Question

Consider the following standard electrode potentials (E° in volts) in aqueous solution :

Element	M3+ / M	M+ / M
Al	−1.66	+0.55
Tl	+1.26	−0.34

Based on these data, which of the following statements is correct ?

• A. \(\mathrm{Tl^+}\) is more stable than \(\mathrm{Al^{3+}}\)
• B. \(\mathrm{Al^+}\) is more stable than \(\mathrm{Al^{3+}}\)
• C. \(\mathrm{Tl^+}\) is more stable than \(\mathrm{Al^+}\)
• D. \(\mathrm{Tl^{3+}}\) is more stable than \(\mathrm{Al^{3+}}\)

Hint:

Compare how easily each ion is reduced to the metal: the more negative the potential for reduction, the more stable the ion in water.

Answer 1

The Correct Option is C

A larger positive reduction potential means the ion is more easily reduced and hence less stable in aqueous solution.
(1) \(\mathrm{Tl^+/Tl}\) = -0.34 V against \(\mathrm{Al^{3+}/Al}\) = -1.66 V: \(\mathrm{Al^{3+}}\) is harder to reduce, hence more stable than \(\mathrm{Tl^+}\). Statement (1) is false.
(2) \(\mathrm{Al^+/Al}\) = +0.55 V against \(\mathrm{Al^{3+}/Al}\) = -1.66 V: \(\mathrm{Al^{3+}}\) is more stable than \(\mathrm{Al^+}\). False.
(3) \(\mathrm{Tl^+/Tl}\) = -0.34 V against \(\mathrm{Al^+/Al}\) = +0.55 V: \(\mathrm{Tl^+}\) is harder to reduce than \(\mathrm{Al^+}\), hence more stable. True.
(4) \(\mathrm{Tl^{3+}/Tl}\) = +1.26 V against -1.66 V for \(\mathrm{Al^{3+}}\): \(\mathrm{Tl^{3+}}\) is much easier to reduce, so less stable. False.
Answer: (3) \(\mathrm{Tl^+}\) is more stable than \(\mathrm{Al^+}\)