Which species in each pair is a better oxidizing agent under standard-state conditions? (a) br2 au3+ (b) h2 ag+ (c) cd2+ cr3+ (d) o2 in acidic media o2 in basic media?

(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.

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.

it is b that is the answer.