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3.1.5Stoichiometry

Bond Energy — Cambridge IGCSE Chemistry 0620 Revision Notes & Topical Questions

5 min read 5 parts Lesson 18 of 28 Unit quiz

📚 Cambridge IGCSE Chemistry 0620 Hub / Topical Revision
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Part 1 of 5Bond Breaking and Making

3.1.5 Bond Energy

A reaction is a process of breaking bonds in the reactants and forming bonds in the products.

Bond breaking
  • Energy must be taken in to break bonds, so bond breaking is an endothermic process.
Bond making
  • Energy is released when new bonds are formed, so bond making is an exothermic process.
Overall change
  • Whether a reaction is exothermic depends on whether more energy is released by bond making than is taken in by bond breaking.
Bond energy
  • The bond energy is the energy needed to break one mole of a particular bond, and it is the same amount released when the bond forms.
BondBond energy (kJ/mol)
H–H436
Cl–Cl242
H–Cl431

Part 2 of 5Calculating an Enthalpy Change

The enthalpy change is found by comparing the energy taken in with the energy given out.

1
Draw the displayed equation and count the number of each type of bond broken in the reactants.
2
Add the bond energies of all the bonds broken: this is the energy taken in.
3
Count the bonds formed in the products and add their bond energies: this is the energy given out.
4
Calculate ΔH = energy taken in − energy given out, so a negative result means the reaction is exothermic.

Part 3 of 5Common Sources of Error

Common Sources of Error
Coefficients
  • Each bond must be multiplied by the number of molecules in the balanced equation, such as two H–Cl bonds for 2HCl.
Signs
  • Bond breaking is endothermic and bond making is exothermic, so their energies are added with opposite signs.
Average values
  • Bond energies are averages, so calculated values are only approximations of the enthalpy change.
Method
  • Drawing out the bonds in each molecule helps avoid missing a bond.

Part 4 of 5Worked Example

Question Use the bond energies H–H = 436 kJ/mol, Cl–Cl = 242 kJ/mol and H–Cl = 431 kJ/mol to calculate the enthalpy change for H₂(g) + Cl₂(g) → 2HCl(g).

  1. Energy taken in (bonds broken)H–H + Cl–Cl = 436 + 242 = 678 kJ.
  2. Energy given out (bonds formed)2 × H–Cl = 2 × 431 = 862 kJ.
  3. ΔH = energy taken in − energy given out = 678 − 862 = −184 kJ/mol.

Answer

  • ΔH = −184 kJ/mol.
  • The reaction is exothermic, because more energy is released when the bonds form than is taken in to break them.

Part 5 of 5Exam Checklist & Tips

3.1.5 – Key Points for the Exam

  • Bond energy The energy needed to break one mole of a particular covalent bond in the gas phase.
  • Bond breaking An endothermic process in which energy is taken in to split a bond.
  • Bond making An exothermic process in which energy is released when a bond forms.
  • Displayed formula A diagram showing all the atoms and bonds in a molecule.

Common Mistakes

  • Forgetting to multiply the bond energy of HCl by 2 in the balanced equation.
  • Subtracting the energy taken in from the energy given out, which reverses the sign.
  • Treating bond breaking as exothermic, when it is always endothermic.
  • Writing the bond energy calculation without stating whether the answer is exothermic or endothermic.

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