrier is broken, the ammonium nitrate dissolves in the water. This is endothermic, so the pack gets cold and can be applied to reduce swelling.
• Cooking and baking — thermal decomposition of sodium hydrogencarbonate (baking soda) is an endothermic process. When heated, it releases CO₂ gas, which makes dough rise.

Exam tip: Dissolving ammonium nitrate in water is one of the most commonly cited examples of an endothermic process. Remember it for the exam — it appears regularly in questions about energy changes.

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Reaction Profile Diagram for Endothermic Reactions

For an endothermic reaction, the reaction profile looks like this:

Description of the Diagram

1. The reactants are shown at a lower energy level on the left side of the diagram.
2. The energy rises to a peak — this is the activation energy (Eₐ), which is measured from the reactant energy level to the top of the peak.
3. The energy then falls to a level that is still higher than where the reactants started — this is the energy of the products.
4. The difference between the energy of the products and the energy of the reactants is the overall energy change (ΔH).
5. Because the products are at a higher energy than the reactants, energy has been absorbed from the surroundings.
6. ΔH is positive for endothermic reactions.

Key Labels on the Diagram

Label	What it shows
Reactants energy level	Starting energy (lower)
Products energy level	Final energy (higher than reactants for endothermic)
Activation energy (Eₐ)	Energy barrier from reactants to the peak
Overall energy change (ΔH)	Difference from reactants to products — positive for endothermic

Exam tip: When drawing a reaction profile for an endothermic reaction, make sure the products line is higher than the reactants line. The ΔH arrow should point upward. A common mistake is to draw the products lower, which would make it exothermic.

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Practical: Measuring an Endothermic Temperature Change

Dissolving Ammonium Nitrate in Water

Equipment

• Polystyrene cup
• Thermometer (or temperature probe)
• Measuring cylinder
• Ammonium nitrate (solid)
• Distilled water
• Balance

Method

1. Measure 50 cm³ of distilled water using a measuring cylinder and pour it into a polystyrene cup.
2. Record the initial temperature of the water.
3. Weigh out 5 g of ammonium nitrate using a balance.
4. Add the ammonium nitrate to the water and stir gently with the thermometer.
5. Record the minimum temperature reached.
6. Calculate the temperature change: ΔT = minimum temperature − initial temperature.

Expected Results

Measurement	Value
Initial temperature of water	20.0 °C
Minimum temperature after dissolving	14.5 °C
Temperature change (ΔT)	−5.5 °C

The temperature decreases, confirming that dissolving ammonium nitrate in water is an endothermic process.

Exam tip: Note that the temperature change (ΔT) is negative because the temperature went down. A negative temperature change means energy was absorbed from the water, making it colder. This confirms the process is endothermic.

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Comparing Exothermic and Endothermic Reactions

This comparison table is extremely useful for the exam. Learn it thoroughly.

Feature	Exothermic	Endothermic
Energy transfer	Energy transferred to the surroundings	Energy transferred from the surroundings
Temperature change	Surroundings get hotter (temperature rises)	Surroundings get cooler (temperature falls)
Bond energy balance	Energy released forming bonds > energy to break bonds	Energy to break bonds > energy released forming bonds
ΔH	Negative	Positive
Reaction profile	Products lower than reactants	Products higher than reactants
Examples	Combustion, neutralisation, oxidation, respiration	Thermal decomposition, photosynthesis, dissolving NH₄NO₃
Everyday use	Hand warmers, self-heating cans	Cold packs

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Side-by-Side Reaction Profiles

Exothermic Reaction Profile (description)

• Reactants at a high energy level on the left
• Energy rises to the activation energy peak
• Energy drops to a lower level for products on the right
• ΔH arrow points downward — energy is released
• The gap between reactants and products represents the energy released to surroundings

Endothermic Reaction Profile (description)