At `25^(@)C`, the following heat of formations are given:
`{:("Compound",SO_(2)(g),H_(2)O(l),,),(Delta_(f)H^(Theta)kJmol^(-1),-296.0,-285.0,,):}`
For the reactions at `25^(@)C`,
`2H_(2)S(g) +Fe(s) rarr FeS_(2)(s) +2H_(2)(g), DeltaH^(Theta) =- 137 kJ mool^(-1)`
`H_(2)S(g) +(3)/(2)O_(2)(g) rarr H_(2)O(l) +So_(2)(g),DeltaH^(Theta) =- 562kJ mol^(-1)`
Calculate the heat of formation of `H_(2)S(g)` and `FeS_(2)(g)` at `25^(@)C`.

Explanation

`4C(s)+5H_(2)+SrarrH-overset(H)overset(|)underset(H)underset(|)(C)-overset(H)overset(|)underset(H)underset(|)(C)-S-overset(H)overset(|)underset(H)underset(|)(C)-overset(H)overset(|)underset(H)underset(|)(C)-H`
`4C(s)+5H_(2)+2SrarrH-overset(H)overset(|)underset(H)underset(|)(C)-overset(H)overset(|)underset(H)underset(|)(C)-S-S-overset(H)overset(|)underset(H)underset(|)(C)-overset(H)overset(|)underset(H)underset(|)(C)-H`
`DeltaH = sum(BE)_(R)-sum(BE)_(P)`
a. `-147.2 =` Heat of atomisation of `4C,10H,1S -BE` of `10(C -H), 2(C-S), 2(C-C)`
b. `-201.9 =`Heat of atomization of `4C, 10H, 2S - BE of 10(C-H), 2(C-S), 2(C-C), (S-S)`
Subtracting equations (i) from (ii), we get
`-201.9 +14.2 =`Heat of atomisation of `1S -BE of (S-S)`
`= 22.8 kJ -BE of (S-S)`
`BE of (S-S) = 277.5 kJ`