Given - Enthalpy of ionization of two acids- - H- HCN - 45-2 kJ mol-1 - H- CH3COOH - 2-1 kJ mol-1Which relationship for the two acids is true-

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Standard XII

Chemistry

Salt of Weak Acid and Strong Base

Given : Entha...

Question

Given : Enthalpy of ionization of two acids:
$Δ H^{\circ} (H C N) = 45.2 k J m o l^{- 1}$
$Δ H^{\circ} (C H_{3} C O O H) = 2.1 k J m o l^{- 1}$
Which relationship for the two acids is true?

p K_{a} (H C N) = p K_{a} (C H_{3} C O O H)

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p K_{a} (H C N) > p K_{a} (C H_{3} C O O H)

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p K_{a} (H C N) < p K_{a} (C H_{3} C O O H)

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p K_{a} (H C N) = \frac{45.2}{2.1} p K_{a} (C H_{3} C O O H)

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Solution

The correct option is B $p K_{a} (H C N) > p K_{a} (C H_{3} C O O H)$
$Δ H^{o} (H C N) = 45.2 K J / m o l$
$Δ H^{o} (C H_{3} C O O H) = 2.1 K J / m o l$
$p K_{a}$ of $H C N \sim 9.2$
$p K_{a}$ of $C H_{3} C O O H \sim 4.7$
i.e. $p K_{a}$ of $H C N > p K_{a}$ of $C H_{3} C O O H$ .

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