Why do reactions either give out or absorb heat?

3 marks · Normal

The reaction below occurs:

Bond energies (in kJ/mol): H–H = 436, Cl–Cl = 243, H–Cl = 431

Calculate the overall enthalpy change (ΔH) for the reaction.

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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–184 kJ/mol (±2)

Also accept: –185 kJ/mol; –183 kJ/mol

Common mistakes: Reversing the signs for bond breaking and making; failing to double H–Cl bond formed.

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Marking points

M1 · 1 mark · Correctly identifies bonds broken and bonds formed.

M2 · 1 mark · Substitutes bond energies into the equation: (bonds broken) – (bonds formed). (follow-through)

A1 · 1 mark · Correct ΔH value with sign and units (±2 kJ/mol). (follow-through)

Worked solution

Bonds broken: 1 H–H and 1 Cl–Cl

Bonds formed: 2 H–Cl

ΔH = (bonds broken) – (bonds formed)

So ΔH ≈ –184 kJ/mol.