4.1.2Reactivity 1 — What drives chemical reactions?
Exothermic and Endothermic Reactions — IB Chemistry HL Revision Notes & Topical Questions
5 min read 5 parts Lesson 6 of 12 Unit quiz
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Section 1 of 5Two Types of Reaction
Exothermic
- Exothermic Energy is transferred from the system to the surroundings, so the temperature of the surroundings rises and ΔH is negative.
Endothermic
- Endothermic Energy is absorbed from the surroundings, so the temperature of the surroundings falls and ΔH is positive.
Enthalpy change
- Enthalpy change ΔH = H(products) − H(reactants), and it is quoted in kJ mol⁻¹ for the reaction as written.
System and surroundings
- System and surroundings The system is the reacting chemicals, and everything else, including the solvent and container, is the surroundings.
Stability
- Stability Exothermic reactions give products lower in enthalpy, which are usually more stable than the reactants.
| Feature | Exothermic | Endothermic |
|---|---|---|
| Surroundings temperature | Rises | Falls |
| ΔH | Negative | Positive |
| Examples | Combustion, neutralisation | Thermal decomposition, photosynthesis |
Section 2 of 5Why Energy Changes
1
To start a reaction, bonds in the reactants must be broken, which always requires energy and is endothermic.
2
Forming new bonds in the products releases energy, which is always exothermic.
3
If more energy is released making bonds than is absorbed breaking bonds, the reaction is exothermic overall.
4
If the energy needed to break bonds is larger, the reaction is endothermic overall.
5
The net change appears as heat flow to or from the surroundings, which can be detected as a temperature change.
Section 3 of 5Everyday Examples
Everyday Examples
Hand warmers
- Hand warmers Crystallisation or oxidation of iron releases heat, making these exothermic.
Cold packs
- Cold packs Dissolving ammonium nitrate in water absorbs energy, making the pack feel cold.
Respiration
- Respiration Glucose is oxidised in cells, releasing energy for the organism.
Cooking an egg
- Cooking an egg Heat must be supplied continuously to drive the endothermic changes in the proteins.
Section 4 of 5Worked Example
Question
- Methane burns with ΔH = −890 kJ mol⁻¹.
- How much energy is released by burning 8.0 g of methane?
- M(CH₄) = 12.01 + 4 × 1.008 = 16.04 g mol⁻¹.
- n = 8.0 ÷ 16.04 = 0.499 mol.
- Energy = 0.499 × 890 kJ mol⁻¹.
- Energy released ≈ 444 kJ, and the sign shows heat is lost from the system.
Answer ≈ 444 kJ released
Section 5 of 5Exam Checklist & Tips
4.1.2 – Key Points for the Exam
- Exothermic A reaction that releases energy to the surroundings, with ΔH < 0.
- Endothermic A reaction that absorbs energy from the surroundings, with ΔH > 0.
- Enthalpy change The heat energy change at constant pressure.
- System The chemicals under study, separated from the surroundings.
Common Mistakes
- Saying that an endothermic reaction does not need an initial energy input.
- Using a positive ΔH for combustion.
- Mixing up the system and surroundings when describing temperature changes.
- Thinking bond breaking releases energy.
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