What is an electrochemical theory of rusting of irons and give two methods of preventing of rusting of iron?
Write the Nernst equation and calculate the emf of following cell at 298 K :
$N i | N i^{2 +}$ (0.01)|| $C u^{2 +} | C u (0.01 M)$
Given $E^{\circ}$ ( $C u^{2 +}$ / Cu) = + 0.34V, $E^{\circ}$ ( $N I^{2 +}$ / Ni) = -0.22V

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Solution

When $F e$ is exposed to air, according to electro chemical theory, corrosion takes place due to formation of anodic and cathodic on the $F e$ surface.
At anode : $F e (s) ⟶ {F e}^{2 +} (a q) + 2 e^{-}$
$H_{2} O ⟶ H^{+} (a q) + {O H}^{(-)} (a q)$
At cathode : ${4 H}^{+} + O_{2} + 4 e^{-} ⟶ 2 H_{2} O$
${F e}^{2 +}$ ions are oxidized further by $O_{2}$ to form rust.
${4 F e}^{2 +} (a q) + O_{2} (g) + {4 H}_{2} O ⟶ 2 F e O_{3} + 8 H^{+}$
$*$ 1. By electropolating $F e$ with some non-corrosive metal such as $N i, C r, C u$ etc.
2. By coating reaction will be ;
$N i + {C u}^{2 +} = {N i}^{2 +} + C u (n = 2)$
Nearest eqn :-
$E_{c e l l} = E_{{C u}^{2 +} / C u}^{0} + E_{{N i}^{2 +} / N i}^{0} - \frac{0.059}{2} l o g (\frac{a_{{N i}^{2 +}}}{a_{{C u}^{2 +}}})$
$∴$ $E_{c e l l} = 0.34 V - (- 0.22 V) - \frac{0.059}{2} l o g (\frac{0.01}{0.01})$
$= 0.66 V$

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