4CH1

Energetics

Physical Chemistry · 3 question types

Practise

Exam Frequency Analysis

Past paper frequency (2018 to 2024)

This topic accounts for approximately 7% of your exam marks.

stable
Low
Stable7%

Calorimetry calculations and energy profile diagrams appear in nearly every series.

Energy in to break bonds, energy out to make bonds

  • During a reaction:
    • Existing bonds in the reactants must be broken — this takes energy IN (an endothermic step)
    • New bonds in the products are formed — this gives energy OUT (an exothermic step)
  • Whether the overall reaction is exothermic or endothermic depends on which of these is larger:
    • If energy released from forming new bonds > energy needed to break old bonds → exothermic (ΔH negative)
    • If energy needed to break old bonds > energy released from forming new bonds → endothermic (ΔH positive)
  • Mnemonic for the direction: bond breaking is the END of the bond, so it is END-othermic; bond making is EX-othermic
Bond breaking: bonded pairs of atoms separate into single atoms, with heat energy taken in from the surroundings, showing this step is endothermic
Source: What is bond energy? by Save My Exams
Bond making: single atoms join into bonded pairs to form new bonds, with heat energy released to the surroundings, showing this step is exothermic
Source: What is bond energy? by Save My Exams

Bond energy values

  • The of a given bond is the energy needed to break one mole of that bond, in the gas phase, in kilojoules per mole (kJ mol⁻¹)
  • Bond energies are quoted in data books as positive values (the amount of heat that has to be supplied to snap that bond apart)
  • Stronger bonds have larger bond energies (e.g. the triple bond N≡N is much larger than the single bond N−N)

Calculating ΔH from bond energies

  • General method:
    • Write out the equation with every bond shown explicitly (a "displayed formula")
    • Energy in = sum of the bond energies of every bond in the reactants
    • Energy out = sum of the bond energies of every bond in the products
    • ΔH = (energy in) − (energy out)
  • A negative answer means the products are at lower energy → exothermic; a positive answer → endothermic
Worked example

Calculating ΔH from bond energies

Hydrogen reacts with chlorine: H₂ + Cl₂ → 2HCl. Use the bond energy data to calculate ΔH. Bond energies: H−H = 436 kJ mol⁻¹, Cl−Cl = 242 kJ mol⁻¹, H−Cl = 431 kJ mol⁻¹.

Solution:

  • M1: energy to break bonds in reactants = 436 (H−H) + 242 (Cl−Cl) = 678 kJ
  • M2: energy released making bonds in products = 2 × 431 (H−Cl) = 862 kJ
  • M3: ΔH = energy in − energy out = 678 − 862 = −184 kJ mol⁻¹
Exam tip

Sign convention in bond energy calculations and the "bonds made = endothermic" trap

What comes up: explain why a reaction is exothermic using bond energies, or spot the sign error in a student's working.

Write (two marks): (1) breaking bonds requires energy (bond breaking is endothermic); (2) more energy is released when the new bonds form in the products than is taken in when the bonds in the reactants break, so the overall reaction is exothermic.

Watch out: the mark scheme awards no credit to a student who states that energy is released when bonds break, or that energy is taken in when bonds form — these are the reverse of the correct direction and the examiner notes treat them as zero.