Answer:
(a) Au³⁺; (b) Ag⁺; (c) Cd²⁺; (d) O₂ in acidic media
Explanation:
You've had Trends in the Periodic Table. Now, here are Trends in Standard Reduction Potentials.
- The strength of oxidizing agents increases from bottom to top on the left-hand side.
- The strength of reducing agents increases from top to bottom on the right-hand side.
Thus, for each pair of half reactions, we need to look only at which one has the more positive standard reduction potential.
(a) Br₂/Au³⁺
Au³⁺(aq) + 3e⁻ ⟶ Au(s) 1.498 V
Br₂(ℓ) + 2e⁻ ⟶ 2Br⁻(aq) 1.066 V
Au³⁺ is the stronger oxidizing agent.
(b) H₂/Ag⁺
Ag⁺(aq) + e⁻ ⟶ Au(s) 0.7996 V
H₂(g) + 2e⁻ ⟶ 2H⁻(aq) -2.33 V
Ag⁺ is the stronger oxidizing agent.
(c) Cd²⁺/Cr³⁺
Cd²⁺(aq) + e⁻ ⟶ Cd(s) -0.4030 V
Cr³⁺(aq) + 3e⁻ ⟶ Cr(s) -0.744 V
Cd²⁺ is the stronger oxidizing agent.
(d) O₂, H⁺/O₂, OH⁻
O₂(g) + 4H⁺(aq) + 4e⁻ ⟶ 2H₂O(ℓ) 1.224 V
O₂(g) + H₂O(ℓ) + 4e⁻ ⟶ 4OH⁻(aq) 0.401 V
O₂ in acid is the stronger oxidizing agent.
