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Hess' Law

Consider the ...

Question

Consider the following reactions:

(a) $H^{+} (a q) + O H^{-} (a q) \to H_{2} O (l) Δ H = - x_{1} k J m o l^{- 1}$

(b) $H_{2} (g) + \frac{1}{2} O_{2} (g) \to H_{2} O (l) Δ H = - x_{2} k J m o l^{- 1}$

(c) $C O_{2} (g) + H_{2 (g)} \to C O_{(g)^{'}} + H_{2} O (l) Δ H = - x_{3} k J m o l^{- 1}$

(d) $C_{2} H_{2} (g) + \frac{5}{2} O_{2} (g) \to 2 C O_{2} (g) + H_{2} O (l) Δ H = x_{4} k J m o l^{- 1}$

Enthalpy of formation of $H_{2} O (l)$ is

A

$+ x_{1} k J . m o l^{- 1}$

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B

$- x_{2} k J . m o l^{- 1}$

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C

$+ x_{3} k J . m o l^{- 1}$

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D

$+ x_{4} K J . m o l^{- 1}$

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Solution

The correct option is B
$- x_{2} k J . m o l^{- 1}$

This question, or rather the answer to this question, is based on the very definition of the enthalpy (or heat) of formation:

It is the amount of heat evolved or absorbed during the formation of 1 mole of a compound from its constituent elements (standard state).

$H_{2} (g) + \frac{1}{2} O_{2} (g) ⟶ H_{2} O (l)$

Hence the heat of formation of $H_{2} O (l) = - x_{2} k J . m o l^{- 1}$ .

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Similar questions

Q. Consider the following reactions:
(a)

H_{(a q)}^{+} + O H_{(a q)}^{-} = H_{2} O_{(I)}, △ H = - X_{1} k J m o l^{- 1}

H_{2 (g)} + \frac{1}{2} O_{2 (g)} = H_{2} O_{(I)}, △ H = - X_{2} k J m o l^{- 1}

C O_{2 (g)} + H_{2 (g)} = C O_{(g)} + H_{2} O_{(I)} - X_{3} k J m o l^{- 1}

C_{2} H_{2 (g)} + \frac{5}{2} O_{2 (g)} = 2 C O_{2 (g)} + H_{2} O_{(I)} + X_{4} k J m o l^{- 1}

.
Enthalpy of formation of

H_{2} O_{(l)}

Q. Consider the following reactions.

H_{(a q)}^{+} + O H_{(a q)}^{-} = H_{2} O_{(l)}, Δ H = - X_{1}

m o l^{- 1}

H_{2 (g)} + \frac{1}{2} O_{2 (g)} = H_{2} O_{(l)}, Δ H = X_{2}

m o l^{- 1}

C O_{2 (g)} + H_{2 (g)} = C O_{(g)} + H_{2} O_{(l)} - X_{3}

m o l^{- 1}

C_{2} H_{2 (g)} + \frac{5}{2} O_{2 (g)} = 2 C O_{2 (g)} + H_{2} O_{(l)} + X_{4}

m o l^{- 1}

Enthalpy of formation of

H_{2} O_{(l)}

Q. Consider the following reactions :

H^{+} (a q) + O H^{-} (a q) = H_{2} O (l); Δ H = - X_{1} k J . m o l^{- 1}

H_{2} (g) + \frac{1}{2} O_{2} (g) = H_{2} O (l); Δ H = - X_{2} k J . m o l^{- 1}

C O_{2} (g) + H_{2} (g) = C O (g) + H_{2} O (l); Δ H = + X_{3} k J . m o l^{- 1}

C_{2} H_{2} (g) + \frac{5}{2} O_{2} (g) = 2 C O (g) + H_{2} O (l); Δ H = + X_{4} k J . m o l^{- 1}

Enthalpy of formation of

H_{2} O (l)

Q. Consider the following reactions:

(i) H^{+} (a q) + O H^{-} (a q) \to H_{2} O (l), Δ H = - X_{1} {kJ mol}^{- 1}

(i i) H_{2} (g) + \frac{1}{2} O_{2} (a q) \to H_{2} O (l), Δ H = - X_{2} {kJ mol}^{- 1}

(i i i) C O_{2} (g) + H_{2} (g) \to C O (g) + H_{2} O (l), Δ H = - X_{3} {kJ mol}^{- 1}

(i v) C_{2} H_{2} (g) + \frac{5}{2} O_{2} (g) \to 2 C O_{2} (g) + H_{2} O (l), Δ H = X_{4} {kJ mol}^{- 1}

Enthalpy of formation of

H_{2} O (l)

Q. Using the following thermochemical data,

C (s) + O_{2} (g) \to C O_{2} (g) Δ H = - 94.0 k c a l

H_{2} (g) + \frac{1}{2} O_{2} (g) \to H_{2} O (l) Δ H = - 68.0 k c a l

C H_{3} C O O H (l) + 2 O_{2} (g) \to 2 C O_{2} (g) + 2 H_{2} O (l) Δ H = - 210.0 k c a l

the heat of formation of acetic acaid is :

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