Practice Problem 11

Which of the following pairs of ions cannot exist simultaneously in aqueous solutions?

(a) Cu+ and Fe3+     (b) Fe3+ and I-     (c) Al3+ and Co2+


(a)  The Cu+ ion is a significantly better reducing agent than the Fe2+ ion and the Fe3+ ion is a better oxidizing agent than Cu2+.   Solutions that contain both Cu+ and Fe3+ ions therefore should undergo a spontaneous oxidation-reduction reaction.  As a result, these ions can't exist simultaneously in the same solution:

Cu+(aq) + Fe3+(aq) Cu2+(aq) + Fe2+(aq)

(b)  The Fe3+ ion is strong enough to oxidize I- ions to I2.  These ions therefore cannot exist in the same solution:

2 Fe3+(aq) + 2 I-(aq) 2 Fe2+(aq) + I2(aq)

(c)  The Al3+ ion is one of the weakest oxidizing agents and the Co2+ ion is one of the weakest reducing agents.  These ions can't undergo an oxidation-reduction reaction and can therefore coexist in aqueous solution:

Al3+(aq) + Co2+(aq) NR

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