Consider the following data: Delta_f H^ominus(textmethane, g) = -X text kJ mol^-1$\Delta_f H^{\ominus}(\text{methane, g}) = -X \text{ kJ mol}^{-1}$ Enthalpy of sublimation of graphite = Y text kJ mol^-1$= Y \text{ kJ mol}^{-1}$ Dissociation enthalpy of textH_2 = Z text kJ mol^-1$\text{H}_2 = Z \text{ kJ mol}^{-1}$ The bond enthalpy of textC-H$\text{C-H}$ bond is given by:

### Core Logic Reaction for formation of methane: textC(texts) + 2textH_2(textg) rightarrow textCH_4(textg)$$\text{C}(\text{s}) + 2\text{H}_2(\text{g}) \rightarrow \text{CH}_4(\text{g})$$ -X = (Delta H_textsub text of carbon) + 2 times (textB.E. of H-H) - 4 times (textB.E. of C-H)$-X = (\Delta H_{\text{sub}} \text{ of carbon}) + 2 \times (\text{B.E. of H-H}) - 4 \times (\text{B.E. of C-H})$ -X = Y + 2Z - 4(textB.E. of C-H)$-X = Y + 2Z - 4(\text{B.E. of C-H})$ ### Step 1: Rearranging for Bond Enthalpy Rearranging the equation for C-H bond enthalpy: textB.E. of C-H = fracX + Y + 2Z4$$\text{B.E. of C-H} = \frac{X + Y + 2Z}{4}$$ ### Pattern Recognition Sees: bond enthalpy derivation from heat of formation, sublimation, and dissociation. Trap: Sign convention errors when substituting formation enthalpies. ### Chapter Mix Class 11 Chemistry: Chemical Thermodynamics