Question

Consider the reactions.

\[\ce{C_{(s)} + 2H2_{(g)} -> CH4_{(g)}}\], ΔH = −x kcal

\[\ce{C_{(g)} + 4H_{(g)} -> CH4_{(g)}}\], ΔH = −x₁ kcal

\[\ce{CH4_{(g)} -> CH3_{(g)} + H_{(g)}}\], ΔH = +y kcal

The average C-H bond enthalpy is:

Options

• y kcal

• x₁ kcal

• `"x"/4` kcal

• `"x"_1/4` kcal

Answer 1

`"x"_1/4` kcal

Explanation:

The average C-H bond enthalpy can be calculated from the second reaction:

\[\ce{C_{(g)} + 4H_{(g)} -> CH4_{(g)}}\], ΔH = −x₁ kcal

∴ \[\ce{CH4_{(g)} -> C_{(g)} + 4H_{(g)}}\], ΔH = x₁ kcal

This reaction gives the energy required to break four C-H bonds.

∴ Average C-H bond enthalpy = `"x"_1/4` kcal