Question

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

Answer 1

Answer:

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

Answer 2

Answer:

So energy released in the given reaction is -228 kJ/mol

Explanation:

$\rm \Delta H^0_r_x$ of the reaction is the net energy of the reaction. For considering the net energy of the reaction, the account of all the bonds broken and formed is considered.

In the above reaction, there is:

Broken bonds -  2 Br-Br bonds

                           1 C $\equiv$ C bond

Bonds formed = 1 C-C bond

                           4 C-Br bonds

The formation of bonds requires energy while breaking of bond releases energy.

From the given energies, total energy used in the breaking of bonds is:

Br-Br bonds = 2 * 193 kJ/mol

                    = 386 kJ/mol

C $\equiv$ C bond = 837 kJ/mol

Total energy for breaking of bond = 386 kJ/mol + 837 kJ/mol

                                                          = 1223 kJ/mol

From the given energies, total energy used in the formation of bonds is:

C-C bond = 347 kJ/mol

4 C-Br bonds = 4 * 276 kJ/mol

                     = 1104 kJ/mol

Total energy for formation of bond = 347 kJ/mol + 1104 kJ/mol

                                                           = 1451 kJ/mol

$\rm \Delta H^0_r_x$ for reaction = Energy for formation of bonds - Energy released for breaking of bonds

$\rm \Delta H^0_r_x$ = 1451 kJ/mol - 1223 kJ/mol

          = 228 kJ/mol

Since there is release of energy, minus sign is imparted.

So energy released in the given reaction is -228 kJ/mol.

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