Transition Metals: Variable Oxidation States

Question 1

Can transition metals exist in variable oxidation states?

Answer 1

• Yes

Question 2

Which oxidation states can vanadium exist in?

Answer 2

• +5
• +4
• +3
• +2

Question 3

Give each of the oxidation states of vanadium, their formulas and their colours

Answer 3

Question 4

What is the formula and colour of ion of vanadium with an oxidation state of +5?

Answer 4

• Formula
- VO2^+

• Colour
- Yellow

Question 5

What is the formula and colour of ion of vanadium with an oxidation state of +4?

Answer 5

• Formula
- VO^2+

• Colour
- Blue

Question 6

What is the formula and colour of ion of vanadium with an oxidation state of +3?

Answer 6

• Formula
- V^3+

• Colour
- Green

Question 7

What is the formula and colour of ion of vanadium with an oxidation state of +2?

Answer 7

• Formula
- V^2+

• Colour
- Violet

Question 8

How can variable oxidation states of vanadium be formed?

Answer 8

• Vanadium(V) can be reduced by adding vanadate(V) ions to zinc in acidic solution

Question 9

Write the redox equation for the reduction of vanadium(V) into vanadium(IV)

Answer 9

• Zn(s) + 2VO2^+(aq) + 4H+(aq) → Zn^2+(aq) + 2VO^2+(aq) + 2H2O(l)

Question 10

Write the redox equation for the reduction of vanadium(IV) into vanadium(III)

Answer 10

• Zn(s) + 2VO^2+(aq) + 4H+(aq) → Zn^2+(aq) + 2V^3+(aq) + 2H2O(l)

Question 11

Write the redox equation for the reduction of vanadium(III) into vanadium(II)

Answer 11

• Zn(s) + 2V^3+(aq) → Zn^2+(aq) + 2V^2+(aq)

Question 12

What is a redox potential?

Answer 12

• Redox potential of an ion tells you how easily it’s reduced to a lower oxidation state
• The more positive the redox potential, the less stable the ion will be and so the more likely it is to be reduced

Question 13

What factors affect the redox potential?

Answer 13

• Ligands

- pH

Question 14

How does pH affect the redox potential?

Answer 14

• The redox potential for a transition metal ion changes if the reaction involves hydrogen or hydroxide ions
• Addition of hydrogen/hydroxide ions can shift equilibrium and therefore affect redox potential
• Generally, redox potentials more positive in acidic solutions because ion is more easily reduced

Question 15

What is the formula of Tollens’ reagent?

Answer 15

• [Ag(NH3)2]+

Question 16

How is Tollens’ reagent prepared?

Answer 16

• By adding just enough ammonia solution to silver nitrate to form colourless solution containing complex ion [Ag(NH3)2]+

Question 17

How does Tollens’ reagent distinguish between aldehydes and ketones?

Answer 17

• When added to aldehydes, a silver mirror forms; no visible change with ketones
• Aldehyde is oxidised to a carboxylate anion and Ag+ ions reduced to silver metal
• RCHO(aq) + 2[Ag(NH3)2]+(aq) + 3OH-(aq) → RCOO-(aq) + 2Ag(s) + 4NH3(aq) + 2H2O(l)
• Tollens’ reagent can’t oxidise ketones, so no silver mirror will form

Question 18

Write the general equation for the redox reaction between Tollens’ reagent and aldehydes

Answer 18

• RCHO(aq) + 2[Ag(NH3)2]+(aq) + 3OH-(aq) → RCOO-(aq) + 2Ag(s) + 4NH3(aq) + 2H2O(l)

Question 19

Which titration reactions do you need to know?

Answer 19

• Reaction of Fe^2+ and C2O4^2- ions with oxidising agent manganate(VII), MnO4^-, in aqueous potassium manganate (VII)

Question 20

Give the reaction for the oxidation of MnO4^-

Answer 20

• MnO4^–(aq) + 8H+(aq) + 5e– → Mn^2+(aq) + 4H2O(l)

Question 21

Give the reaction between Fe^2+ and MnO4-

Answer 21

• MnO4^–(aq) + 8H+(aq) + 5Fe^2+(aq) → Mn^2+(aq) + 4H2O(l) + 5Fe^3+(aq)

Question 22

Give the reaction between C2O4^2- with MnO4-

Answer 22

2MnO4^-(aq) + 16H+(aq) + 5C2O4^2-(aq) → 2Mn^2+ (aq) + 8H2O(l) + 10CO2(g)

Question 23

What colour does the solution turn at the end point of the titration reactions with MnO4^-?

Answer 23

• Purple