reactions of ions in aqueous solution

Question 1

Are 2+ ions or 3+ ions more acidic in solution? why?

Answer 1

3+ ions are much more acidic

3+ ions are smaller and more highly charged, so have a higher charge density. They attract the electrons from the oxygen of the ligands much more strongly. This weakens the O-H bonds, so the complex readily releases a H+ ion into the solution, making it acidic.

Question 2

Write an equation for the hydrolysis of a 3+ ion to release a proton

Answer 2

[Fe(H2O)6]3+ (aq) ⇌ [Fe(H2O)5(OH)]2+ (aq) + H+ (aq)

Question 3

What kind of acid is the complex ion acting as? Why?
[Fe(H2O)6]3+ (aq) ⇌ [Fe(H2O)5(OH)]2+ (aq) + H+ (aq)

Answer 3

Bronsted-Lowry - donates a proton

Question 4

Define a Lewis acid

Answer 4

Electron pair acceptor

Question 5

Define a Lewis base

Answer 5

Electron pair donor

Question 6

What is acting as the Lewis acid and what is acting as the Lewis base during the
formation of complex ions?

Answer 6

The metal ion is acting at the Lewis acid and the ligands as Lewis bases

Question 7

Fe 2+ ion in aqueous solution
complex and colour

Answer 7

[Fe(H2O)6]2+ (aq)

Green

Question 8

Fe 2+ with NaOH added dropwise and in excess

Answer 8

[Fe(H2O)4(OH)2] (s)

Green ppt, goes brown on standing in air

Question 9

Fe 2+ with NH3 added dropwise in excess

Answer 9

[Fe(H2O)4(OH)2] (s)

Green ppt, goes brown standing in air

Question 10

Fe 2+ with Na2CO3

Answer 10

FeCO3

Green ppt

Question 11

Why does the green ppt go brown on standing in air in the reaction of Fe2+ with
NH3?

Answer 11

Oxygen in the air oxidises Fe2+ to Fe3+, and [Fe(OH)3(H2O)3] is brown, so colour changes