Q.Standard electrode potential values, E°V for Al3+/Al is –1.66 V and that of Tl3+/Tl is +1.26 V. Predict about the formation of M3+ ion in solution and compare the electropositive character of the two metals.

Step 1 - Read the sign and magnitude of $E^\circ$

A large negative $E^\circ$ for $M^{3+}/M$ means the reverse process, $M \rightarrow M^{3+} + 3e^-$, is thermodynamically favourable - the metal oxidises easily and $M^{3+}$ is the stable species in solution. A positive $E^\circ$ means the forward reduction $M^{3+} + 3e^- \rightarrow M$ is favoured - $M^{3+}$ is a strong oxidising agent and is not the stable form.

Step 2 - Apply to Al

The strongly negative value shows Al loses its three valence electrons ($3s^23p^1$) easily; Al$^{3+}$(aq) is thermodynamically stable and Al is a strong reducing agent.

Step 3 - Apply to Tl

The positive value shows Tl$^{3+}$ readily gains electrons, i.e. it is a strong oxidising agent that is reduced back to the far more stable $\text{Tl}^+$ (the $6s^2$ pair resists bonding - the inert pair effect - so it stays as a lone pair rather than being used to form the third bond).

Step 4 - Compare electropositive character

Electropositive character is measured by how readily a metal loses electrons to form its cation. Al does this far more readily (very negative $E^\circ$) than Tl does for the +3 state (positive $E^\circ$), so Al is more electropositive than Tl as far as the $M^{3+}$ state is concerned; for Tl the electropositive character instead shows up in the $+1$ state.

Read the sign of E deg(M3+/M): negative means M oxidises easily and M3+ is stable; positive means M3+ is oxidising/unstable. Link Tl's positive value to the inert pair effect (6s2 electrons resisting bonding), which favours Tl+ over Tl3+, and contrast with Al which shows no such effect.

• Treating a positive E deg as meaning the ion is "more stable" in the everyday sense, rather than recognising it means the ion is a strong oxidiser that gets reduced.
• Forgetting to connect Tl's behaviour to the inert pair effect (6s2 electrons).
• Confusing "electropositive" (tendency to lose electrons) with "electronegative".