Question

Which of the following reactions is non-spontaneous?

• A. \(2F_2 + 2H_2O \rightarrow 4HF + O_2\)
• B. \(Cl_2 + H_2O \rightarrow HCl + HOCl\)
• C. \(Br_2 + H_2O \rightarrow HBr + HOBr\)
• D. \(2I_2 + 2H_2O \rightarrow 4HI + O_2\)

Hint:

Oxidizing power of halogens decreases from \(F_2 > Cl_2 > Br_2 > I_2\).

Answer 1

The Correct Option is D

Step 1: Understand spontaneity for these reactions.
A reaction is spontaneous if it has a positive cell potential, meaning the species can actually carry out the oxidation it is supposed to. Here the halogen would have to oxidise water to release $O_2$, or just disproportionate.
Step 2: Look at the fluorine reaction.
Fluorine is the strongest oxidising agent of all, so it easily oxidises water to oxygen: $2F_2 + 2H_2O \rightarrow 4HF + O_2$. This is spontaneous.
Step 3: Look at the chlorine and bromine reactions.
Chlorine and bromine do not oxidise water to $O_2$; instead they disproportionate in water to give a halide and a hypohalous acid, like $Cl_2 + H_2O \rightarrow HCl + HOCl$. These happen, so they are feasible.
Step 4: Note the trend down the group.
Oxidising power falls steadily as we go down from F to Cl to Br to I. Iodine is the weakest oxidiser of the four.
Step 5: Test the iodine reaction.
For $2I_2 + 2H_2O \rightarrow 4HI + O_2$, iodine is too weak to pull oxygen out of water. In fact the reverse happens: oxygen can oxidise $I^-$ back to $I_2$. So this forward reaction is non spontaneous.
Step 6: Conclude.
The non spontaneous reaction is the iodine one.
\[ \boxed{2I_2 + 2H_2O \rightarrow 4HI + O_2} \]