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Hess' Law

Given HCNaq+...

Question

Given
$H C N (a q) + N a O H (a q) ⟶ N a C N (a q) + H_{2} O + 44 k J$
$H C N (a q) + {N H}_{4} O H (a q) ⟶ {N H}_{4} C N (a q) + H_{2} O + 40 k J$
${(Δ_{f} H)}_{{O H}^{-} (a q)} = - 229.5 k J {m o l}^{- 1}$
${(Δ_{f} H)}_{H_{2} O (l))} = - 286.2 k J {m o l}^{- 1}$
Which is correct statement?

A

Δ_{i o n} H (H C N) = 13.3 k J {m o l}^{- 1}

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B

Δ_{i o n} H ({N H}_{4} O H) = 4 k J m o l^{- 1}

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C

Enthalpy of neutralisation of

H C l

and

N a O H

is

- 57.3 k J

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D

None of these

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Solution

The correct option is B $Δ_{i o n} H ({N H}_{4} O H) = 4 k J m o l^{- 1}$
Given
1: $H C N (a q) + N a O H (a q) ⟶ N a C N (a q) + H_{2} O + 44 k J$
2: $H C N (a q) + {N H}_{4} O H (a q) ⟶ {N H}_{4} C N (a q) + H_{2} O + 40 k J$
${(Δ_{f} H)}_{{O H}^{-} (a q)} = - 229.5 k J {m o l}^{- 1}$
${(Δ_{f} H)}_{H_{2} O (l))} = - 286.2 k J {m o l}^{- 1}$
Difference between reaction 1 and 2 is due to the heat of ionization of weak base $N H_{4} O H$ .
Therefore $Δ_{i o n} H (N H_{4} O H) = 44 - 40 = 4 k J / m o l$
Enthalpy of neutralization of strong acid base is calculated as:
$H^{+} + O H^{-} \to H_{2} O$
$Δ_{n e u t} H = Δ_{f} H (H_{2} O) - Δ_{f} H (O H^{-}) - Δ_{f} H (H^{+}) = - 286.2 - (- 229.5) - 0 = - 56.7 k J / m o l$
For reaction 1:
$Δ_{i o n} H (H C N) = Δ_{r} H - Δ_{n e u t} H = - 44 - (- 56.7) = 12.7 k J / m o l$
Therefore option B is correct.

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Similar questions

H C l + N a O H ⟶ N a C l + H_{2} O; Δ H = - 57.1 k J m o l^{- 1}

Δ H

is the heat of neutralisation of

H C l

N a O H

solution.If true enter 1 else o

Q. 100 ml 0.5 N

H_{2} S O_{4}

(strong acid) is neutralised with 200 ml 0.2 M

N H_{4} O H

in a constant pressure calorimeter which results in temperature rise of

1.4^{o} C

. If heat capacity of calorimeter content is

1.5 k J /^{o} C

. Which of the following statements is/are correct?
Given:

H C l + N a O H \to N a C l + H_{2} O + 57 k J

C H_{3} C O O H + N H_{4} O H \to C H_{3} C O O N H_{4} + H_{2} O + 48.1 k J

Q. Enthalpy of neutralisation of a strong acid by strong base:
(1) has a constant value of

- 57.32 k J

(2) is independent of the nature of strong acid and strong base
(3) results in heat change accompanied by the reaction

H^{+} (a q) + {O H}^{-} (a q) \to H_{2} O

Which of the above statements is\are correct?

Calculate the enthalpy change for the following reaction
$C H_{4} (g) + 2 O_{2} (g) \to C O_{2} (g) + 2 H_{2} O (l)$
Given enthalpies of formation of $C H_{4}, C O_{2} & H_{2} O$ are $- 74.8 K J m o l^{- 1}, - 393.5 K J m o l^{- 1}$ and $- 286.2 K J m o l^{- 1}$ respectively.

Q. Enthalpy of neutralisation of the reaction between

C H_{3} C O O H (a q .) a n d N a O H (a q .) i s - 13.2 k c a l e q^{- 1}

and that of the reaction between

H_{2} S O_{4} (a q .)

and KOH (aq.) is - 13.7 kcal

e q^{- 1} .

The enthalpy of dissociation of

C H_{3} C O O H (a q .)

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