Oxidation-Reduction Volumetric Analysis

Question 1

Name an oxidising agent used in Oxidation-Reduction titrations

Answer 1

KMnO4/
Potassium Permanganate/
Potassium Manganate (VII)

Question 2

Describe KMNO4

Answer 2

• Purple coloured solid
• Not a primary standard as it cannot be obtained in a state of high purity
• Unstable and decomposes in the presence of sunlight and heat

Question 3

Write the half equations in relation to KMnO4 where KMnO4 acts as an oxidising agent.

Answer 3

MnO4- + 8H+ +5e- -> Mn2+ + 4H20

Question 4

Why is it necessary to add an acid, and which acid, to KMnO4

Answer 4

To provide H+ ions for Mn7+ to reduce to Mn2+ (purple to colourless, otherwise it would be Mn4+, a brown colour)
Sulfuric acid is used because Nitric acid would interfere (it’s an strong oxidising agent) and in HCl, Cl- would oxidise to Cl2.

Question 5

Does an indicator need to be used in the titration involving KMnO4?

Answer 5

No, Potassium Permanganate is self-indicating

Question 6

Why is KMnO4 read from the top of the meniscus and not from the bottom?

Answer 6

because of it’s intense colour, the bottom would be difficult to see.

Question 7

What primary standard solution is used in Oxidation-Reduction titrations?

Answer 7

Ammonium Iron (II) Sulfate

Question 8

Write the half equation of Iron from Fe2+ to Fe3+

Answer 8

Fe2+ -> Fe3+ + e-

Question 9

What is the molecular formula for ammonium Iron (II) Sulfate

Answer 9

(NH4)2SO4.FeSO4.6H20

Question 10

What is necessary to add when making up a solution of ammonium Iron (II) Sulfate and why

Answer 10

A small amount of Sulfuric acid is added to prevent the oxidation of Fe2+ to Fe3+

Question 11

What is haemoglobin

Answer 11

It is the molecule that is responsible for the transport of oxygen in our blood

Question 12

Why are iron tablets sometimes medically prescribed

Answer 12

To treat Amaenia

Question 13

Name reducing agent used in oxidation-reduction reactions

Answer 13

Sodium thiosulfate/ Na2S2O3

Question 14

What are some uses of Sodium thiosulfate

Answer 14

Analysing the amount of certain substances in water, extracting gold, antidote to cyanide poisoning, for developing photographs by chemical means

Question 15

Describe Na2S2O3

Answer 15

• colourless crystalline

- common formula Na2S203.5H20

Question 16

What is oxidised to the tetrathionate ion in the reaction between Iodine and a thiosulfate ion

Answer 16

the thiosulfate

Question 17

Why is thiosulfate not a primary standard?

Answer 17

It cannot be obtained in a sufficiently pure state and the crystals are efflorescent

Question 18

Why is Iodine not a primary standard?

Answer 18

because iodine slightly vaporises even at room temp. it is also completely insoluble in water

Question 19

Give two reasons why excess potassium iodide is added to the reaction between acidified potassium ions with iodide ions

Answer 19

• all the KMnO4 reacts completely with the I- ions to form the maximum amount of I2
• Iodine is only sparingly soluble in water but is readily soluble in potassium iodide solution. When iodine reacts with the potassium iodide ion it forms tri-iodide ions which have the same chemical behaviour as Iodine

Question 20

What is the colour change of Iodine in the reaction between thiosulfate and iodine. (without the indicator)

Answer 20

Reddish brown to colourless

Question 21

What indicator is used in the reaction of thiosulfate and iodine, describe how its used and its colour change

Answer 21

strach
add it once most of the iodine has disappeared
black-blue to colourless

Question 22

Why is the starch indicator only added when most of the iodine has reacted?

Answer 22

If the starch were added at an earlier stage the iodine present would become strongly adsorbed on to the starch and this would make the titration less accurate since the iodine would be desorbed slowly to react with the thiosulfate.

Question 23

In the experiment to standardise a solution of sodium thiosulfate, why is it not necessary to know the precise mass of the sodium thiosulfate crystals?

Answer 23

The purpose of the experiment is to standardise the sodium thiosulfate, from the titration results we will be able to find the origional mass. Also it is not a primary substance.