Question

Identify change in oxidation state of oxidising agent in following redox reaction.
\(3\text{H}_3\text{AsO}_{3(aq)}+\text{BrO}_{3(aq)}^-⟶\text{Br}_{(aq)}^-+3\text{H}_3\text{AsO}_{4(aq)}\)

• A. \(-1 \text{to} +5\)
• B. \(+5 \text{to} -1\)
• C. \(+3 \text{to} +5\)
• D. \(+5 \text{to} +3\)

Hint:

The oxidising agent is reduced; find the oxidation number of bromine before and after.

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
An oxidising agent gains electrons, so its oxidation number goes down. We find which species is reduced.

Step 2: Oxidation Numbers:
In \(\text{H}_3\text{AsO}_3\): \(3 + x - 6 = 0\), so As is \(+3\). In \(\text{H}_3\text{AsO}_4\): \(3 + x - 8 = 0\), so As is \(+5\). Arsenic is oxidised.
In \(\text{BrO}_3^-\): \(x - 6 = -1\), so Br is \(+5\). In \(\text{Br}^-\), Br is \(-1\). Bromine is reduced.

Step 3: Identify the Oxidising Agent:
Bromate, \(\text{BrO}_3^-\), is the oxidising agent because it takes electrons from arsenious acid. Its oxidation state changes from \(+5\) to \(-1\).

Step 4: Check the Other Options:
(C) \(+3\) to \(+5\) is the change for arsenic, the reducing agent. (A) and (D) have the wrong direction or wrong values. So (B) is correct.

Final Answer:
The oxidising agent is bromate and Br goes from +5 to -1, option (B). \[ \boxed{\text{(B) } +5 \text{ to } -1} \]