24.5 Redox And Qualitative Analysis

Question 1

What is MnO4- ions reduced to during the redox reaction between Fe2+ and MnO4- under acid conditions?

Answer 1

Mn2+

Question 2

What colour changes occur during the redox reaction between Fe2+ and Fe3+?

Answer 2

PURPLE MnO4- solution goes to COLOURLESS Mn2+ solution.

The MnO4- is decolourised by the Fe2+ ions.

Question 3

What is the equation for the redox reaction between MnO4- and Fe2+ under acid conditions?

(And can also be used as a basis for a REDOX TITRATION)

Answer 3

MnO4-(aq) + 8H+(aq) + 5Fe2+(aq) -> Mn2+(aq) + 5Fe3+(aq) + 4H2O(l)

Question 4

How is Fe3+ reduced to Fe2+ NS what are the colour observations?

Answer 4

2Fe3+(aq) + 2I(aq) -> 2Fe2+(aq) + I2(aq)

ORANGE-BROWN -> PALE GREEN + BROWN

Question 5

What is the formula for dichromate (VI)?

Answer 5

Cr2O72-

Question 6

How is Cr2O72- reduced to Cr3+ AND what are the colour observations?

In fact, can it be reduced further?

Answer 6

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

Yes, to Cr2+.

Zn(aq) + 2Cr3+(aq) -> Zn2+(aq) + 2Cr2+(aq)

GREEN -> PALE BLUE

Question 7

What is the equation for the oxidation of Cr3+ to CrO42-?

Answer 7

Hot alkaline H2O2 is a powerful oxidising agent that is used.

3H2O2 + 10OH- + 2Cr3+ -> 2CrO42- + 8H2O

Question 8

How is Cu2+ reduced to Cu+ AND what are the colour observations?

Answer 8

2Cu2+(aq) + 4I-(aq) -> 2CuI(aq) + I2(s)

PALE BLUE -> WHITE PRECIPITATE + BROWN