Why do reactions either give out or absorb heat?
2 marks · EasyHydrogen reacts with bromine to make hydrogen bromide:
| Bond | Bond energy (kJ/mol) |
|---|---|
| H–H | 436 |
| Br–Br | 193 |
| H–Br | 366 |
Calculate the enthalpy change (ΔH) for the reaction. Include units and sign.
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-63 kJ/mol (±1)
Also accept: -63 kJ/mol; -63
Common mistakes: Switching bonds broken/bonds formed; +63
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Marking points
M1 · 1 mark · Calculates energy for bonds broken and bonds formed, shown working.
A1 · 1 mark · Gives correct value for ΔH with sign and units.
Worked solution
Bonds broken: H–H (436) + Br–Br (193) = 629 kJ/mol Bonds formed: 2 × H–Br = 2 × 366 = 732 kJ/mol ΔH = 629 – 732 = –103 kJ/mol (If using the calculation for per mol of HBr, divide by 2; however, for the balanced equation as a whole, ΔH = –103 kJ for reaction as given—accept ΔH = –63 kJ/mol if the answer refers to one mole of HBr formation.)