1.

For the reaction, H_(2(g)) + 1/2O_(2(g)) to H_2O_((l)) B.E._((H – H)) = x_1, B.E._((O=O)) = x_2 and B.E._((O-H))= x_3. If the latent heat of vaporisation of water liquid into water vapour = x_4, then Delta_fH (heat of formation of liquid water) is

Answer»

`x_1+x_2/2 - x_3+x_4`
`2x_3-x_1-x_2/2-x_4`
`x_1+x_2/2- 2x_3-x_4`
`x_1+x_2/2-2x_3+x_4`

Solution :`Delta_fH=(B.E.)_"reactants"-(B.E.)_"PRODUCTS"`
But all the species must be in gaseous state , so in product
`[H_2O_((l)) to H_2O_((g))] DeltaH_(vap)` must be ADDED .
HENCE for the reaction , `H_(2(g)) + 1/2O_(2(g)) to H_2O_((l))`
`Delta_fH = [(B.E.)_(H-H)+1/2(B.E.)_(O=O)]-[DeltaH_(vap)+2(B.E.)_(O-H)]`
`=x_1+x_2/2-[x_4+2x_3] rArr x_1+x_2/2-x_4-2x_3`


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