Topic 15.2 Transition Metal Reactions

Question 1

Reactions of Cr

Answer 1

• [Cr(H2O)6]^3+ –> hexaaquachromium(III).
• Ions in this complex are fairly acidic (pH: 2-3).
• When an ion reacts with water molecule, a hydrogen ion is lost from one water ligand attached to the central metal.

Question 2

Reactions of Cr: Reactions with hydroxide

Answer 2

HYDROXIDE IONS remove H+ from [Cr(H2O)6]^3+ water ligands.
-Once the OH- ion has been removed, you are left with a complex with no charge.
-This is insoluble in water and forms a precipitate.

Question 3

Reactions of Cr: Reactions with ammonia

Answer 3

-Ammonia acts as a base and a ligand.

Question 4

The oxidation of chromium(III) to chromium(IV)

Answer 4

• An excess of sodium hydroxide solution is added to a solution of the hexaaquachromium(III) ions to produce a solution of green hexahydroxochromate(III) ions.
• This is then oxidised by warming it with hydrogen peroxide solution. You eventually get a bright yellow solution containing chromate(IV) ions.

Question 5

The chromate(VI)-dichromate(VI) equilibrium

Answer 5

-If you add dilute hydrochloric acid to the yellow solution it turns orange.
-If you add sodium hydroxide solution to the orange solution it turns yellow.

Question 6

Reduction of dichromate (VI) ions with zinc and acid

Answer 6

-These can be reduced to chromium (III) then chromium (II) using zinc and acid.
-Hydrogen is also produced as a side reaction and must be allowed to escape whilst disallowing any air to enter the reaction.
-Oxygen will rapidly re-oxidise the chromium (II) and (III).
–> An easy way of doing this is to put a bit of cotton wool in the top of the flask; this allows hydrogen to escape, but stops air getting in against the flow of hydrogen.

Question 7

Explanation of redox reactions using Ecell values

Answer 7

High negative E values at the top to high positive values at the bottom.
-The best reducing agent at the top and on the right.
-The best oxidising agent is at the bottom and on the left.

Question 8

Potassium dichromate

Answer 8

• Used as an oxidising agent.
• Strong agent (but not so strong that it takes the molecule apart).
  It’s used to oxidise:
  -Primary alcohols to aldehydes
  -Primary alcohols to carboxylic acids
  -Secondary alcohols to ketones

Question 9

Chromium ion colours summary

Answer 9

Question 10

Vanadium colours: Oxidation state +5

Answer 10

-Yellow
-VO2^+
-Dioxovanadium(V)
–> Can be reduced by zinc to V^2+.
…

Question 11

Vanadium colours: Oxidation state +4

Answer 11

-Blue
-VO^2+
-Oxovanadium(IV)
…

Question 12

Vanadium colours: Oxidation state +3

Answer 12

-Green
-V^3+
-Vanadium(III)
…

Question 13

Vanadium colours: Oxidation state +2

Answer 13

-Violet
-V^2+
-Vanadium(II)
…

Question 14

The colour of vanadium between oxidation states +5 and +4

Answer 14

A green solution will form between the yellow +5 oxidation state and the blue +4 oxidation state which is just the mixture of colours between the two.

Question 15

Re-oxidation of the vanadium(II)

Question 16

Reactions with ammonia: Cr3+

Answer 16

Metal-aqua ion: Violet solution; [Cr(H2O)6]3+
With NH3: Green precipitate; Cr(OH)3(H2O)3
With excess NH3: Purple solution; [Cr(NH3)6]3+

Question 17

Reactions with ammonia: Fe2+

Answer 17

Metal-aqua ion: Green solution; [Fe(H2O)6]2+
With NH3: Green precipitate; Fe(OH)2(H2O)4
With excess NH3: No change

Question 18

Reactions with ammonia: Fe3+

Answer 18

Metal-aqua ion: Yellow solution; [Fe(H2O)6]3+
With NH3: Brown precipitate; Fe(H2O)3(OH)3
With excess NH3: No change

Question 19

Reactions with ammonia: Co2+

Answer 19

Metal-aqua ion: Pink solution; [Co(H2O)6]2+
With NH3: Blue precipitate; Co(OH)2(H2O)4
With excess NH3: Yellow solution; [Co(NH3)6]2+

Question 20

Reactions with ammonia: Cu2+

Answer 20

Metal-aqua ion: Blue solution; [Cu(H2O)6]2+
With NH3: Blue precipitate; Cu(OH)2(H2O)4
With excess NH3: Dark blue solution; [Cu(NH3)4(H2O)2]2+

Question 21

Types of reaction

Answer 21

-Redox: the oxidation number of the transition metal ion changes.
-Acid-base reaction: one or more of the ligands gains or loses and hydrogen ion.
-Ligand exchange: one or more of the ligands around the transition metal ion is replaced by a different ligand.
-Coordination number change: the number of ligands changes.

Question 22

The formation of [Cu(NH3)4(H2O)2]^2+ : acid-base reaction

Answer 22

-When aqueous sodium hydroxide is added to copper(II) sulfate solution.
-Observation is pale blue solution forms a blue precipitate
-Reaction: [Cu(H2O)6]^2+ + 2OH- –> [Cu(H2O)4(OH)2] +2H2O
-It is an acid-base reaction as the two hydroxide ions have removed hydrogen ions from two of the water ligands and converted them into water molecules.
-Described as amphoteric behaviour.
-When an acid is added to the blue precipitate, the hydrogen ions from the acid react with the blue precipitate, the hydrogen ions from the acid react with the hydroxide ion ligands and convert them back to water molecules.

Question 23

The formation of [Cu(NH3)4(H2O)2]^2+ : ligand exchange reaction

Answer 23

-When ammonia is added: [Cu(h2O)6]^2+ + 2NH3 –> [Cu(H2O)4(OH)2] + 2NH4^+
-Blue precipitate dissolves to form a deep blue solution.

Question 24

Amphoteric …

Answer 24

??

Question 25

The formation of [CuCl4]^2- : change in coordination number

Answer 25

-Also involve a change of ligand.
-Eg. reaction between copper(II) sulfate solution and concentrated HCl.
-When the acid is added (slowly and continuously) the colour gradually changes from blue to green, and finally to yellow).
-The reaction: [Cu(H2O)6]^2+ + 4Cl ⇌ [CuCl4]^2- + 6H2O.
-All sox water ligands have been substituted by four chloride ions.
-Which is also a change in coordination number.

Question 26

Reaction with alkalis