Calculate the enthalpy of formation of methane, given that the enthalpiedof combustion of methane, graphite and hydrogen are 890.2 kJ, 393.4 kJ and 285.7 kJ mol^(-1) respectively.

Calculate the enthalpy of formation of methane, given that the enthalp

1.	Calculate the enthalpy of formation of methane, given that the enthalpiedof combustion of methane, graphite and hydrogen are 890.2 kJ, 393.4 kJ and 285.7 kJ mol^(-1) respectively.
Answer» SOLUTION :We are given `:(i) CH_(4) + 2O_(2) rarr CO_(2)+ 2H_(2)O,DeltaH== -890.2 kJ mol^(-1)` (II) `C+ O_(2) rarr CO_(2), Delta H = - 393.4 kJ mol^(-1)``(iii) H_(2)+(1)/(2)O_(2) rarr H_(2)O , Delta H = - 285.7 kJ mol^(-1)` We aim at `: C+2 H_(2)rarr CH_(4), Delta H = ?` In order to get thisthermochemical equation, multiplyeqn. (iii) by 2 and it to eqn. (ii) and then subtract eqn. (i) from their sum. We get `:` `C+2H_(2) rarr CH_(4), Delta H = -393.4+2(-285.7) - (-890.2) kJ mol^(-1) = -74.6 kJ mol^(-1)` Hence, the heat of formation of methane is `:Delta _(F) H= -74.6 kJ mol^(-1)`