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5.3Chemical Energetics

Bond energies and enthalpy change — Cambridge IGCSE Chemistry 0620 Revision Notes & Topical Questions

5 min read 8 parts Lesson 3 of 3 1 quiz

📚 Cambridge IGCSE Chemistry 0620 Hub / Topical Revision
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5.3 Bond energies and enthalpy change — IGCSE CHEMISTRY notes: Explain enthalpy changes in terms of bond breaking and bond making and calculate ΔH from bond energies.Covalent bond — a strong attraction between atoms, so separating atoms takes energy and joining atoms releases it.…

  • Exothermic reaction
  • Endothermic reaction
  • Transferred energy
  • Bond energy
  • Typical values (kJ/mol)
  • Interpretation

Section 1 of 8Bond breaking and bond making

Covalent bond
  • Covalent bond a strong attraction between atoms, so separating atoms takes energy and joining atoms releases it.
Bond breaking
  • Bond breaking endothermic, because energy must be supplied to separate the atoms.
Bond making
  • Bond making exothermic, because energy is given out when new bonds form.
FeatureBond breakingBond making
Energy changeEnergy absorbedEnergy released
Type of processEndothermicExothermic
Applies toBonds in reactantsBonds in products
Sign in the ΔH sumAdded (+)Subtracted (−)

Section 2 of 8Linking bonds to heat changes

Exothermic reaction
  • Exothermic reaction more energy is released by bond making than is used in bond breaking.
Endothermic reaction
  • Endothermic reaction more energy is used in bond breaking than is released by bond making.
Transferred energy
  • Transferred energy the difference is the heat transferred to or from the surroundings.
Fuels
  • Fuels give out heat because the bonds formed in CO2 and H2O are stronger than those broken in the fuel and oxygen.

Section 3 of 8Bond energy

Bond energy
  • Bond energy the energy needed to break one mole of a particular covalent bond, in kJ/mol.
  • The same amount is released when one mole of that bond forms.
Typical values (kJ/mol)
  • H–H 436, Cl–Cl 242, H–Cl 431, C–H 412, O=O 498, C=O 805, O–H 464, N≡N 945 and N–H 391.
  • In exams, use the values given in the question.

Section 4 of 8Calculating ΔH

  • Equation
    • ΔH = total energy of bonds broken − total energy of bonds formed.
    • Bonds broken are all the bonds in the reactants.
    • Bonds formed are all the bonds in the products.
1
Write the balanced equation.
2
Draw displayed formulae so that every bond is visible.
3
Count each type of bond and multiply by the balancing numbers.
4
Add the energy for bonds broken and for bonds formed.
5
Subtract and give the sign and the unit kJ/mol.
  • Interpretation
    • a negative result means exothermic and a positive result means endothermic.

Section 5 of 8Finding an unknown bond energy

Method
  • Method write the equation ΔH = bonds broken − bonds formed with the unknown bond energy as x, then rearrange it.
Information needed
  • Information needed the overall ΔH and all the other bond energies.
Check
  • Check the answer must be a positive bond energy in kJ/mol.

Section 6 of 8More from the specification

Endothermic energy intake by system
  • Endothermic energy intake by system absorbing thermal energy from the surroundings increases chemical potential energy stored within the thermodynamic system.
Reaction profile
  • A diagram showing the relative energies of reactants and products
Pathway
  • A sequence of reactions converting one compound into another.
Hydrogen
  • A gas that gives a squeaky pop with a lighted splint.
Reaction pathway
  • Sequence of reactions leading to a product.
Energy level diagram
  • Energy level diagram products sit lower than reactants for exothermic reactions and higher for endothermic reactions.
Activation energy
  • Activation energy the minimum energy colliding particles need to react, shown as a peak above the reactants' level.
Temperature change
  • Temperature change exothermic reactions raise the temperature of the surroundings, while endothermic reactions lower it.

Section 7 of 8Key terms

TermMeaning
Bond energyEnergy to break one mole of a particular covalent bond.
Bond breakingEndothermic process in which energy is absorbed.
Bond makingExothermic process in which energy is released.
Enthalpy change (ΔH)Bonds broken minus bonds formed, in kJ/mol.
ExothermicMore energy released making bonds than absorbed breaking bonds.
EndothermicMore energy absorbed breaking bonds than released making bonds.

Section 8 of 8Exam technique

1
Saying breaking a bond releases energy — breaking bonds always absorbs energy.
2
Reversing the formula as bonds formed − bonds broken.
3
Forgetting to multiply bond counts by the balancing numbers, for example in 2HCl.
4
Leaving out the sign or unit of ΔH — give the sign and kJ/mol.

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