The enthalpy changes for the following processes are listed below: $\mathrm{Cl}_{2}(\mathrm{~g})=2 \mathrm{Cl}(\mathrm{g}), \quad 242.3 \mathrm{~kJ} \mathrm{~mol}^{-1}$ $\mathrm{I}_{2}(\mathrm{~g})=2 \mathrm{I}(\mathrm{g}), \quad 151.0 \mathrm{~kJ} \mathrm{~mol}^{-1}$ $\mathrm{ICl}(\mathrm{g})=\mathrm{I}(\mathrm{g})+\mathrm{Cl}(\mathrm{g}), 211.3 \mathrm{~kJ} \mathrm{~mol}^{-1}$ $\mathrm{I}_{2}(\mathrm{~s})=\mathrm{I}_{2}(\mathrm{~g}), \quad 62.76 \mathrm{~kJ} \mathrm{~mol}^{-1}$ Given that the standard states for iodine and chlorine are $\mathrm{I}_{2}(\mathrm{~s})$ and $\mathrm{Cl}_{2}(\mathrm{~g})$, the standard enthalpy of formation for $\mathrm{ICl}(\mathrm{g})$ is

(C) $+16.8 \mathrm{~kJ} \mathrm{~mol}^{-1}$

(A) $-14.6 \mathrm{~kJ} \mathrm{~mol}^{-}$

(B) $-16.8 \mathrm{~kJ} \mathrm{~mol}^{-1}$

(D) $+244.8 \mathrm{~kJ} \mathrm{~mol}^{-1}$

JEE Main · 2006 · 30 Apr 2006 · Offline · Chemical Thermodynamics

The enthalpy changes for the following processes are listed below:

Cl_{2} (g) = 2 Cl (g), 242.3 kJ mol^{- 1}

I_{2} (g) = 2 I (g), 151.0 kJ mol^{- 1}

ICl (g) = I (g) + Cl (g), 211.3 kJ mol^{- 1}

I_{2} (s) = I_{2} (g), 62.76 kJ mol^{- 1}

Given that the standard states for iodine and chlorine are

I_{2} (s)

and

Cl_{2} (g)

, the standard enthalpy of formation for

ICl (g)

(A) $- 14.6 kJ mol^{-}$
(B) $- 16.8 kJ mol^{- 1}$
(C) $+ 16.8 kJ mol^{- 1}$ ✓ Correct
(D) $+ 244.8 kJ mol^{- 1}$

Answer: (C)

Solution

The formation of

ICl (g)

\frac{1}{2} I_{2} (s) + \frac{1}{2} C l_{2} (g) \to ICl (g)

, and Hess's law lets it be assembled from the listed steps. Half a mole of iodine must be sublimed and dissociated and half a mole of chlorine dissociated to atoms, after which one mole of

I - Cl

bonds forms, releasing the dissociation enthalpy of ICl:

Δ_{f} H = \frac{1}{2} (62.76) + \frac{1}{2} (151.0) + \frac{1}{2} (242.3) - 211.3

. This evaluates to

31.38 + 75.5 + 121.15 - 211.3 = + 16.73

, about

+ 16.8 kJ mo l^{- 1}

JEE Main · 2006 · 30 Apr 2006 · Offline · Single correct option

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JEE Main · 2006 · 30 Apr 2006 · Offline · Chemical Thermodynamics

Solution

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