The table shows bond energies.

| Bond | Bond energy / kJ mol⁻¹ |
|------|------------------------|
| H–H | 436 |
| Cl–Cl| 242 |
| H–Cl | 431 |

What is the enthalpy change for the reaction: H₂ + Cl₂ → 2HCl?

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Question

The table shows bond energies.

| Bond | Bond energy / kJ mol⁻¹ |
|------|------------------------|
| H–H  | 436                    |
| Cl–Cl| 242                    |
| H–Cl | 431                    |

What is the enthalpy change for the reaction: H₂ + Cl₂ → 2HCl?

Accepted Answer

–184 kJ mol⁻¹ — Award 1 mark for correct calculation. Bonds broken: H–H (436) + Cl–Cl (242) = +678 kJ mol⁻¹. Bonds formed: 2 × H–Cl (2 × 431) = –862 kJ mol⁻¹. ΔH = 678 + (–862) = –184 kJ mol⁻¹. B is incorrect — this has the wrong sign. C is incorrect — this only considers bonds formed. D is incorrect — this has the wrong sign and ignores bonds broken.

Answer

–862 kJ mol⁻¹

Answer

–184 kJ mol⁻¹

Answer

+862 kJ mol⁻¹

Answer

+184 kJ mol⁻¹

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