Titration - standardisation of potassium manganate using ammionium iron sulphate crystalline solution. Flashcards
What is the equation for ammonium iron (II) sulphate?
(NH₄)₂SO₄FeSO₄.6H₂0
Is ammonium iron (II) sulphate a primary or a secondary standard?
Primary
Why is it necessary that ammonium iron (II) sulphate is a primary standard?
So it can be used to standardise potassium manganate (VII).
What is first done to the ammonium iron (II) sulphate?
Pipetted into the conical flask
Is Fe(+2) easily oxidised to Fe(+3)?
Yes
What is Fe(+2) oxidised to?
Fe(+3)
How is Fe(+2) oxidised to Fe(+3)?
By atmospheric oxygen
Why is ammonium iron (II) sulphate used?
To prevent the Fe(+2) from being oxidised by atmospheric oxygen to Fe(+3).
Why can ammonium iron (II) sulphate prevent Fe(+2) from being oxidised by atmospheric oxygen to Fe(+3)?
As it is quite stable in air.
Does ammonium iron (II) sulphate have a high or low molecular mass?
High
What does ammonium iron (II) sulphate’s high molecular mass ensure?
Ensures accuracy when weighing.
What is added to the volumetric flask when making up ammonium iron (II) sulphate?
Sulphuric acid.
Why is sulphuric acid added to the volumetric flask when making up the ammonium iron (II) sulphate?
To prevent aerial oxidation of Fe(+2) to Fe(+3).
What is reduced during the titration?
Potassium manganate (VII)
What is the equation for potassium manganate (VII)?
KMnO₄
What is the oxidising agent?
Potassium manganate (VII)
Is potassium manganate a primary or secondary standard?
Secondary
Why is potassium manganate (VII) a secondary standard?
As it decomposes in strong sunlight and is also easily reduced.
Does potassium manganate (VII) dissolve easily in water?
Yes
What colour is potassium manganate (VII)?
Deep purple
Why is potassium manganate (VII) deep purple?
Due to Mn(+7)
Where is the potassium manganate (VII) placed?
Always placed in the burette.
What conditions are titrations always carried out in?
Acidic conditions.
Why are titrations always carried out in acidic conditions?
To ensure the complete reduction of Mn(+7) to Mn(+2). The sulfuric acid is added to the conical flask to ensure this.
What does the formation of Mn(+2) act as in the reaction?
Autocatalyst
What is the colour change in the reaction?
Purple to colourless
What colour is Mn(+2)?
Colourless
What colour is Mn(+7)?
Purple
What happens if insufficient acid (H₂SO₄) is added to the conical flask?
The Mn(+7) is incompletely reduced to Mn(+4), which appears in the conical flask as a brown ppt of MnO₂.
What is done to rectify if insufficient acid (H₂SO₄) is added?
Add more sulphuric acid to the conical flask.
Can HCl or HNO₃ be used to acidify the solution in the conical flask?
No
Why can’t HNO₃ be used to acidify the solution in the conical flask?
As nitric acid is a powerful oxidising agent itself and will take part in the reaction.
Why can’t HCl be used to acidify the solution in the conical flask?
As the hydrochloric acid will be oxidised by the manganate (VII) solution.
What is the first step in the titration?
The potassium manganate (VII) is placed into the burette observing the usual precautions.
What happens after the potassium manganate placed into the burette?
The acidified ammonium iron (II) sulphate solution is pipetted into the conical flask observing the usual safety precautions.
What happens after the acidified ammonium iron (II) sulphate solution is pipetted into the conical flask?
Dilute sulphuric acid is added to the conical flask.
What happens when the purple manganate (VII) is added to the conical flask from the burette?
The colour change that occurs is purple to colourless.
Why is the acidified ammonium iron (II) sulphate solution pipetted into the conical flask?
To prevent aerial oxidation of Fe(+2) to Fe(+3).
Why is the dilute sulphuric acid added to the conical flask?
To ensure the complete reduction of Mn(+7) to Mn(+2).
Does the colour change happen immediately?
Yes
What does the fact that the colour change is immediate show?
That the reaction is very fast at room temperature.
When is the end-point indicated?
When the colourless solution in the conical flask retains a permanent pink tinge.
Why does the permanent pink tinge represent the end-point?
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).
Is there an indicator needed?
No
Why is there no indicator needed?
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.
What is the reaction ratio between MnO₄- and Fe₂+?
5:1
What happens when Fe(+2) is oxidised to Fe(+3)?
It loses 1e
What is another variation of this experiment?
To ask about the purity of the crystals.
What is the balanced ionic equation?
MnO₄- + 5Fe₂+ + 8H+ → Mn₂+ + 5Fe₃+ + 4H₂0
What happens when Mn(+7) is reduced to Mn(+2)?
Mn(+7) loses 5e
