CH₄ + 2Cl₂ → CH₂Cl₂ + 2HCl [The bond energies of C − H, Cl − Cl, C − Cl and H − Cl are 414, 244, 326 and 431 kJ/mol respectively] What is the value of ΔH in the reaction?

Correct answer: d 198

Break 2 C-H and 2 Cl-Cl = 1316 kJ, form 2 C-Cl and 2 H-Cl = 1514 kJ; ΔH = −198 kJ, magnitude 198.

Why the other options are wrong

• a 92: No step gives this; broken 2(414) + 2(244) = 1316, formed 2(326) + 2(431) = 1514, difference 198.
• b 99: 99 is the book's one-Cl₂ example; here there are 2 Cl₂, so everything doubles: 198.
• c 192: An arithmetic slip; 1316 − 1514 = −198, magnitude 198.