Consider the following reactions:
(i) `H^(+)(aq)+OH^(-)(aq)rarrH_(2)O(l)` ,
`DeltaH==-X_(1)Kjmol^(-1)`
(ii) `H_(2)(g)+(1)/(2)O_(2)(g)rarrH_(2)O(l),DeltaH=-X_(2)Kjmol^(-1)`
(iii) `CO_(2)(g)+H_(2)(g)rarrCO(g)+H_(2)O(l)` ,
`DeltaH=-X_(3)KJmol^(-1)`
(iv) `C_(2)H_(2)(g)+(5)/(2)O_(2)(g)rarr2CO_(2)(g)+H_(2)O(l)` ,
`DeltaH=+X_(4)KJmol^(-1)`
Enthanlpy of formation of `H_(2)O(l)` is

Explanation

(A) Allthought the meachanism of photoynthesis is very complex but broadly speaking it may be visualized to occur in two steps in the first step `H_(2)O` decomposes to give `H_(2)` and `O_(2)` in presence of chlorophyll as the `H_(2)` thus produced reduces `CO_(2)` to `C_(6)H_(12)O_(16)`in the second step during the second step some `H_(2)O` molecules are also produced as show below
`12H_(2)O(I)rarr12H_(2)(g)+6O_(2)(g)`
`6CO_(2)(g)+12H_(2)(g)rarrC_(6)H_(12)O_(6)(s)+6H(2)O(l)`
`6CO_(2)(g)+12H_(2)O(l)rarrC_(6)H_(12)O_(6)H_(12)O_(6)(s)+6H_(2)O(l)+6O_(2)(g)`
therefore it is more appropriate to write the equation for photosythesis as (iii) because it imphasis that `12 H_(2)O` are used per molecule of cabohydrate formed and 6