Potassium Permanganate (oxidising agent)

Question 1

What is another name for potassium permanganate?

Answer 1

potassium manganate(VII)

Question 2

Explain the appearance of potassium permanganate

Answer 2

It is a purple-coloured solid

Question 3

Is potassium permanganate a primary standard? Give reason for your answer?

Answer 3

It is not a primary standard as it cannot be obtained in a state of very high purity.

Question 4

Is potassium permanganate stable or unstable? Explain your answer further.

Answer 4

It is unstable and decomposes in the presence of sunlight and also in the presence of heat

Question 5

In the presence of what does Potassium permanganate decompose?

Answer 5

It decomposes in the presence of sunlight and also in the presence of heat

Question 6

Since Potassium permanganate decomposes in the presence of sunlight and also in the presence of heat, how does this affect its concentration?

Answer 6

therefore, the concentrations of potassium permanganate solutions vary over time

Question 7

Since the concentrations of potassium permanganate solutions vary over time, what must be done to them?

Answer 7

They must be standardised by titration against a primary standard solution

Question 8

Does potassium permanganate act as an oxidising agent or a reducing agent?

Answer 8

It acts as an oxidising agent by gaining five electrons in acidic solution.

Question 9

Give the equation for the colour change of potassium permanganate in acidic solution

Answer 9

MnO4 + 8H+ + 5e- —-> Mn2+ + 4H2O

purple —> colourless

Question 10

Why is HCl not used to supply H+ ions?

Answer 10

If HCl were used, the Cl- ions would be oxidised to Cl2 by the KMnO4

Question 11

Why is nitric acid not used to supply H+ ions?

Answer 11

If nitric acid were used, it could interfere with the reaction as nitric acid is itself a strong oxidising agent

Question 12

What is used in the titration to standardise potassium permanganate that supplies H+ ions?

Answer 12

sulfuric acid

Question 13

Why does the potassium permanganate appear as a brown colour if instructions are not followed?

Answer 13

In neutral or basic solution, the above reaction does not take place. The manganese ion changing from an oxidation state of +7 (purple) to +3 (colourless), an intermediate oxidation state of +4 is formed.
This oxidation state appears as a brown colour and is due to the formation of manganese dioxide, MnO2.

Question 14

Why does an indicator not need to be added to the potassium permanganate?

Answer 14

It acts as its own indicator and is said to be self-indicating

Question 15

When reading a volume of potassium permanganate, what must you do?

Answer 15

read the top of the meniscus rather than the bottom of the meniscus

Question 16

What do we use a source of Fe2+ ions?

Answer 16

ammonium iron (II) sulfate

Question 17

Why do we use ammonium iron(II) sulfate?

Answer 17

This can be obtained in a high degree of purity, is not affected by the air, and so it can be used as a primary standard

Question 18

what is the formula for ammonium iron(II) sulfate?

Answer 18

(NH₄)₂Fe(SO₄)₂·6H₂O

Question 19

What was the old name used to refer to ammonium iron sulfate?

Answer 19

ferrous ammonium sulfate

Question 20

Why is ammonium iron(II) sulfate used as a primary standard?

Answer 20

It is very stable and does not spontaneously lose its water of crystallisation to the air

Question 21

At the end point of titrating potassium permanganate against ammonium iron(II) sulfate, what colour change is observed?

Answer 21

pale green to a faint permanent pink colour

Question 22

How is a standard solution of iodine obtained?

Answer 22

a standard solution of iodine is obtained by reacting a standard solution of potassium permanganate with excess potassium iodide solution