Question

The standard reduction potentials at 298K for the following half cells are given below. Find the strongest reducing agent. Zn²⁺ _(aq.) + 2e ↔Zn _(s), Ev = - 0.76 V; Cr³⁺ 3 e ↔Cr_(s), Ev = -0.74 V₂H+ + 2e ↔ H2 _(g), Ev = 0.0 V; Fe³⁺ + e ↔ Fe²⁺ (aq.), Ev = 0.77 V

• A. Fe²⁺_(aq)
• B. Zn_(s)
• C. Cr_(s)
• D. H_2(g)

Answer 1

The Correct Option is B

To determine the strongest reducing agent from the given half-cell reactions, we analyze their standard reduction potentials (E°).

1. Understanding Reducing Agent Strength:
The strength of a reducing agent is determined by its tendency to lose electrons (oxidize).
- Lower (more negative) E° = Stronger reducing agent
- This is because substances with more negative E° values more readily undergo oxidation.

2. Given Half-Cell Reactions and Potentials:
The standard reduction potentials at 298 K are:
(i) Zn²⁺ + 2e⁻ ⇌ Zn(s); E° = -0.76 V
(ii) Cr³⁺ + 3e⁻ ⇌ Cr(s); E° = -0.74 V
(iii) 2H⁺ + 2e⁻ ⇌ H₂(g); E° = 0.0 V
(iv) Fe³⁺ + e⁻ ⇌ Fe²⁺; E° = +0.77 V

3. Identifying the Strongest Reducing Agent:
To find the strongest reducing agent, we consider the reverse (oxidation) reactions:
- The species being oxidized (on the left side of the reverse reaction) is the reducing agent
- The most negative E° for reduction corresponds to the easiest oxidation

4. Comparing the Potentials:
The reduction potentials in order:
Zn²⁺/Zn: -0.76 V (most negative)
Cr³⁺/Cr: -0.74 V
H⁺/H₂: 0.0 V
Fe³⁺/Fe²⁺: +0.77 V (most positive)

5. Conclusion:
Since Zn has the most negative reduction potential (-0.76 V), its oxidized form (Zn metal) is:
- The easiest to oxidize (Zn → Zn²⁺ + 2e⁻)
- Therefore, the strongest reducing agent

Final Answer:
The strongest reducing agent is $\boxed{\text{Zn (s)}}$.