Consider the reactions :
(a) `6 CO_(2) (g) + 6 H_(2)O (1) to C_(6) H_(12) O_(6) (aq) + 6 O_(2)(g)`
(b) `O_(3)(g) + H_(2)O_(2)(1) + 2 O_(2)(g)`
Why it is more appropriate to write these reactions as :
(a) `6 CO_(2) (g) + 12 H_(2)O(1) to C_(6)H_(12)O_(6) (aq) + 6 H_(2)(1) + 6 O_(2)(g)`
(b) `O_(3)(g) + H_(2)O_(2)(1) to H_(2)(1) + O_(2)(g) + O_(2)(g)`
Also suggest a technique to investigate the path of the above (a) and (b) redox reactions .

Explanation

Although the mechanism ot photosynthesis is very complex but broadly speaking it may be visualised to occure in two steps. In the first step, `H_(2)O` decpmposes to give `H_(2)` and `O_(2)` in presence of chlorophyll and the `H_(2)` thus produced reduces `CO_(2)` to `C_(6)H_(12)O_(6)` in the second step. During the second step, some `H_(2)O` molecules are also produced as shown below:
`12H_(2)O(l) rarr 12H_(2)(g)+6O_(2)(g) ...(i)`
`6CO_(2)(g)+12H_(2)(g) rarr C_(6)H_(12)O_(6)(s)+6H_(2)O(l) ...(ii)`
`ulbar(6CO_(2)(g)+12H_(2)O(l) rarr C_(6)H_(12)O_(6)(s)+6H_(2)O(l)+6O_(2)(g) ...(iii))`
Therefore, it is more appropriate to write the equation for photosynthesis as (iii) because it emphasises that `12H_(2)O` are used per molecle of carbohydrate formed and `6H_(2)O` are produced during the process.
b. The purpose of writing `O_(2)` two tome suggests thai `O_(2)` is being obtained from each of the two reactants.
`O_(3)(g) rarr O_(2)(g)+O_(2)(g)` ...(i)
`H_(2)O_(2)+O(g) rarr H_(2)O(l)+O_(2)(g)` ...(ii)
`ulbar(O_(3)(g)+H_(2)O_(2)(l) rarr H_(2)O(l)+O_(2)(g)+O_(2)(g)` ...(iii))
The path of reactions `(a)` and `(b)` can be determined by using `H_(2)O^(18)` in reaction `(a)` or by using `H_(2)O_(2)^(18)` or `O_(3)^(18)` in reaction `(b)`.