Assume that this is agar medium and 'Fa' in picture should be 'Fe'

I want to know which metals oxidize and which is reduced and changes in medium and observations?

According to my knowledge more corrosion will take place in Zn and Fe also corrode. Then copper should reduce. So, for both reactions, will copper acts as the cathode? I need general but complete explanation. Thank you


1 Answer 1


You have correctly identified zinc and iron as the anodes, since they are both less noble than copper. In the corrosion reaction, they are both oxidized and dissolved in the aqueous gel.

\begin{aligned} \ce{Zn ->& Zn^2+ + 2e-} \\ \ce{Fe ->& Fe^2+ + 2e-} \\ \end{aligned}

The electrons flow to the copper cathode, where oxygen, which is dissolved in the liquid medium, is reduced to hydroxide.

$$\ce{O2 +2H2O + 4e- -> 4OH-}$$

As a consequence, the pH of the solution around the copper cathode rises, and the phenolphthalein indicator changes its color from colorless to violet-red in this area.

enter image description here

$\ce{Fe^2+}$ reacts with $\ce{K3[Fe(CN6)]}$ to form a deep blue iron(II,III) hexacyanoferrate(II,III).

$$\ce{K+ + Fe^2+ + [Fe(CN6)]^3- -> KFe[Fe(CN6)]}$$

Therefore, the solution near the iron surface turns blue. Zinc does not react with $\ce{K3[Fe(CN6)]}$ and the solution around the zinc anode remains colorless (source).


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