Consider the reactions
a) `6CO_(2)(g)+6H_(2)O(I)toC_(6)H_(12)O_(6)(aq)+6O_(2)(g)`
b) `O_(3)(g)+H_(2)O_(2)(I)toH_(2)O(I)+2O_(2)(g)`
Why it is more appropriate to write these reaction as
a) `6CO_(2)(g)+12H_(2)O(I)toC_(6)H_(12)O_(6)+6H_(2)O(I)+6O_(2)(g)`
b) `O_(3)(g)+H_(2)O_(2)(I)toH_(2)O(I)+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)`.