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Chemistry (Tara) > Titration - standardisation of potassium manganate using ammionium iron sulphate crystalline solution. > Flashcards

Titration - standardisation of potassium manganate using ammionium iron sulphate crystalline solution. Flashcards

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1
Q

What is the equation for ammonium iron (II) sulphate?

A

(NH₄)₂SO₄FeSO₄.6H₂0

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2
Q

Is ammonium iron (II) sulphate a primary or a secondary standard?

A

Primary

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3
Q

Why is it necessary that ammonium iron (II) sulphate is a primary standard?

A

So it can be used to standardise potassium manganate (VII).

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4
Q

What is first done to the ammonium iron (II) sulphate?

A

Pipetted into the conical flask

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5
Q

Is Fe(+2) easily oxidised to Fe(+3)?

A

Yes

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6
Q

What is Fe(+2) oxidised to?

A

Fe(+3)

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7
Q

How is Fe(+2) oxidised to Fe(+3)?

A

By atmospheric oxygen

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8
Q

Why is ammonium iron (II) sulphate used?

A

To prevent the Fe(+2) from being oxidised by atmospheric oxygen to Fe(+3).

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9
Q

Why can ammonium iron (II) sulphate prevent Fe(+2) from being oxidised by atmospheric oxygen to Fe(+3)?

A

As it is quite stable in air.

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10
Q

Does ammonium iron (II) sulphate have a high or low molecular mass?

A

High

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11
Q

What does ammonium iron (II) sulphate’s high molecular mass ensure?

A

Ensures accuracy when weighing.

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12
Q

What is added to the volumetric flask when making up ammonium iron (II) sulphate?

A

Sulphuric acid.

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13
Q

Why is sulphuric acid added to the volumetric flask when making up the ammonium iron (II) sulphate?

A

To prevent aerial oxidation of Fe(+2) to Fe(+3).

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14
Q

What is reduced during the titration?

A

Potassium manganate (VII)

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15
Q

What is the equation for potassium manganate (VII)?

A

KMnO₄

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16
Q

What is the oxidising agent?

A

Potassium manganate (VII)

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17
Q

Is potassium manganate a primary or secondary standard?

A

Secondary

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18
Q

Why is potassium manganate (VII) a secondary standard?

A

As it decomposes in strong sunlight and is also easily reduced.

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19
Q

Does potassium manganate (VII) dissolve easily in water?

A

Yes

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20
Q

What colour is potassium manganate (VII)?

A

Deep purple

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21
Q

Why is potassium manganate (VII) deep purple?

A

Due to Mn(+7)

22
Q

Where is the potassium manganate (VII) placed?

A

Always placed in the burette.

23
Q

What conditions are titrations always carried out in?

A

Acidic conditions.

24
Q

Why are titrations always carried out in acidic conditions?

A

To ensure the complete reduction of Mn(+7) to Mn(+2). The sulfuric acid is added to the conical flask to ensure this.

25
Q

What does the formation of Mn(+2) act as in the reaction?

A

Autocatalyst

26
Q

What is the colour change in the reaction?

A

Purple to colourless

27
Q

What colour is Mn(+2)?

A

Colourless

28
Q

What colour is Mn(+7)?

A

Purple

29
Q

What happens if insufficient acid (H₂SO₄) is added to the conical flask?

A

The Mn(+7) is incompletely reduced to Mn(+4), which appears in the conical flask as a brown ppt of MnO₂.

30
Q

What is done to rectify if insufficient acid (H₂SO₄) is added?

A

Add more sulphuric acid to the conical flask.

31
Q

Can HCl or HNO₃ be used to acidify the solution in the conical flask?

A

No

32
Q

Why can’t HNO₃ be used to acidify the solution in the conical flask?

A

As nitric acid is a powerful oxidising agent itself and will take part in the reaction.

33
Q

Why can’t HCl be used to acidify the solution in the conical flask?

A

As the hydrochloric acid will be oxidised by the manganate (VII) solution.

34
Q

What is the first step in the titration?

A

The potassium manganate (VII) is placed into the burette observing the usual precautions.

35
Q

What happens after the potassium manganate placed into the burette?

A

The acidified ammonium iron (II) sulphate solution is pipetted into the conical flask observing the usual safety precautions.

36
Q

What happens after the acidified ammonium iron (II) sulphate solution is pipetted into the conical flask?

A

Dilute sulphuric acid is added to the conical flask.

37
Q

What happens when the purple manganate (VII) is added to the conical flask from the burette?

A

The colour change that occurs is purple to colourless.

38
Q

Why is the acidified ammonium iron (II) sulphate solution pipetted into the conical flask?

A

To prevent aerial oxidation of Fe(+2) to Fe(+3).

39
Q

Why is the dilute sulphuric acid added to the conical flask?

A

To ensure the complete reduction of Mn(+7) to Mn(+2).

40
Q

Does the colour change happen immediately?

A

Yes

41
Q

What does the fact that the colour change is immediate show?

A

That the reaction is very fast at room temperature.

42
Q

When is the end-point indicated?

A

When the colourless solution in the conical flask retains a permanent pink tinge.

43
Q

Why does the permanent pink tinge represent the end-point?

A

As there are no more Fe(+2) present to reduce the purple Mn(+7) to colourless Mn(+2) as it has all been oxidised to Fe(+3).

44
Q

Is there an indicator needed?

A

No

45
Q

Why is there no indicator needed?

A

As the manganate (VII) acts as its own indicator due to the colour changes that occur during and at the end-point of the reaction.

46
Q

What is the reaction ratio between MnO₄- and Fe₂+?

A

5:1

47
Q

What happens when Fe(+2) is oxidised to Fe(+3)?

A

It loses 1e

48
Q

What is another variation of this experiment?

A

To ask about the purity of the crystals.

49
Q

What is the balanced ionic equation?

A

MnO₄- + 5Fe₂+ + 8H+ → Mn₂+ + 5Fe₃+ + 4H₂0

50
Q

What happens when Mn(+7) is reduced to Mn(+2)?

A

Mn(+7) loses 5e

Chemistry (Tara) (50 decks)

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