Calculate the standard enthalpy change for the reaction `CH_4(g) +2O_2(g) rarr CO_2(g)+2H_2O(l)` Given that the standard enthalpies of formation of `CH_4(g),CO_2(g),` and `H_2O(l)` are -74.8 kJ mol^-1, -393.5 kJ mol^-1` and -285.8 kJ `mol^-1` respectively.

Explanation

The combustion of methance is represented as:
`CH _(4) (g) + 2 O _(2) (g) to CO _(2) (g) + 2 H _(2) O (l) , Delta H =?`
`Delta H ^(@) = Delta H _(f) ^(@) (CO _(2)) + 2 Delta H _(f) ^(@) (H _(2) O) - Delta H _(f) ^(@) (CH _(4)) -0`
`= (-393.5 kJ mol ^(-1)) + 2 ( - 285. 9 kJ mol ^(-1)) - (- 74. 8 kJ mol ^(-1))=-890.5 kJ mol ^(-1)`
Thus , enthalpy of combustion of methane at `25 ^(@)C` and 1 atm pressure `=-890.5 kJ mol ^(-1)`