Sodium thiosulphate and iodine titrations

Question 1

What will be oxidised in the reaction?

Answer 1

Sodium thiosulphate

Question 2

What is the equation for sodium thiosulphate?

Answer 2

Na2S2O3.5H20

Question 3

What is sodium thiosulphate?

Answer 3

A reducing agent

Question 4

What does Na2S2O3.5H20 react with?

Answer 4

Iodine that has been liberated from solutions containing an excess of potassium iodide, KI

Question 5

Is sodium thiosulphate a primary standard?

Answer 5

No

Question 6

Why is sodium thiosulphate not a primary standard?

Answer 6

As it is not available in a pure state.

Question 7

Where does the sodium thiosulphate go?

Answer 7

Into the burette.

Question 8

Is iodine soluble or insoluble in water?

Answer 8

Insoluble

Question 9

Why is iodine insoluble in water?

Answer 9

As it is non-polar and water is a polar solvent.

Question 10

What does iodine undergo at room temperature?

Answer 10

Sublimation

Question 11

What standard is iodine?

Answer 11

Secondary standard

Question 12

Why is iodine a secondary standard?

Answer 12

As it is not stable in air.

Question 13

How is a standard solution of iodine obtained?

Answer 13

By reacting a standard solution of KMno4 with excess potassium iodine.

Question 14

What is the oxidising agent in the titration?

Answer 14

KMnO4

Question 15

Why is it important that the potassium iodide is in excess?

Answer 15

1. so that the maximum amount of iodine is released due to the amount of oxidising agent used.
2. To keep iodine in solution

Question 16

What is the limiting reactant in this reaction?

Answer 16

The oxidising agent = iodine

Question 17

What is the amount of iodine determined by?

Answer 17

By the amount of KMnO4 used (limiting reactant).

Question 18

What / How much does 2 KMnO4 produce

Answer 18

5, Iodine2 molecules

Question 19

What happens when the I2 is liberated?

Answer 19

When liberated it reacts with the I- ions in solution (from KI) to form the tri-iodide ion I3-.

Question 20

Why may we regard the solution formed as a solution of I2?

Answer 20

As the I3- ion is similar in its behaviour to I2.

Question 21

What is the first step in the titration?

Answer 21

The sodium thiosulphate is placed in the burette, observing the usual precautions.

Question 22

What happens after the sodium thiosulphate is placed into the burette?

Answer 22

The iodine solution is placed in the conical flask.

Question 23

What colour is the iodine when it is first placed in the conical flask?

Answer 23

Red/brown

Question 24

Why is iodine red/brown when first placed into the conical flask?

Answer 24

Due to the liberated iodine.

Question 25

What happens after the iodine is placed in the conical flask?

Answer 25

The sodium thiosulphate is added to the conical flask until the solution in the conical flask becomes straw-yellow colour.

Question 26

What happens after the sodium thiosulphate is added and the solution in the conical flask becomes straw-yellow colour?

Answer 26

A few drops of starch indicator is added.

Question 27

What colour did the solution turn after the starch indicator was added? (before & after)

Answer 27

Straw-yellow –> blue/black

Question 28

What would happen if the starch was added before this stage (stage added)?

Answer 28

The blue/black complex that is formed is too concentrated and too stable to decompose fast enough to give an accurate end-point.

Question 29

What happens after the starch indicator is added?

Answer 29

More sodium thiosulphate is added until the blue-black colour becomes colourless.

Question 30

What indicates the end point?

Answer 30

When the solution becomes colourless.

Question 31

What are the four colours in the conical flask?

Answer 31

Red/brown - straw/yellow - blue/black - colourless

Question 32

Sodium thiosulphate is not a primary standard? Explain fully a primary standard. (L.C)

Answer 32

Pure from which solutions of known concentration can be made.

Question 33

Describe how the crystalline thiosulfate was dissolved, and how the solution was transferred to the volumetric flask and made up exactly 500cm³. (L.C)

Answer 33

• Rinse from clock glass into beaker containing deionised water
• Stir
• Dissolve
• Pour through funnel into volumetric flask
• Add rinsings of beaker
• Add deionised water until bottom of meniscus on mark

Question 34

Pure iodine is almost completely insoluble in water. What must be added to bring iodine into aqueous solution? (L.C)

Answer 34

Source of iodide

Question 35

What sequence of colours was observed in the conical flask from the start of the titration until the end point was reached? (L.C)

Answer 35

Red / brown → Straw coloured →- Blue-black → colourless

Question 36

Explain how iodine, a non-polar substance of very low water solubility, is brought into aqueous solution. (L.C)

Answer 36

Reaction with potassium iodide

Question 37

The iodine solution was made up in a 500cm³ volumetric flask. Describe the procedure for measuring 25.0cm³ of this solution into a conical. (L.C)

Answer 37

• Pour some iodine into a clean, dry beaker
• Use pipette
• Previously rinsed with deionised water and iodine solution
• Fill using pipette filler until bottom of meniscus is on mark
• Read at eye-level4

Question 38

Name a suitable indicator for this titration. (L.C)

Answer 38

Starch

Question 39

At what stage is the indicator added? (L.C)

Answer 39

When colour is in conical flask is straw coloured / when close to end point

Question 40

State the colour change at the end point in the presence of the indicator. (L.C)

Answer 40

Blue-black → colourless

Question 41

Explain why the use of distilled water instead of deionised water throughout this experiment would be likely to ensure a more accurate result. (L.C)

Answer 41

Deionised water could contain non-ionic substances that could be oxidised or reduced.