Redox Reactions And Qualitative Analysis

Question 1

What happens when Fe2+ is oxidised to Fe3+?

Answer 1

MnO4- is reduced to Mn2+ which turns the purple solution colourless.

Question 2

What happens when Fe3+ is reduced to Fe2+?

Answer 2

I- is oxidised to I2 which turns the orange brown solution pale green and forms the brown I2 precipitate

Question 3

What happens when Cr2O72- is reduced to Cr3+?

Answer 3

1. Adding zinc drop wise, orange solution turns green
2. Adding excess of zinc, green solution turns pale blue as Cr3+ is further reduced to Cr2+

Question 4

What happens when Cr3+ is oxidised to CrO42-?

Answer 4

1. Hot alkaline H2O2 is added and a yellow solution is formed

Question 5

What happens when Cu2+ is reduced to Cu+?

Answer 5

Reacts with excess I- ions:
- pale blue solution reacts to form white precipitate and brown I2

Question 6

What happens when Copper (l) oxide reacts with hot dilute sulphuric acid?

Answer 6

Disproportionation reaction:
1. Cu2O is reduced to Cu(s) to form a brown precipitate
2. Cu2O is oxidised to CuSO4 or form a blue solution

Question 7

What is the test for ammonium ions?

Answer 7

• solution is heated gently with NaOH(aq)
• damp red pH paper will turn blue if NH4+ is present

Question 8

What is the test for carbonate ions?

Answer 8

• add dilute nitric acid
• effervescence forms as carbon dioxide is produced

Question 9

What is the test for sulphate ions?

Answer 9

• add Ba2+ ions
• white precipitate forms

Question 10

What is the test for halide ions?

Answer 10

1. Add Ag+ ions
   - Cl- = white precipitate
   - Br- = cream precipitate
   - I- = yellow precipitate