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