4.1.2Reactivity 1 — What drives chemical reactions?
Exothermic and Endothermic Reactions — IB Chemistry SL Revision Notes & Topical Questions
5 min read 6 parts Lesson 5 of 8 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.
Part 1 of 5Two Types of Reaction
Exothermic
- Exothermic The system releases heat to the surroundings, so the surroundings get warmer and the enthalpy change ΔH is negative.
Endothermic
- Endothermic The system absorbs heat from the surroundings, so the surroundings get cooler and the enthalpy change ΔH is positive.
Sign convention
- Sign convention ΔH is defined as the enthalpy of products minus the enthalpy of reactants, so releasing energy makes the products lower in enthalpy.
Observation
- Observation A thermometer in the reaction mixture rises for an exothermic change and falls for an endothermic change, at constant pressure.
| Feature | Exothermic | Endothermic |
|---|---|---|
| Heat flow | System to surroundings | Surroundings to system |
| Temperature of surroundings | Increases | Decreases |
| ΔH | Negative | Positive |
| Stability of products | Usually more stable | Usually less stable |
| Examples | Combustion, neutralisation | Thermal decomposition of calcium carbonate |
Part 2 of 5Bonds and Energy
The overall sign of ΔH comes from a balance between two processes.
1
Breaking a bond requires energy to pull the atoms apart, so bond breaking is always an endothermic process.
2
Making a bond releases energy as the atoms move into a lower energy arrangement, so bond making is always exothermic.
3
If more energy is released making the new bonds than was absorbed breaking the old ones, the overall reaction is exothermic.
4
If more energy is needed to break the old bonds than is released forming the new ones, the overall reaction is endothermic.
Part 3 of 5Reading and Writing ΔH
1
Write the balanced equation with state symbols, because the value of ΔH applies to the amounts of substances in the equation as written.
2
State ΔH in kJ mol⁻¹ with its sign, since the sign tells the reader whether energy is released or absorbed.
3
Scale the energy by the amount of substance actually reacting if the quantity differs from the equation.
4
Reverse an equation and the sign of ΔH reverses while its magnitude stays the same.
Part 4 of 5Worked Example
Question
- Methane burns with ΔH = −890 kJ mol⁻¹.
- How much heat is released when 4.0 g of methane burns completely?
- (Mr of CH₄ = 16.05)
- Amount of CH₄ = 4.0 ÷ 16.05 = 0.249 mol.
- Heat released = 0.249 × 890 = 222 kJ (3 s.f.).
- The reaction is exothermic, so this energy leaves the system as heat.
Answer About 222 kJ is released.
Part 5 of 5Exam Checklist & Tips
4.1.2 – Key Points for the Exam
- Exothermic reaction A reaction that releases heat to the surroundings, with a negative ΔH.
- Endothermic reaction A reaction that absorbs heat from the surroundings, with a positive ΔH.
- Enthalpy change (ΔH) The heat energy change at constant pressure, equal to the enthalpy of products minus that of reactants.
- System The reacting chemicals being studied.
- Surroundings Everything outside the system, including the container and the air around it.
Common Mistakes
- Saying exothermic reactions give out cold because the temperature rises; it is the surroundings that warm up.
- Believing bond breaking releases energy; breaking bonds always requires energy.
- Writing a positive ΔH for combustion, which is always exothermic.
- Forgetting to scale ΔH when the amount reacting differs from the amount in the equation.
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