The enthalpy of formation of methane at constant pressure and 300 K is-78.84 kJ. What will be the enthalpy of formation at constant volume?

Explanation

The equation representing the enthalpy of formation of methane is :
`C_((s)) + 2H_(2(g)) to CH_(4(g)) , DeltaH`=-78.84 kJ
`DeltaH`=78.84 kJ , `Delta^(ng)`=1-2=-1 mol
`R=8.314xx10^(-3) kJ K^(-1) "mol"^(-1)`, T = 300 K
According to the relation,
`DeltaH=DeltaU+Delta^(ng)RT`
`DeltaU=DeltaH-Delta^(ng)RT`
=(-78.84 kJ) -( 1 mol) x (8.314 x `10^(-3)` kJ `K^(-1) "mol"^(-1)` ) x 300 K
=-78.84-2.49 =-81.35 kJ