(a) Most accepted mechanism of photosynthesis is supposed to occur in two steps. In the first step, H2O decomposes to give H2 and O2 in presence of chlorophyll and the H2 thus produced reduces CO2 to C6H12O6 and H2O molecules are also produced as shown below :
12H2O(l) → 12H2(g)+6O2(g) ... (i)
6CO2(g)+12H2(g) → C6H12O6(s)+6H2O(l) ... (ii)
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6CO2(g)+12H2(g) → C6H12O6(s)+6H2O(l)+6O2(g)
Therefore, it is more appropriate to write the equation for photosynthesis as (iii) because it emphasises that 12 molecules of H2O are used per molecule of carbohydrate formed and 6 molecules of H2O are produced during the process.
(b) The purpose of writing O2 two times suggests that O2 is being obtained from each of the two reactants.
O3(g) → O2(g)+O(g)
H2O2(g)+O(g) → H2O(l)+O2(g)
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O3(g)+H2O2(l) → H2O(l)+O2(g)+O2(g)
The path of reactions (a) and (b) can be determined by tracer technique method using labelled H or O, H2O18 or D2O in reaction (a) or by using H2O218 or O318 in reaction (b).