ThermochemistryHard

Question

Consider the following data:

$\Delta_{f}H^{\Theta}($ methane, g$) = - XkJ{mol}^{- 1}$

Enthalpy of sublimation of graphite $= YkJ{mol}^{- 1}$

Dissociation enthalpy of $H_{2} = ZkJ{mol}^{- 1}$

The bond enthalpy of $C - H$ bond is given by:

Options

A.$\frac{X + Y + 2Z}{4}$
B.$\frac{X + Y + 4Z}{2}$
C.$X + Y + Z$
D.$\frac{- X + Y + Z}{4}$

Solution

$C(s) + 2H_{2}(\text{ }g) \rightarrow {CH}_{4}(\text{ }g)$

$- x = \left( \Delta H_{\text{sub~}} \right.\ $ of carbon $) + 2 \times ($ B.E. of $H - H)$

$- 4 \times ($ B.E. of $C - H)$

$- x = y + 2z - 4($ B.E. of $C - H)$

B.E. of $C - H = \frac{y + 2z + x}{4}$.

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