Question

When 6.0 g of graphite reacts with dihydrogen to give methane gas, 37.4 kJ of heat is liberated. What is standard enthalpy of formation of CH₄ (g)?

Options

• - 74.8 kJ mol^-1

• - 37.4 kJ mol^-1

• - 112.2 kJ mol^-1

• 112.2 kJ mol^-1

Answer 1

- 74.8 kJ mol^-1

Explanation:

Given,

When 6.0 g of graphite reacts with dihydrogen to gives methane gas.

\[\ce{\underset{\text{6g}}{C} + H2 -> CH4, \Delta H = - 37.4}\]kJ   ...(i)

Standard enthalphy of formation CH₄(g)

\[\ce{\underset{\text{12g}}{C} + 2H2 -> \underset{\text{16 g}}{CH4}, - \Delta H_{std}}\] ...(ii)

On comparing both reactions (i) and (ii), we find that reaction (i) is half of reaction (ii), So on multiplying by half.

\[\ce{\underset{\text{(12/2 = 6g)}}{1/2 C} + 1/2 2H2 -> 1/2  (CH4)}\];

`= 1/2 xx Delta "H"_"std"`    ...(iii)

Comparing the (i) and (iii) equations,

`- 37.4 = 1/2 xx Delta "H"_"std"`

`=> "H"_"std" = - 74.8`kJ mol^-1