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Hess' Law

Calculate the...

Question

Calculate the lattice enthalpy of $K C l$ from the following data at standard states-
Enthalpy of sublimation of $K = 89 k J m o l^{- 1}$
Enthalpy of dissociation of $C l_{2} = 244 k J m o l^{- 1}$
Ionisation energy of $K = 425 k J m o l^{- 1}$
Electron gain enthalpy of $C l = - 355 k J m o l^{- 1}$
Enthalpy of formation of $K C l = - 438 k J m o l^{- 1}$

A

719 k J m o l^{- 1}

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B

- 719 k J m o l^{- 1}

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C

- 832 k J m o l^{- 1}

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D

832 k J m o l^{- 1}

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Solution

The correct option is B $- 719 k J m o l^{- 1}$
$K (s) \to K (g); Δ H_{s u b} = 89 k J m o l^{- 1}$

$\frac{1}{2} C l_{2} (g) \to C l (g); \frac{B D E}{2} = \frac{244}{2} = 122 k J m o l^{- 1}$

$K (g) \to K^{+} (g); I E = 425 k J m o l^{- 1}$

$C l (g \to C l^{-} (g); E_{A} = - 355 k J m o l^{- 1}$

$K (s) + \frac{1}{2} C l_{2} (g) \to K C l (s); Δ H_{f}^{0} = - 438 k J m o l^{- 1}$

According to Born-Haber cycle
$H_{f}^{0} = Δ H_{s u b} + \frac{B D E}{2} + I E + E A + U$
$- 438 = 89 + 122 + 425 - 355 + U$
$U = - 719 k J m o l^{- 1}$

hence, lattice energy of $K C l$ = $- 719 k J m o l^{- 1}$

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Enthalpy of sublimation of

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Enthalpy of dissociation of

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Ionisation energy of

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Electron gain enthalpy of

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