Question

What's the correct answer and why?

Answer 1

Answer:

$\boxed{\text{b) reduction potential less than that of oxygen}}$

Explanation:

Electrochemical corrosion of a metal is its spontaneous reaction with oxygen.

Let's compare the standard reduction potentials for a hypothetical metal M with that of oxygen.

                                                             E°/V

O₂(g) + 2H₂O(ℓ) + 4e⁻ ⇌  4OH⁻(aq)     0.4

M²⁺(aq) + 2e⁻ ⇌ M(s)                           -0.1

To react spontaneously with O₂, the metal must have a $\boxed{\textbf{reduction potential less than that of oxygen}}$

                                                                                          E°/V

2 × [M(s) ⇌ M²⁺(aq) + 2e⁻ ]                                                 0.1

1 × [O₂(g) +2H₂O(ℓ) + 4e⁻ ⇌ 4OH⁻(aq)]                              0.4

       2M(s) + O₂(g) +  2H₂O(ℓ) ⇌ 2M²⁺(aq) + 4OH⁻(aq)     0.5

Only then will we get a positive for the overall cell potential and a spontaneous reaction.