Average bond energies ( ): H--H: H--I: H--Br: Br--Br: I--I:

CHEMISTRY · Multi-part · A Level · Chemistry · Energetics · Energetics

Problem

Average bond energies (

\text{kJ mol}^{-1}

): H--H:

436\,\text{kJ mol}^{-1}

H--I:

299\,\text{kJ mol}^{-1}

H--Br:

366\,\text{kJ mol}^{-1}

Br--Br:

193\,\text{kJ mol}^{-1}

I--I: ? I--Br: ? Two gas-phase reactions were studied calorimetrically at

298\,\text{K}

. Reaction 1:

2\,\text{HI(g)} \to \text{H}_2\text{(g)} + \text{I}_2\text{(g)}

\Delta H^\circ = +53\,\text{kJ}

Reaction 2:

\text{HI(g)} + \text{Br}_2\text{(g)} \to \text{HBr(g)} + \text{IBr(g)}

\Delta H^\circ = -26\,\text{kJ}

(a) For Reaction 1, set up an expression for

\Delta H^\circ

in terms of the unknown bond energy of the

I-I

bond,

B(I-I)

, and then determine

B(I-I)

. (b) Using the value of

B(I-I)

found in part (a), set up an expression for

\Delta H^\circ

of Reaction 2 in terms of the unknown bond energy of the

I-Br

bond,

B(I-Br)

, and calculate

B(I-Br)

. (c) Based on your results, rank the three single bonds

H-I

I-Br

, and

I-I

from strongest to weakest. Justify your ranking with reference to your calculated bond energies and periodic trends.