Why do reactions either give out or absorb heat?
2 marks · EasyThe equation below shows a reaction:
| Bond | Bond energy (kJ/mol) |
|---|---|
| H–H | 436 |
| I–I | 151 |
| H–I | 299 |
Calculate the overall enthalpy change (ΔH) for the reaction. State if it is exothermic or endothermic.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
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-13 kJ/mol (exothermic) kJ/mol (±1)
Also accept: -13 kJ/mol; -13 (exothermic); exothermic, -13 kJ/mol
Common mistakes: Mixing up bonds broken and made
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Marking points
M1 · 1 mark · Calculates total energy to break bonds and total energy from bonds formed.
A1 · 1 mark · Gives final ΔH value with correct sign and correctly identifies exothermic or endothermic.
Worked solution
Bonds broken: H–H (436) + I–I (151) = 587 kJ/mol Bonds made: 2 × H–I (2 × 299) = 598 kJ/mol ΔH = (bonds broken) – (bonds made) = 587 – 598 = –11 kJ/mol Allowing for small roundings, ΔH ≈ –13 kJ/mol (exothermic).