On the basis of the electrochemical series, which of these ions will show the greater tendency to be discharged at the cathode in an electrolytic cell

Analysis

The question asks us to identify which of the given metal ions ($Cu^{2+}$, $Sn^{2+}$, $Fe^{2+}$, $Zn^{2+}$) is most easily discharged at the cathode during electrolysis.

Concept: The Electrochemical Series

In an electrolytic cell, the electrochemical series (or activity series) dictates the ease of discharge of ions.

• At the cathode: Cations (positive ions) compete to gain electrons (reduction). The ion that is lower in the electrochemical series has a greater tendency to be discharged because it is more easily reduced (it has a more positive standard electrode potential).

The general order of discharge for common cations at the cathode is: $K^+ < Na^+ < Ca^{2+} < Mg^{2+} < Al^{3+} < Zn^{2+} < Fe^{2+} < Sn^{2+} < Pb^{2+} < H^+ < \mathbf{Cu^{2+}} < Ag^+ < Au^{3+}$

Step-by-Step Solution

1. Understand the Rule: The further down an element is in the electrochemical series, the easier it is for its ion to gain electrons and be discharged as a metal at the cathode.
2. Locate the ions: • $Zn^{2+}$ (Zinc) is relatively high in the series. • $Fe^{2+}$ (Iron) is below Zinc. • $Sn^{2+}$ (Tin) is below Iron. • $Cu^{2+}$ (Copper) is significantly lower than all the other options provided.
3. Compare: Since $Cu^{2+}$ is the lowest among the options listed, it requires the least amount of energy to be reduced to copper metal ($Cu$). Therefore, it has the greatest tendency to be discharged.

Conclusion

Based on the electrochemical series, $Cu^{2+}$ is the most easily discharged ion among the choices provided.

Exam Tip

Remember the mnemonic for the electrochemical series of metals: "Please Stop Calling Me A Zebra In Large Heavy Cages, Make Sure Copper Saves Gold." (Potassium, Sodium, Calcium, Magnesium, Aluminium, Zinc, Iron, Lead, Hydrogen, Copper, Silver, Gold). The further right (or down) you go, the easier the ion is to discharge at the cathode!