When `Fe` powder reacts with chlorine . `FeCl_(3(s))` is obtained in two steps `:`
`Fe((s))+Cl_(2(g)) rarr FeCl_(2(s))`
`FeCl_(2(s))+(1)/(2)Cl_(2(s)) rarr FeCl_(3(s))`
If standard enthalpy of formation `(DeltaH_(f)^(@))` of `FeCl_(2)` is `-341.8kJ mol^(-1)` and heat of reaction for conversion of `FeCl_(2)` into `FeCl_(3)` is `-57.7 kJ mol^(-1)`, then calculate the enthalpy of formation of `FeCl_(3(s))`.

Explanation

We have to calculate `DeltaH` value for following reaction
`Fe_((s))+(3)/(2)Cl_(2(g))rarr FeCl_(3(s)),DeltaH=?`
Given that `:`
`Fe_((s))+Cl_(2(g))rarr FeCl_(3(s)), .....(i)`
`DeltaH=-341.8kJ mol^(-1)`
`ul(FeCl_(2(s))+(1)/(2)Cl_(2(g))rarr FeCl_(3(s)), " ",,,,(ii)`
`DeltaH=-57.7kJ mol^(-1)`)
Adding `(i)` and `(ii)`
`Fe_((s))+(3)/(2)Cl_(2(g))rarr FeCl_(3(g)),`
`DeltaH=-341.8+(-57.7)`
`=-399.5kJ mol^(-1)`