Sodium thiosulphate and iodine titrations Flashcards Preview

Chemistry (Tara) > Sodium thiosulphate and iodine titrations > Flashcards

Flashcards in Sodium thiosulphate and iodine titrations Deck (41)
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1
Q

What will be oxidised in the reaction?

A

Sodium thiosulphate

2
Q

What is the equation for sodium thiosulphate?

A

Na2S2O3.5H20

3
Q

What is sodium thiosulphate?

A

A reducing agent

4
Q

What does Na2S2O3.5H20 react with?

A

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

5
Q

Is sodium thiosulphate a primary standard?

A

No

6
Q

Why is sodium thiosulphate not a primary standard?

A

As it is not available in a pure state.

7
Q

Where does the sodium thiosulphate go?

A

Into the burette.

8
Q

Is iodine soluble or insoluble in water?

A

Insoluble

9
Q

Why is iodine insoluble in water?

A

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

10
Q

What does iodine undergo at room temperature?

A

Sublimation

11
Q

What standard is iodine?

A

Secondary standard

12
Q

Why is iodine a secondary standard?

A

As it is not stable in air.

13
Q

How is a standard solution of iodine obtained?

A

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

14
Q

What is the oxidising agent in the titration?

A

KMnO4

15
Q

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

A
  1. so that the maximum amount of iodine is released due to the amount of oxidising agent used.
  2. To keep iodine in solution
16
Q

What is the limiting reactant in this reaction?

A

The oxidising agent = iodine

17
Q

What is the amount of iodine determined by?

A

By the amount of KMnO4 used (limiting reactant).

18
Q

What / How much does 2 KMnO4 produce

A

5, Iodine2 molecules

19
Q

What happens when the I2 is liberated?

A

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

20
Q

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

A

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

21
Q

What is the first step in the titration?

A

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

22
Q

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

A

The iodine solution is placed in the conical flask.

23
Q

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

A

Red/brown

24
Q

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

A

Due to the liberated iodine.

25
Q

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

A

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

26
Q

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

A

A few drops of starch indicator is added.

27
Q

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

A

Straw-yellow –> blue/black

28
Q

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

A

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

29
Q

What happens after the starch indicator is added?

A

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

30
Q

What indicates the end point?

A

When the solution becomes colourless.

31
Q

What are the four colours in the conical flask?

A

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

32
Q

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

A

Pure from which solutions of known concentration can be made.

33
Q

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

A
  • 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
34
Q

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

A

Source of iodide

35
Q

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

A

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

36
Q

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

A

Reaction with potassium iodide

37
Q

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)

A
  • 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
38
Q

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

A

Starch

39
Q

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

A

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

40
Q

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

A

Blue-black → colourless

41
Q

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)

A

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

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