Question

Consider the reactions: 
(a) \(6 \,CO_2(g) + 6H_2O(l) \rightarrow C_6H_{12}O_6(aq) + 6O_2(g)\) 
(b) \(O_3(g) + H_2O_2(l) \rightarrow  H_2O(l) + 2O_2(g)\) 
Why it is more appropriate to write these reactions as: (a) \(6\,CO_2(g) + 12H_2O(l) \rightarrow  C_6H_{12}O_6(aq) + 6H_2O(l) + 6O_2(g)\) 
(b) \(O_3(g) + H_2O_2(l) \rightarrow H_2O(l) + O_2(g) + O_2(g)\) 
Also suggest a technique to investigate the path of the above (a) and (b) redox reactions.

Answer 1

(a) Photosynthesis Reaction

Usually written as:

\[ 6CO_{2}(g) + 6H_{2}O(l) \rightarrow C_{6}H_{12}O_{6}(aq) + 6O_{2}(g) \]

More appropriate redox form:

\[ 6CO_{2}(g) + 12H_{2}O(l) \rightarrow C_{6}H_{12}O_{6}(aq) + 6H_{2}O(l) + 6O_{2}(g) \]

• This explicitly shows that 12 water molecules are involved, of which 6 end up as product water and 6 supply oxygen atoms for the 6O\(_2\).
• It clearly separates the water that is consumed (as source of O\(_2\) and H) from the water that appears again, making the oxidation of water and reduction of CO\(_2\) as a redox process more evident.

(b) Reaction of O\(_3\) and H\(_2\)O\(_2\)

Given as:

\[ O_{3}(g) + H_{2}O_{2}(l) \rightarrow H_{2}O(l) + 2O_{2}(g) \]

More appropriate way to write:

\[ O_{3}(g) + H_{2}O_{2}(l) \rightarrow H_{2}O(l) + O_{2}(g) + O_{2}(g) \]

• Writing 2O\(_2\) as \(O_{2} + O_{2}\) emphasizes that both oxidant species (ozone and hydrogen peroxide) contribute to the formation of separate O\(_2\) molecules.
• This highlights the redox nature: one O\(_2\) comes effectively from O\(_3\) and the other from H\(_2\)O\(_2\), clarifying electron loss and gain from each reactant.

Technique to Investigate the Reaction Path

A suitable technique is isotopic labelling using oxygen isotopes (e.g. \(^ {18}O\)).

• For reaction (a): use \(^ {18}O\)-labelled water (\(H_{2}^{18}O\)) or \(^ {18}O\)-labelled CO\(_2\), then track in which product (O\(_2\) or C\(_6H_{12}O_{6}\)) the \(^ {18}O\) appears; this shows whether the O\(_2\) originates from water or CO\(_2\).
• For reaction (b): label O\(_3\) or H\(_2\)O\(_2\) with \(^ {18}O\) and analyse the formed O\(_2\) by mass spectrometry to see which reactant supplies each O\(_2\) molecule.