Question

Iron or steel is often covered by a thin larger of a second metal to prevent rusting. Tin cans consist of steel covered with tin, and galvanised iron is made by coating iron with a larger of zinc. If the protective larger is broken, however iron will rust more readily in a tin can than in galvanised iron. Explain this observation by comparing the half-cell potentials of iron, tin and zinc.

Short answer

Since Zinc is more readily oxidized than iron, galvanized iron is not susceptible to rusting. While iron is more readily oxidized than tin and thus a tin can becomes susceptible to rusting.

Step-by-step solution

Step-1:To compare the cell potentials of iron and zinc.

The 2 half-reactions involved with iron and zinc are:

$$\begin{gathered} Z{n}^{2+}\left(aq\right)+2e^{-}\to Zn\left(s\right),E^o=-0.760V \\ F{e}^{2+}\left(aq\right)+2e^{-}\to Fe\left(s\right),E^o=-0.440V \end{gathered}$$

Here, zinc undergoes oxidation in galvanised form. And hence the iron will not be exposed to oxidation.

Step-2: To compare the cell potentials of iron and tin.

The 2 half-reactions with their cell potentials are;

$$\begin{gathered} S{n}^{2+}\left(aq\right)+2e^{-}\to Sn\left(s\right),E^o=-0.140V \\ F{e}^{2+}\left(aq\right)+2e^{-}\to Fe\left(s\right),E^o=-0.440V \end{gathered}$$

Here, iron would get oxidised more readily as compared to tin.

Hence, on comparing the cell potentials of iron in both tin in zinc, we observed that iron would get oxidised more readily with tin.