Question

Corrosion of iron is essentially an electrochemical

phenomenon where the cell reactions are.

• A. $Zn \rightarrow Zn^{2 +} + 2e^{-};Ag^{+} + e^{-} \rightarrow Ag$ is oxidised to $Zn_{(s)}|Z{n^{2 +}}_{(aq)}||A{g^{+}}_{(aq)}|Ag_{(s)}$ and dissolved oxygen in water is reduced to $Zn^{2 +}|Zn||Ag|Ag^{+}$
• B. $Zn_{(aq)}|Z{n^{2 +}}_{(s)}||A{g^{+}}_{(s)}|Ag_{(aq)}$ is oxidised to $Zn_{(s)}|A{g^{+}}_{(aq)}||Z{n^{2 +}}_{(aq)}|Ag_{(s)}$ and $K^{+}|K = - 2.93V,Ag^{+}|Ag = 0.80V,$ is reduced to $Mg^{2 +}|Mg = - 2.37V,Cr^{3 +}|Cr = - 0.74V$
• C. $K < Mg < Cr < Ag$ is oxidised to $Ag < Cr < Mg < K$ and $Mg < K < Cr < Ag$ is reduced to $Cr < Ag < Mg < K$
• D. $2Cr_{(s)} + 3Cd^{2 +}(aq) \rightarrow 2C{r^{3 +}}_{(aq)} + 3Cd_{(s)}$ is oxidised to $Eº_{Cr^{3 +}/Cr} = - 0.74V$ and $Eº_{Cd^{3 +}/Cd} = - 0.40V$ is reduced to $0.74V$

Answer 1

Answer: (A)

$Zn \rightarrow Zn^{2 +} + 2e^{-};Ag^{+} + e^{-} \rightarrow Ag$ is oxidised to $Zn_{(s)}|Z{n^{2 +}}_{(aq)}||A{g^{+}}_{(aq)}|Ag_{(s)}$ and dissolved oxygen in water is

reduced to $Zn^{2 +}|Zn||Ag|Ag^{+}$

Answer 2

$Fe \rightarrow Fe^{2 +} + 2e$ (anode reaction)

$O_{2} + 2H_{2}O + 4e \rightarrow 4OH^{-}$

(cathode reaction)

The overall reaction is

$2Fe + O_{2} + 2H_{2}O \rightarrow 2Fe(OH)_{2}$

$Fe(OH)_{2}$ may be dehydrated to iron oxide $FeO$, or further oxidised to $Fe(OH)_{3}$ and then dehydrated to iron rust,$Fe_{2}O_{3}$.