Standard enthalpy of combustion of $C H_{4}$ is $- 890 k J m o l^{- 1}$ and standard enthalpy of vaporisation of water is 40.5 kJ $m o l^{- 1}$ . Calculate the enthalpy change for the reaction :
$C H_{4 (g)} + 2 O_{2 (g)} \to C O_{2 (g)} + 2 H_{2} O_{(g)}$

$- 890 k J m o l^{- 1}$

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$- 809 k J m o l^{- 1}$

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$809 k J m o l^{- 1}$

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$- 971 k J m o l^{- 1}$

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Solution

The correct option is B
$- 809 k J m o l^{- 1}$

$C H_{4 (g)} + 2 O_{2 (g)} \to 2 C O_{2 (g)} + 2 H_{2} O_{ℓ} . Δ H = - 890 k J$ ...(i)
$2 H_{2} O_{(ℓ)} \to 2 H_{2} O_{(g),} Δ H = 2 \times 40.5 = 81 k J$ ...(ii)
$(i) + (i i), C H_{4 (g)} + 2 O_{2 (g)} \to 2 C O_{2 (g)} + 2 H_{2} O_{(g)^{'}} Δ H = - 890 + 81 = - 809 k J$

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Standard enthalpy of combustion of $C H_{4}$ is $- 890 k J m o l^{- 1}$ and standard enthalpy of vaporisation of water is 40.5 kJ $m o l^{- 1}$ . Calculate the enthalpy change for the reaction :
$C H_{4 (g)} + 2 O_{2 (g)} \to C O_{2 (g)} + 2 H_{2} O_{(g)}$