Redox and qualitative analysis

Question 1

What are the reactions of Fe2+ and Fe3+?

Answer 1

• oxidation of Fe2+ to Fe3+ using Mn04- ions
• reduction of Fe3+ to Fe2+ using I- ions

Question 2

What are the reactions of Cr207 2- to Cr3+

Answer 2

• reduction of Cr207 2- to cr3+ using Zn
• oxidation of cr3+ to cr207 2- using hydrogen peroxide

Question 3

What are the reactions of cu2+ and Cu+

Answer 3

• reduction of cu2+ to Cu+ using excess I- ions
• disproportion of Cu+ ions

Question 4

describe the oxidation of Fe2+ to Fe3+ using Mn04- ions?

Answer 4

• Fe2+ is oxidised to Fe3+
• Mn04- is reduced to Mn2+
• the colour change Is purple and is decolourised by Fe2+ forming a colourless solution containing Mn2+ ions
• 5Fe2+ + Mn04- + 8H+ = 5Fe3+ + Mn2+ + 4H20

Question 5

describe the reduction of Fe3+ to Fe2+ using I- ions?

Answer 5

• Fe3+ is reduced to Fe2+
• I- is oxidised to I2
• the colour change is orange/brown to brown
• 2Fe3+ + 2I- = 2Fe2+ + I2

Question 6

Describe the -reduction of Cr207 2- to cr3+ using Zn

Answer 6

• Cr207 2- is reduced to Cr3+
• Zinc is oxidised to Zn2+
• Cr2072- + 14H+ + 2Zn = 2cr3+ + 7H20 + 3Zn2+
• This causes a colour change from orange to green
• when excess zinc is added, chromium ions are further reduced to chromium (II)

Zn + 2Cr3+ = Zn2+ + 2Cr2+
this is a colour change from green to pale blue

Question 7

Describe the reduction of cu2+ to Cu+ using excess I- ions?

Answer 7

• I- is oxidised to brown iodine I2
• cu2+ is reduced to cu+
• the cu+ forms a white precipitate of copper iodide

2Cu2+ + 4I- = 2CuI (s) + I2
blue to white precipitate in brown liquid of I2

Question 8

Describe the disproportion of Cu+ ions

Answer 8

when solid copper oxide Cu2O reacts with sulfuric acid forms a brown precipitate of Cu(s) in blue solid CuSO4 (aq)

Cu20 + H2SO4 = Cu(s) + CuSo4 +H20

+1 0 +2

Question 9

What is the test for NH4+?

Answer 9

• Add NaOH
• warm gently
• damp red litmus turns blue

Question 10

What is the test for Cu2+?

Answer 10

• Add NaOH
• forms blue precipitate
• insoluble in excess NAOH

Question 11

What is the test for Fe2+?

Answer 11

• Add NaOH
• forms dark green precipitate
• insouble in excess NaOH
• turns brown as oxidised to Fe3+

Question 12

What is the test for Fe3+?

Answer 12

• Add NaOH
• forms orange brown precipitate
• insoluble in excess NaOH

Question 13

What is the test for Mn2+?

Answer 13

• Add NaOH
• forms pale brown precipitate
• insoluble in excess NaOH