1.8.5Physical Chemistry
Bond Enthalpy — Edexcel AS Chemistry Revision Notes & Topical Questions
5 min read 6 parts Lesson 37 of 42
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 6Bond Enthalpy Basics
Definition
- Definition Bond enthalpy is the energy needed to break one mole of a particular covalent bond in the gaseous state.
Mean bond enthalpy
- Mean bond enthalpy Averaged over many molecules, because the same bond has slightly different energies in different compounds.
Breaking bonds
- Breaking bonds Always endothermic, with energy absorbed from the surroundings.
Making bonds
- Making bonds Always exothermic, with energy released to the surroundings.
Part 2 of 6Calculation Method
1
Draw out the displayed formulae of all reactants and products so that every bond is visible.
2
List and count the bonds broken in the reactants and add up their mean bond enthalpies.
3
List and count the bonds formed in the products and add up their mean bond enthalpies.
4
Use ΔH = Σ(bonds broken) − Σ(bonds formed).
5
Give the sign and units, kJ mol⁻¹, and comment on whether the reaction is exothermic or endothermic.
| Bond | Mean bond enthalpy (kJ mol⁻¹) |
|---|---|
| H–H | 436 |
| Cl–Cl | 243 |
| H–Cl | 432 |
| C–H | 413 |
| O=O | 498 |
| C=O in CO₂ | 805 |
| O–H | 464 |
Part 3 of 6Limitations
Limitations
Mean values
- Mean values Mean bond enthalpies are averages, so calculated values are only estimates.
Gaseous state
- Gaseous state Bond enthalpies apply to gaseous species, so liquids or solids need extra enthalpy of vaporisation.
Experimental value
- Experimental value Results often differ from calorimetric data for these reasons.
Stronger or weaker bonds
- Stronger or weaker bonds Higher bond enthalpy means a stronger bond requiring more energy to break.
Part 4 of 6Interpreting Results
Exothermic
- Exothermic More energy is released making bonds than is needed to break them, so ΔH is negative.
Endothermic
- Endothermic More energy is needed to break bonds than is released on forming new ones, so ΔH is positive.
Check
- Check Counting bonds carefully is the most common place to lose marks, especially in molecules with several identical bonds.
Compare
- Compare Combustion of methane using bond data gives about −818 kJ mol⁻¹ when water is a gas.
Part 5 of 6Worked Example
Question Use the table to calculate ΔH for H₂(g) + Cl₂(g) → 2HCl(g).
- Bonds broken1 × H–H (436) + 1 × Cl–Cl (243) = 679 kJ.
- Bonds formed2 × H–Cl = 2 × 432 = 864 kJ.
- ΔH = 679 − 864 = −185 kJ mol⁻¹.
Answer ΔH = −185 kJ mol⁻¹ (exothermic)
Part 6 of 6Exam Checklist & Tips
1.8.5 – Key Points for the Exam
- Bond enthalpy Energy required to break one mole of a bond in the gas phase.
- Mean bond enthalpy Average value of a bond enthalpy across different molecules.
- Exothermic Releases net energy.
- Endothermic Absorbs net energy.
Common Mistakes
- Using the formula the wrong way round, formed minus broken.
- Forgetting to multiply by the number of each type of bond.
- Applying bond enthalpies to liquid water without correcting for state.
- Forgetting double bonds when counting C=O or O=O.
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