Endothermic Reactions

This lesson covers endothermic reactions — reactions that absorb energy from the surroundings — including examples, temperature changes, everyday uses and energy profile diagrams as required by the Edexcel GCSE Combined Science specification (1SC0).

---

What Is an Endothermic Reaction?

An endothermic reaction is one that takes in (absorbs) energy from the surroundings, usually as heat. The temperature of the surroundings (and the reaction mixture) decreases.

Term	Meaning
Endothermic	"Endo" = in; "thermic" = heat. Energy is absorbed
Temperature change	The temperature of the surroundings falls
Energy transfer	From the surroundings to the chemical system

---

Examples of Endothermic Reactions

Reaction	Detail
Thermal decomposition	Breaking down compounds by heating — e.g. CaCO₃ → CaO + CO₂. Requires continuous energy input
Citric acid + sodium hydrogencarbonate	The fizzing reaction in sherbet — the mixture gets cold
Photosynthesis	6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ — energy absorbed from sunlight
Dissolving ammonium nitrate in water	NH₄NO₃(s) → NH₄⁺(aq) + NO₃⁻(aq) — solution gets very cold
Electrolysis	Electrical energy is continuously supplied to decompose compounds

---

Detecting Endothermic Reactions

Observation	Interpretation
Temperature decreases	The reaction is endothermic
The reaction mixture or container feels cold	Energy is being absorbed from the surroundings

Practical Example: Citric Acid and Sodium Hydrogencarbonate

1. Add a spatula of citric acid powder to a polystyrene cup.
2. Add a spatula of sodium hydrogencarbonate powder.
3. Add 20 cm³ of water and stir.
4. The mixture fizzes and the temperature drops — the reaction is endothermic.

---

Reaction Profile for an Endothermic Reaction

graph TD
    subgraph "Endothermic Reaction Profile"
        R["Reactants<br/>(lower energy)"] -->|"Activation<br/>energy (Eₐ)"| T["Transition state<br/>(energy peak)"]
        T --> P["Products<br/>(higher energy)"]
    end

    style R fill:#2980b9,color:#fff
    style T fill:#f39c12,color:#fff
    style P fill:#e74c3c,color:#fff

Key features

Feature	Description
Reactants	At a lower energy level
Products	At a higher energy level
Activation energy (Eₐ)	The energy barrier from reactants to the top of the curve — usually larger than for exothermic reactions
Overall energy change (ΔH)	Positive — energy is absorbed from the surroundings
Energy absorbed	Equals the difference in energy between products and reactants

Exam Tip: In an endothermic reaction profile, the products are higher than the reactants. The arrow for ΔH points upwards. Be careful: the activation energy is still measured from the reactants to the peak — not from the products.

---

Comparing Exothermic and Endothermic Reactions

Feature	Exothermic	Endothermic
Energy transfer	To the surroundings	From the surroundings
Temperature change	Increases	Decreases
ΔH	Negative (−)	Positive (+)
Products vs reactants	Products at lower energy	Products at higher energy
Bond energy	Energy out > energy in	Energy in > energy out

---

Everyday Uses of Endothermic Reactions

Sports Injury Packs (Instant Cold Packs)

An instant cold pack contains water and a compartment of solid ammonium nitrate (or sometimes urea). When the inner bag is squeezed and broken, the ammonium nitrate dissolves in the water — an endothermic process. The pack absorbs heat from the injured area, reducing swelling and pain.

Feature	Detail
Chemical	Ammonium nitrate (NH₄NO₃)
Process	Dissolving in water (endothermic)
Effect	Temperature drops to approximately 0–5 °C
Use	Applied to sprains, bruises, swelling
Reusable?	No — single use

Exam Tip: The cold pack works because dissolving ammonium nitrate is endothermic — it absorbs heat from the skin and injured tissue, reducing swelling. Make sure you explain the energy transfer direction.

---

Energy and Bond Breaking/Making

In an endothermic reaction, the energy needed to break bonds in the reactants is greater than the energy released when new bonds form in the products.

Process	Energy
Breaking bonds	Requires energy (endothermic)
Making bonds	Releases energy (exothermic)
Endothermic overall	Energy required breaking bonds > energy released making bonds

---

Thermal Decomposition

Thermal decomposition is an important type of endothermic reaction. A compound is broken down into simpler substances by heating.

Examples

Compound	Equation	Products
Calcium carbonate	CaCO₃ → CaO + CO₂	Calcium oxide + carbon dioxide
Copper carbonate	CuCO₃ → CuO + CO₂	Copper oxide + carbon dioxide
Zinc carbonate	ZnCO₃ → ZnO + CO₂	Zinc oxide + carbon dioxide

The Bunsen burner must remain lit throughout — if you stop heating, the reaction stops. This confirms that energy is continuously being absorbed.

---

Summary

• Endothermic reactions absorb energy from the surroundings.
• The temperature of the surroundings decreases.
• Examples: thermal decomposition, photosynthesis, dissolving ammonium nitrate, citric acid + sodium hydrogencarbonate.
• On a reaction profile, the products are higher than the reactants and ΔH is positive.
• Energy needed to break bonds is greater than energy released forming new bonds.
• Everyday uses include instant cold packs for treating sports injuries.

---

Extended Worked Example 1: Bond Energy Confirming Endothermic

Question: Thermal decomposition of calcium carbonate: CaCO₃ → CaO + CO₂. Qualitatively explain why this reaction is endothermic using the concept of bond energy.

Answer:

1. In CaCO₃, strong ionic Ca²⁺–CO₃²⁻ bonds and C–O bonds within the carbonate ion must be broken.
2. Breaking bonds requires energy (endothermic step).
3. In the products, CaO contains Ca–O ionic bonds and CO₂ contains two C=O double bonds.
4. Making bonds releases energy.
5. For CaCO₃ decomposition, the energy absorbed breaking the reactant bonds exceeds the energy released making the product bonds.
6. Net ΔH is positive — energy must be continuously supplied (from the Bunsen burner) to keep the reaction going. This is why the reaction stops the instant heating stops.

---

Extended Worked Example 2: Temperature Drop in a Cold Pack