Chapter 15 - Redox Volumetric Analysis

Question 1

Is potassium permanganate a primary standard?

Answer 1

No - it’s not available in pure state and its solubility is low

Question 2

Why must the ammonium iron (II) sulfate be dissolved in water that has been acidified?

Answer 2

Dissolved in sulfuric acid as the acid prevents the Fe²⁺ ions from being oxidised to Fe³⁺ ions by oxygen in the air

Question 3

Why is dilute sulfuric acid added in excess to the conical flask containing the ammonium iron (II) sulfate solution?

Answer 3

To ensure a sufficient supply of H⁺ ions are present so that Mn⁺⁷ fully reduces to Mn⁺², not Mn⁺⁴

If sufficient acid was not added, a brown precipitate of MnO₂ would form

Question 4

What is the colour change observed in the titration of ammonium iron (II) sulfate and potassium permanganate?

Answer 4

colourless → faint permanent pink

Question 5

Why can’t HCl or nitric acid be used in the titration of ammonium iron (II) sulfate and potassium permanganate?

Answer 5

The Cl⁻ ions would be oxidised to Cl₂ gas

Nitric acid is an oxidising agent

Question 6

Why are the iron tablets crushed and dissolved in dilute sulfuric acid and water?

Answer 6

Dissolved in sulfuric acid as the acid prevents the Fe²⁺ ions from being oxidised to Fe³⁺ ions by oxygen in the air

Question 7

Why is dilute sulfuric acid added in excess to the conical flask containing the iron tablet solution?

Answer 7

To ensure a sufficient supply of H⁺ ions are present so that Mn⁺⁷ fully reduces to Mn⁺², not Mn⁺⁴

If sufficient acid was not added, a brown precipitate of MnO₂ would form

Question 8

What is the colour change observed in the titration of iron tablets and potassium permanganate?

Answer 8

colourless → faint permanent pink

Question 9

What is sodium thiosulfate used for?

Answer 9

To develop photos

Question 10

Is sodium thiosulfate a primary standard?

Answer 10

No - it is not available in a pure state and its crystals are efflorescent (spontaneous loss of water of crystalisation to the air)

Question 11

Is iodine a primary standard?

Answer 11

No - it sublimes (changes directly from a solid to vapour) even at room temperature

Question 12

Which is the reducing agent and which is the oxidising agent: sodium thiosulfate and iodine?

Answer 12

sodium thiosulfate: reducing agent

iodine: oxidising agent

Question 13

How do you create I₂ ions in the titration of sodium thiosulfate and iodine experiment?

Answer 13

By reacting a standard solution of acidified potassium permanganate with excess potassium iodide

Question 14

In the titration of sodium thiosulfate and iodine, why must deionised water be used?

Answer 14

Tap water contains small amounts of chlorine that may interfere with the result by increasing the amount of iodine (chlorine displaced iodine from iodine solutions)

Question 15

Why is dilute sulfuric acid added in excess to the conical flask containing potassium permanganate solution in the titration of sodium thiosulfate and iodine experiment?

Answer 15

To supply sufficient H⁺ ions for the complete reduction of Mn⁺⁷ to Mn⁺², not Mn⁺⁴

Question 16

Why is potassium iodide solution added in excess to the conical flask containing potassium permanganate solution in the titration of sodium thiosulfate and iodine experiment? What is observed?

Answer 16

The solution turns red/brown as the iodine has been formed

The iodine is insoluble in water and so the KI helps to keep it in solution; and also to ensure all the KMnO₄ is reacted (ie max amount of iodine produced)

Question 17

State the colour changes observed in the titration of sodium thiosulfate and iodine experiment

Answer 17

purple -(add potassium iodide)→ reddish/brown -(titrate)→ straw/pale yellow -(add starch indc.)→ blue/black -(titrate)→ colourless

Question 18

Why is the indicator only added in half way through the titration of sodium thiosulfate and iodine experiment?

Answer 18

This is when most of the iodine has disappeared. If added earlier, the iodine present would become strongly adsorbed on to the starch, which would make the final reading less accurate as the iodine would be desorbed slowly so react with the sodium thiosulfate

Question 19

What is the oxidising agent in the bleach experiment?

Answer 19

The hypochlorite ion is the oxidising agent that converts iodide ions to iodine

Question 20

Why is the bleach diluted in for the titration?

Answer 20

As, otherwise, a very large volume of sodium thiosulfate would be required to react with the large amount of I₂ liberated

Question 21

Why is dilute sulfuric acid added in excess to the conical containing the bleach solution in that titration?

Answer 21

To supple sufficient H⁺ ions so that all of the hypochlorite ions (ClO⁻) are completely reduced to chloride ions (Cl⁻)

Question 22

Why is potassium iodide solution added in excess to the conical flask containing the bleach solution in the bleach titration? What is observed?

Answer 22

The solution turns red/brown as the iodine has been formed

The iodine is insoluble in water and so the KI helps to keep it in solution; and also to ensure all the KMnO₄ is reacted (ie max amount of iodine produced)

Question 23

Why is the indicator only added in half way through the bleach titration?

Answer 23

This is when most of the iodine has disappeared. If added earlier, the iodine present would become strongly adsorbed on to the starch, which would make the final reading less accurate as the iodine would be desorbed slowly so react with the sodium thiosulfate

Question 24

State the colour changes observed in the bleach titration

Answer 24

colourless -(add potassium iodide)→ reddish/brown -(titrate)→ straw/pale yellow -(add starch indc.)→ blue/black -(titrate)→ colourless