topic 15B

Question 1

why do transition metals form variable oxidation states

Answer 1

• they are able to donate and receive electrons
• are able to oxidise and reduce
• their ions contain partially filled sub-shells of d electrons that can easily lose or gain electrons
• energy differences between the oxidation states are small

Question 2

general trends to know of

Answer 2

• relative stability of +2 state with respect to +3 state increases across the period
• compounds with high oxidation states tend to be oxidising agents
• compounds with low oxidation states tend to be reducing agents

Question 3

e.g. of oxidising agent

Answer 3

MnO4-

Question 4

e.g. of reducing agent

Answer 4

V2+ & Fe2+

Question 5

four oxidation states of vanadium

Answer 5

+5 - VO2^+
+4 - VO^2+
+3 - V^3+
+2 - V^2+

Question 6

colour of VO2^+

Answer 6

yellow solution

Question 7

colour of VO^2+

Answer 7

blue solution

Question 8

colour if V3+

Answer 8

green solution

Question 9

colour if V2+

Answer 9

violet solution

Question 10

what does the addition of zinc do to the vanadium in solution

Answer 10

• vanadium is reduced down each successive state
• colour would successively change from yellow to blue to green to violet

Question 11

interconversions of Vanadium ions - redox equations

Answer 11

1. V2+ + 2e- ⇌ V
2. Zn2+ + 2e- ⇌ Zn
3. V3+ + e- ⇌ V2+
4. VO^2+ + 2H+ + e- ⇌ V3+ + H2O
5. VO2^+ + 2H+ + e- ⇌ V2+ + H2O

Question 12

electrode potential values of 1-5

Answer 12

1. -1.18V
2. -0.76V
3. -0.26V
4. +0.34V
5. +1.00V

Question 13

how can the dichromate (VI) ion be reduced

Answer 13

by zinc in acidic conditions

Question 14

reduction of Cr2O7- to Cr3+

Answer 14

Cr2O72- (aq) + 14H+ (aq) + 3Zn (s) → 2Cr3+ (aq) + 7H2O (l) + 3Zn2+ (aq)

carried out in acidic conditions due to the presence of H+

Question 15

reduction of Cr2O7- to Cr2+

Answer 15

Cr2O72- (aq) + 14H+ (aq) + 4Zn (s) → 2Cr2+ (aq) + 7H2O (l) + 4Zn2+ (aq)