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3 May, 18:38

Use the bond energies provided to estimate ΔH°ᵣₓₙ for the reaction below.

2 Br2 (l) + C2H2 (g) → C2H2Br4 (l)

Br-Br = 193 kJ/mol; C≡C = 837 kJ/mol; C-C = 347 kJ/mol; C-Br = 276 kJ/mol;

C-H = 414 kJ/mol

a) + 407 kJ b) - 228 kJ c) + 573 kJ d) - 648 kJ e) - 324 kJ

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  1. 3 May, 20:10
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    The answer is b), - 228 kJ

    Explanation:

    First, we consider which bonds are broken and formed during the reaction

    Broken, Br - Br (02 molecules), C ≡ C

    Formed, C - C, C - Br (4 bonds)

    Noted that when breaking bonds, the molecules NEED energy, while forming bond, they release energy. So, secondly, we calculate

    Breaking energy = 2 x (Br-Br) + 1 x (C≡C) = 2 x 193 + 1 x 837 = 1,223 kJ

    Forming energy = - (4 x (C-Br) + 1 x (C-C) = - (4 x 276 + 1 x 347) = - 1,451 kJ (minus means that the energy is released during the reaction)

    Hence the ΔH°ᵣₓₙ is 1,223 kJ + (-1,451) kJ = - 228 kJ
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