Question

Using the following information and the fact that the average C―H bond enthalpy is 414 kJ/mol, estimate the standard enthalpy of formation of methane (CH4). C(s) → C(g) ΔH o rxn = 716 kJ/mol 2H2(g) → 4H(g) ΔH o rxn = 872.8 kJ/mol

Answer 1

Answer:

The standard enthalpy of formation of methane is -67.2 kJ/mol.

Explanation:

$C(s) \rightarrow C(g) \Delta H^o_{1,rxn} = 716 kJ/mol$...[1]

$2H_2(g)\rightarrow 4H(g) ,\Delta H^o_{2,rxn} = 872.8 kJ/mol$...[2]

$C(g)+4H\rightarrow CH_4, \Delta H^o_{3,rxn} =?$...[3]

Since, the bond formation is taking place energy will release that id reaction will be an exothermic reaction

$\Delta H^o_{3,rxn} =4\times (-414 kJ/mol)=-1656 kJ/mol$

$C(s)+2H_2(g)\rightarrow CH_4, \Delta H^o_{4,rxn} =?$...[4]

Using Hess's law:

[4] = [1] + [2] + [3]

$\Delta H^o_{4,rxn} =\Delta H^o_{1,rxn} +\Delta H^o_{2,rxn} +\Delta H^o_{3,rxn}$

$=716 kJ/mol+872.8 kJ/mol +(-1656 kJ/mol)=-67.2 kJ/mol$

The standard enthalpy of formation of methane is -67.2 kJ/mol.