The enthalpy of formation of methane at constant pressure and $300 K$ is $- 75.83 K J$ . What will be the heat of formation at constant volume? $(R = 8.3 J K^{- 1} m o l^{- 1}$ )

-73.34 kJ

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73.34 kJ

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72.34 kJ

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-72.34 kJ

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Solution

The correct option is A
-73.34 kJ

The equation for the formation of methane is
$C (s) + 2 H_{2} (g) \to C H_{4} (g); △ H = - 75.83 k J$
$△$ n = 1 - 2 = -1 (only gaseous components are taken into account)
Given $△ H = - 75.83 K J, R = 8.3 \times 10^{- 3}$ $k J K^{- 1} m o l^{- 1}, T = 300 K$
The enthalpy change can be given by the equation:
$△ H = △ U + △ n R T$
$△ U = △ H - △ n R T$
$△ U = - 75.83 - (- 1 \times 8.3 \times 10^{- 3} \times 300)$
$△ U = - 75.83 + 2.49$
$△ U = - 73.34 k J$

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The enthalpy of formation of methane at constant pressure and 300K is −75.83KJ. What will be the heat of formation at constant volume? (R=8.3JK−1mol−1)

The enthalpy of formation of methane at constant pressure and $300 K$ is $- 75.83 K J$ . What will be the heat of formation at constant volume? $(R = 8.3 J K^{- 1} m o l^{- 1}$ )