H_(2)(g) +(1)/(2)O_(2)(g) rarr H_(2)O(l) BE (H-H) = x_(1), BE (O=O)=x_(2) BE(O=H)=x_(3) Latent heat of vaporisation of water liquid into water vapour =x_(4), then Delta_(f)H (heat of formation of liquid water) is

H_(2)(g) +(1)/(2)O_(2)(g) rarr H_(2)O(l) BE (H-H) = x_(1), BE (O=O)=x

1.	H_(2)(g) +(1)/(2)O_(2)(g) rarr H_(2)O(l) BE (H-H) = x_(1), BE (O=O)=x_(2) BE(O=H)=x_(3) Latent heat of vaporisation of water liquid into water vapour =x_(4), then Delta_(f)H (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 :`DeltaH = (BE)_("REACTANT") -(BE)_("products")` [But all the species must be in gaseous state. In product, `[H_(2)O(L) rarr H_(2)O(g)] DeltaH` must be added. Hence, `H_(2)(g) +(1)/(2)O_(2)(g) rarr H_(2)O(l)` `DeltaH = [(BE)_(H-H)+(1)/(2)(BE)_(O=O)]` `= [(DeltaH)_(vap) +2(BE)_(O-H)]` `= x_(1) +(x_(2))/(2) -[x_(4)+2x_(3)]` `= x_(1) +(x_(2))/(2) -x_(4) - 2x_(3)`