Question

In which of the following reactions is ${H_2O}$ oxidized?
I. Photosynthesis
II. ${H_2O + F_2}$
III. ${H_2O + H_2}$
IV. ${H_2O + P_4O_{10}}$

• A. I, II
• B. I, III
• C. II, IV
• D. III, IV

Hint:

Oxidation of Water. H2O is oxidized if oxygen's oxidation state increases (e.g., from -2 in H2O to 0 in O2).

Answer 1

The Correct Option is A

In (I) and (II), oxygen in \( \mathrm{H_2O} \) changes from -2 to 0 (in \( \mathrm{O_2} \)), indicating oxidation.

\[ 6 \mathrm{CO_2} + 6 \mathrm{H_2O} \rightarrow \mathrm{C_6H_{12}O_6} + 6 \mathrm{O_2} \quad (\text{Photosynthesis}) \]

\[ 2 \mathrm{F_2} + 2 \mathrm{H_2O} \rightarrow 4 \mathrm{HF} + \mathrm{O_2} \quad (\mathrm{H_2O} \text{ is oxidized by fluorine}) \]

In III and IV, oxygen remains at -2. No oxidation of \( \mathrm{H_2O} \) occurs.

Answer 2

Step 1: Understanding oxidation of water
Oxidation means loss of electrons or increase in oxidation state. In the context of water (H₂O), oxidation involves water losing electrons and forming oxygen or other oxidized species.

Step 2: Analysis of each reaction
I. Photosynthesis: Water is oxidized to oxygen (O₂) during photosynthesis, releasing electrons and protons. This is a classic example of water oxidation.

II. H₂O + F₂: Fluorine is a very strong oxidizing agent and oxidizes water to oxygen while itself being reduced to fluoride ions. So, water is oxidized here.

III. H₂O + H₂: Water does not get oxidized in this reaction; hydrogen and water do not react to oxidize water.

IV. H₂O + P₄O₁₀: Phosphorus pentoxide reacts with water to form phosphoric acid. Water acts as a reactant but does not get oxidized; phosphorus changes oxidation state.

Step 3: Conclusion
Therefore, water is oxidized only in reactions I (photosynthesis) and II (reaction with fluorine).