Combustion enthalpy of carbon is .-x. kJ, Enthalpy of formation of water is .-y. kJ and combustion enthalpy of methane is .-z. kJ calculate the enthalpy of formation of methane.

Options

A. `(-x-y+z)` kJ
B. `(-z+2y-x)` kJ
C. `(-x-2y+z)` kJ
D. `(-x-2y-z) kJ

Answer

C

Explanation

Enthalpy of combustion reaction of carbon :
(i) `C_((s)) + O_(2(g)) = CO_(2(g)) , Delta H_((i)) = -x` kJ
Enthalpy of formation of water :
(ii) `H_(2(g)) + (1)/(2) O_(2(g)) = H_(2) O_((l)) , Delta H_(ii) =-y "kJ"`
Combustion enthalpy of methane :
(iii) `CH_(4(g)) + 2O_(2(g)) = CO_(2(g)) + 2H_(2) O_((l)) DeltaH_((iii)) =z "kJ"`
Enthalpy of formation of methane :
(iv) `C_((s)) + 2H_(2(g)) = CH_(4(g)), Delta H_((iv)) = ?`
According to Hess. rule : (equation) (i)) + (2 · equation (ii)) - (equation (iii)) = equation (iv)
eq. (i) : `C_((s)) + O_(2(g)) = CO_(2(g)), DeltaH_((i)) =-x "kJ"`
eq. (ii) `:2H_(2(g)) + O_(2(g)) = 2H_(2) O_((l)) , DeltaH_((ii)) = -2y "kJ"`
eq. (iii) : `[-CH_(4(g)) + 2O_(2(g)) = CO_(2(g)) + 2H_(2) O_((l)),DeltaH_((iii)) = -z "kJ"`]
eq. (iv) : `C_((s)) + 2H_(2(g)) to CH_(4(g)) , DeltaH_((iv)) = DeltaH_((iv)) = (-x-2y+z)` kJ