2.6 Reactions of Ions in Aqueous Solution

Question 1

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

Answer 1

3+ 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 this complex ion acting as, why

Answer 3

[Fe(H2O)6]3+(aq) <–> [Fe(H2O)5(OH)]2+(aq) + H+(aq)
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 ion

Answer 6

The metal ion is acting as the lewis acid and the ligands as lewis bases

Question 7

What is the complex ion formed and colour for these tests for Fe2+
In aqueous solution
Add NaOH
Add NH3
Add Na2CO3

Answer 7

In aqueous solution - [Fe(H2O)6]2+(aq) - Green
Add NaOH - Fe(H2O)4(OH)2 - Green ppt, goes brown in air
Add NH3 - Fe(H2O)4(OH)2 - Green ppt, brown in air
Add Na2CO3 - FeCO3 - green ppt

Question 8

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

Answer 8

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

Question 9

What is the complex ion formed and colour for these tests for Cu2+
In aqueous solution
Add NaOH
Add NH3 (dropwise)
Add NH3 (excess)
Add Na2CO3
Add conc HCl

Answer 9

In aqueous solution - [Cu(H2O)6]2+ - Blue
Add NaOH - Cu(OH)2(H2O4) - Blue ppt
Add NH3(dropwise) - Cu(H2O)4(OH)2 - blue ppt
Add NH3(excess) - [Cu(H2O)2(NH3)4]2+(aq) - Deep blue solution
Add Na2CO3 - CuCO3 - blue green ppt
Add conc HCl - [CuCl4]2-(aq) - Pale green/yellow solution

Question 10

What is the complex ion formed and colour for these tests for Fe3+
In aqueous solution
Add NaOH
Add NH3
Add Na2CO3

Answer 10

In aqueous solution - [Fe(H2O)6]3+ - purple solution, may look yellow-brown due to [Fe(H2O)5(OH)]2+(aq)
Add NaOH - Fe(H2O)3(OH)3 - Brown ppt may look orange-brown
Add NH3 - Fe(H2O)3(OH)3 - Brown ppt may look orange-brown
Add Na2CO3 - Fe(H2O)3(OH)3 - Brown ppt, may look orange-brown, effervesence

Question 11

What is the complex ion formed and colour for these tests for Al3+
In aqueous solution
Add NaOH (dropwise)
Add NaOH (excess)
Add NH3
Add Na2CO3

Answer 11

In aqueous solution - [Al(H2O)6]3+ - colourless solution
Add NaOH(dropwise) - [Al(H2O)3(OH)3] (s) - White ppt
Add NaOH(excess) - [Al(OH)4]-(aq) - Colourless solution
Add NH3 - Al(H2O)3(OH)3 - White ppt
Add Na2CO3 - Al(H2O)3(OH)3 - White ppt, effervescence

Question 12

What is the complex ion formed and colour for these tests for Co2+
In aqueous solution
Add NaOH
Add NH3
Add Na2CO3
Add HCl

Answer 12

In aqueous solution - [Co(H2O)6]2+ - Pink
Add NaOH - Co(H2O)4(OH)2 - Blue ppt
Add NH3 - [Co(NH3)6]2+(aq) - Pale brown solution
Add Na2CO3 - CoCO3(s) - Purple ppt
Add HCl - [CoCl4]2-(aq) - Blue solution