The molar enthalpies of combustion of C_(2)H_(2)(g), C("graphite") and H_(2)(g) are - 310.62 kcal, -94.05 kcal and -68.32 kcal respectively. Calculate the standard enthalpy of formation of C_(2)H_(2)(g).

The molar enthalpies of combustion of C_(2)H_(2)(g), C("graphite") and

1.	The molar enthalpies of combustion of C_(2)H_(2)(g), C("graphite") and H_(2)(g) are - 310.62 kcal, -94.05 kcal and -68.32 kcal respectively. Calculate the standard enthalpy of formation of C_(2)H_(2)(g).
Answer» Solution :The given equations are : (i)`C_(2)H_(5)(g) + 5/2O_(2)(g) to 2CO_(2)(g) + H_(2)O(g)` `DELTAH = -310.62 kcal` (ii) `C("graphite") + O_(2) to CO_(2)(g)DeltaH = -94.05 kcal` (iii) `H_(2)(g) + 1/2O_(2)(g) to H_(2)O(g) DeltaH = -68.32 kcal` The required EQUATION is `2C("graphite") + H_(2)(g) to C_(2)H_(2)(g)DeltaH = ?` MULTIPLY eq.(ii) by 2 and add eqn (iii) and (iv) `2C("graphite") + 2O_(2)(g) to 2CO_(2)(g) DeltaH = -188.10 kcal` `H_(2)(g) + 1/2O_(2)(g) to H_(2)O(g)DeltaH = -68.32 kcal` (iv)`2C("graphite") + H_(2)(g) + 5/2O_(2)(g) to 2CO_(2)(g) + H_(2)O(g)` `DeltaH= -256.42kcal` Subtract eq.(i) `(i) C_(2)H_(2)(g) + 5/2 O_(2)(g) to 2CO_(2)(g) + H_(2)O(g)` `DeltaH = -310.62 kcal` Subtracting `2C("graphite") + H_(2)(g) to C_(2)H_(2)(g)` `DeltaH = 54.20 kcal`.