Calculation of enthalpy changes of reaction using bond energies
3 marks · NormalExplain why the reaction between hydrogen and chlorine to form hydrogen chloride is exothermic, making reference to bond energies:
Bond energies (kJ/mol):
| Bond | Bond energy |
|---|---|
| H–H | 436 |
| Cl–Cl | 243 |
| H–Cl | 432 |
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Breaking H–H and Cl–Cl bonds requires energy (436 + 243 = 679 kJ). Forming 2 H–Cl bonds releases more energy (2×432 = 864 kJ). Since more energy is released than absorbed, the reaction is exothermic.
Also accept: Because making H–Cl bonds releases more energy than is needed to break H–H and Cl–Cl bonds so overall heat is released.; The bonds formed are stronger (release more energy) than the bonds broken, making the reaction exothermic.
Common mistakes: Not linking energy released to exothermicity; Not referring to bond energies
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Marking points
M1 · 1 mark · Correctly describes energy needed to break bonds: H–H and Cl–Cl.
M2 · 1 mark · Correctly describes energy released when new bonds are formed: 2 H–Cl.
A1 · 1 mark · Explains overall that more energy is released than absorbed; links to exothermic reaction. (follow-through)
Worked solution
Breaking H–H and Cl–Cl bonds absorbs 436 kJ and 243 kJ, total 679 kJ.
Forming two H–Cl bonds releases 2×432 = 864 kJ.
Because the energy released in making bonds (864 kJ) is greater than energy absorbed (679 kJ), the reaction is exothermic overall.