Redox Titrations

Question 1

Describe the procedure To use a standard solution of hydrated ammonium iron (II)
sulfate to standardise a solution of potassium manganate (VII) by titration

Answer 1

1- filling the burette with the potassium managanate vii solution.
fill KMnO4 in burette, fill above 0 cm mark first reading @ eyelevel.
UNTIL TOP OF MENISCUS ON 0CM MARK

Question 2

Making the hydrated ammonium iron (II) sulfate up into a standard solution

To use a standard solution of hydrated ammonium iron (II)
sulfate to standardise a solution of potassium manganate (VII) by titration

Answer 2

Dilute sulfuric acid (H2SO4) is added to the deionised water when making up the solution of hydrated amonium sulfate.

Question 3

describe the process of carring out the titration

To use a standard solution of hydrated ammonium iron (II)
sulfate to standardise a solution of potassium manganate (VII) by titration

Answer 3

BEFORE TITRATION BEGINS, EXCESS DILUTE SULFURIC ACID H2SO4 IS ADDED TO THE HYDRATED AMMONIUM IRON II SULFATE IN THE CONICAL FLASK
the purple drops will decolourise during the titration, at the end point a permanent pink colour is observed in the conical flask.

Question 4

what is the suitable indicator for this reaction?

To use a standard solution of hydrated ammonium iron (II)
sulfate to standardise a solution of potassium manganate (VII) by titration

Answer 4

no indicator required, potassium manganate acts as its own indicator (self indicating)

Question 5

describe the colour chage observed in this reaction

To use a standard solution of hydrated ammonium iron (II)
sulfate to standardise a solution of potassium manganate (VII) by titration

Answer 5

during the titration- the potassium mananate is added to the acidified hydrated ammonium iron II sulfate, its purple colour decolourises
at end point- a permanent pink colour is observed in the conical flask

Question 6

explain the colour change that occours during and at end point

To use a standard solution of hydrated ammonium iron (II)
sulfate to standardise a solution of potassium manganate (VII) by titration

Answer 6

during titration- Mn 7+ ions cause the purple colour in KMnO4.
Fe2+ ions are oxidised to Fe3+ ions : the Mn7+ ions are reduced to Mn 2+ ions
as Mn2+ ions are formed the purple colour decolourises

at end point- There is no more Fe2+ ions to reduce the MN7+ ions The last drop of KmnO4 is added and leaves a permanent pink colour.

Question 7

Explain why a standard solution of potassium manganate (VII) CANNOT be directly made up i.e. why must potassium manganate (VII) be standardised by titration?

To use a standard solution of hydrated ammonium iron (II)
sulfate to standardise a solution of potassium manganate (VII) by titration

Answer 7

Potassium manganate VII is not a primary standard,it cannot be obtained in a pure state

Question 8

Explain why a standard solution can be directly made up from hydrated ammonium iron (II) sulfate

To use a standard solution of hydrated ammonium iron (II)
sulfate to standardise a solution of potassium manganate (VII) by titration

Answer 8

Hydrated ammonium iron ii sulfate is a primary standard, it can be obtained in a pure form that is stable in air and can be dissolved in water to make up a solution of accurately known conc.

Question 9

Why is ammonium iron (II) sulfate used as a primary standard instead of iron (II) sulfate?

To use a standard solution of hydrated ammonium iron (II)
sulfate to standardise a solution of potassium manganate (VII) by titration

Answer 9

Iron ii sulfate is easily oxidised by oxgen in the air, not a suitable primary standard
Ammonium II sulfate can be obtained in a pure state that is stable in air and has a high mm and can be dissolved directly in water to make a solution of known conc

Question 10

It is noted during the titration that the first few drops of KMnO4 are slow to decolourise, but
subsequent drops decolourise rapidly. Explain

To use a standard solution of hydrated ammonium iron (II)
sulfate to standardise a solution of potassium manganate (VII) by titration

Answer 10

Mn2+ ions act as a auto catalyst, as more are formed they speed up thye reaction and cause the purple colour to decolourise more rapidly.- auto catalysis

Question 11

At what two occasions is dilute sulfuric acid required to be added and explain why it is required on each occasion.

To use a standard solution of hydrated ammonium iron (II)
sulfate to standardise a solution of potassium manganate (VII) by titration

Answer 11

1- sulfuric acid is added when making up and dissolving the hydrated ammonium iron II sullfate into solution- Prevent Fe2+ ions being oxidised to Fe3+ ions by oxygen in the air

2- excess sulfuric is added to the hydrated ammonium II sulfate solution in the conical flask before titrating against potassium manganate VII - only in an acidic enviorment are MN 7+ ions reduced to Mn2+ ions and NOT Mn 4+ ions

Question 12

if a brown precipitate forms during the titrations, what conclusion can be drawn?

To use a standard solution of hydrated ammonium iron (II)
sulfate to standardise a solution of potassium manganate (VII) by titration

Answer 12

not enough dilute sulfuric acid has been added to the hydrated ammonium iron II sulfate before the titration_ Mn+7 ions have ONLY been reduced to Mn+4 ions and an insoluble brown percipitate has been formed

Question 13

How is the potassium permanganate read in the burette during the titrations? And why?

To use a standard solution of hydrated ammonium iron (II)
sulfate to standardise a solution of potassium manganate (VII) by titration

Answer 13

The potassium permanganate is read from the top of the meniscus eye level .
The intense purple colour of potassium manganate (VII) makes it difficult to read from the bottom of the meniscus.
(Also for this reason, dilute solutions of potassium manganate (VII) are used)

Question 14

Describe the procedure for dertiming the amount of iron in an iron tablet

Answer 14

1 fill burette with standard solution of potassium manganate VII. Read from top of meniscus at eye level
2. making the iron tables up into solution. The iron tablets need to be crushed with a pestle and mortar and dilute sulfuric acid is added to the deionied water when making up the iron tablet solution.

Question 15

Describe how you carried out the titration

To dertimine the amount of iron in an iron tablet

Answer 15

before titration begins, excess dilute sulfuric acid is added to the iron tablet solution in the conical flask.
the purple KMnO4 decolourises and a permant pink colour forms.

Question 16

what is the average amount of iron in an iron tablet

To dertimine the amount of iron in an iron tablet

Answer 16

25- 50%

Question 17

what is the active compound in iron tables

To dertimine the amount of iron in an iron tablet

Answer 17

Iron (II) sulfate

Question 18

why are iron tablets somtimes medically percibed?

To dertimine the amount of iron in an iron tablet

Answer 18

Iron is part of haemoglobin in our red blood cells which carry oxugen and provide energy. A person suffering from anameia may be perscribes iron tablets for tirednedd and fatigue

Question 19

how was it possible to have a standard solution of KMnO4 to use in this titration despite the fact its not a promary standard

To dertimine the amount of iron in an iron tablet

Answer 19

KMnO4 perviously titred against a standard sln of Hydrated ammonium iron II sulfate

Question 20

At what two occations is diltute sulfuric acid added and why is it required

To dertimine the amount of iron in an iron tablet

Answer 20

1- sulfuric acid is added when making up and dissolving the hydrated ammonium iron II sullfate into solution- Prevent Fe2+ ions being oxidised to Fe3+ ions by oxygen in the air

2- excess sulfuric is added to the hydrated ammonium II sulfate solution in the conical flask before titrating against potassium manganate VII - only in an acidic enviorment are MN 7+ ions reduced to Mn2+ ions and NOT Mn 4+ ions

Question 21

Why Is it important to use the previously standardised KMnO4 immediately to dertimine the % Fe in Iron tablets?

To dertimine the amount of iron in an iron tablet

Answer 21

KMnO4 is unstable, it decoposes in the presence of light and heat

Question 22

Describe the procedure of using a standard solution of iodine to standardise a solution of sodium thiosulfate

Answer 22

Procedure: making the dosium thiosulfate up into a solution if required.
2: filling te burette with the sodium thisulfate solution.
3. making a standard soultion of iodine using potassium permanganate - irodine is not a primary standard.
A set volume of perviously standardised solution of potassium maganate is reacted with EXCESS potassium iodide (KI) and EXCESS sulfuric acid (H2SO4) to form iodine.
The colour change at THIS point is purple to red-brown. (2:5) ratio.
Carry out titration, Colour change during titration. goes from red-brown colour to yellow to pale yellow. When pale yellow, add starch solution indicator. The solution in conical flask turns blue- black. At the end point all iodine is used up and solutio becomes colourless.

Question 23

what is suitable indicator for this solution?

Describe the procedure of using a standard solution of iodine to standardise a solution of sodium thiosulfate

Answer 23

startch solution is added when close to the end point of a titration, when a pale yellow colour forms

Question 24

why should the indicator be freshly prepared.

Describe the procedure of using a standard solution of iodine to standardise a solution of sodium thiosulfate

Answer 24

starch is biodegradable

Question 25

describe the colour chage during this titration

Describe the procedure of using a standard solution of iodine to standardise a solution of sodium thiosulfate

Answer 25

red-brown to yellow to pale yellow. Now add starch - a blue black colour forms. Blue-black to colourless

Question 26

explain the colour change during and at the end point of the titration.

Describe the procedure of using a standard solution of iodine to standardise a solution of sodium thiosulfate

Answer 26

During the titration the sodium thosufate is added to and reacts with the iodine, whos colour becomes less intense as its being used up in the reaction.
At the end point starch solution indicator is added and a blue black colour forms due to small amount of iodine left, as soon iodine has been conpeltely used up the blue-black colour decolourises.

Question 27

Why is the stach only added when a pale-yellow colour forms in the conical flask.

Describe the procedure of using a standard solution of iodine to standardise a solution of sodium thiosulfate

Answer 27

1) waiting until pale yellow colour tells us end point is very near.the sodium thiosulfate can be added in slowly in small drops, resulting in accurate end point
2) Iodine absorbs onto starch, preventing it reactig with sodium thiosulfate, adding too early results in innacuratly large end pt

Question 28

Explain why a standard solution of sodium thiosuldate cannot be directly made up, why must it be standardised by titration

Describe the procedure of using a standard solution of iodine to standardise a solution of sodium thiosulfate

Answer 28

sodium thiosulfate is not a primary standard- it is a hydrated compound and loses some of their water of crystallisation in dry air- cannot be obtained in a pure state, so a percise mass cannot be weighed out

Question 29

why cant a standard solution of iodine be directly made up?

Describe the procedure of using a standard solution of iodine to standardise a solution of sodium thiosulfate

Answer 29

iodine is not a primary standard
Iodine sublimes (changes directly drom a solid to a vapour at room temperatire)
poorly soluble in water

Question 30

How is a standard aqueous solution of iodine obtained? What colour change occours in conical flask as a redult of this reaction?

Describe the procedure of using a standard solution of iodine to standardise a solution of sodium thiosulfate

Answer 30

reacted with excess potassium iodide and excess sulfuric acid (it has molar ratio 2:5)
Colour change is purple to red brown.

Question 31

why when making up the iodine, is excess sulfuric acid added to the conical flask?

Describe the procedure of using a standard solution of iodine to standardise a solution of sodium thiosulfate

Answer 31

Only in an acidic enviorment are Mn7+ ions fully reduced to Mn2+ ion, meaning that the I- ions will be fully oxidised to form iodine Produces the max amount of iodine

Question 32

In making up iodine, why cant nitric acid or hydrocloric acid be used to provide an acidic enviorment

Describe the procedure of using a standard solution of iodine to standardise a solution of sodium thiosulfate

Answer 32

nitric acid is an oxidising agent- will interfere
Hydrocloric acid will be oxidised by potassium manganate forming Cl2

Question 33

Why is excess potassium iodide added to the conical flask?

Describe the procedure of using a standard solution of iodine to standardise a solution of sodium thiosulfate

Answer 33

1- potassium iodide in excess ensure that the potassium manganate is the limiting reagent- ensuring all of the potassium mangante reacts- produces max amount of iodine

2- the iodine that is produced will react with potassium iodine in excess forming the triiodide ion I3- a soluble version iodine- keeps iodine in aqueous solution.

Question 34

explain how iodine, a non-polar substance of very low water solubility , is brought into aqueous solution

Describe the procedure of using a standard solution of iodine to standardise a solution of sodium thiosulfate

Answer 34

reacting the acdified potassium manganate with EXCESS potassium iodine. The iodine that is produced will react with the potassium iodide in excess forming the triiodide ion I3-. keeps iodine in aqeous solution

Question 35

Describe the procedure to dertime the % w/v of sodium hyprochlorite in bleach

Answer 35

1.making the sodium thiosulfate up into a standard solution (if required)
2.Filling the burette with the sodium thosulfate solution.
3. diluting the bleach
4.making a solution of iodine using the diltued bleach- a set volume of the diluted bleach containing sodium hypochloride (NaCLO) is pipetted to a conical flask and reacted excess potassium (KI) and excess sulfuric acid (H2SO4) The colour wil change from colourless to red-brown, the molar ratio means the moles of sodium hypocloride can be known.
The titration is carried out red-brown to yellow to pale yellow. Starch indicator is added, solution turns blue black, at ent point, titration, solution becomes colourless.

Question 36

what is the typical w/v of sodium hypochlorite in bleach.

Describe the procedure to dertime the % w/v of sodium hyprochlorite in bleach

Answer 36

3-10%

Question 37

how was it possible to have a standard solution of sodium thiosulfate to use in this titration despite the fact it is not a primary standard

Describe the procedure to dertime the % w/v of sodium hyprochlorite in bleach

Answer 37

the sodium thisolfate was previously standardised by titration it against a standard solution of iodine

Question 38

why is the bleach diltuted before use in this titration?

Describe the procedure to dertime the % w/v of sodium hyprochlorite in bleach

Answer 38

the orginal bleach was too concentrated meaming the end point of the titration would take too long to occour. a large volume of sodium thiosulfate would be required.

Question 39

how is an aqueous solution of iodine obtained from the bleach

Describe the procedure to dertime the % w/v of sodium hyprochlorite in bleach

Answer 39

the diluted bleach containing sodium hypochlorite is reacted with excess potasium iodide and excess sulfuric acid

Question 40

when making up the iodine what colour change occours in the conical flask as a result of the reaction?

Describe the procedure to dertime the % w/v of sodium hyprochlorite in bleach

Answer 40

colourless to red-brown

Question 41

why is excess sulphuric acid added to the conical flask when making the I2

Describe the procedure to dertime the % w/v of sodium hyprochlorite in bleach

Answer 41

Only in an acidic enviorment are CLO- ions in sodium hypochlorite fully reduced to Cl- ion. I- ions will be fully oxidised to form iodine (i2) produces the maxium amount of iodine

Question 42

why is excess potassium iodide added to the conical flask

Describe the procedure to dertime the % w/v of sodium hyprochlorite in bleach

Answer 42

1.Potassium iodide in excess ensures that the sodiim hypochlorite is the limiting reagent - the ensures all of the sodium hypochlorite reacts- produces max iodine.

2. The iodine that is produces will react with the extra potassium iodide in excess forming the triiodine ion I3- keeps iodine in aqueous solution

Question 43

explain how iodine, a non-polar substance of very low water solubility, is brought until aqueous solution

Describe the procedure to dertime the % w/v of sodium hyprochlorite in bleach

Answer 43

reacting the acidified sodium hypochlorite with excess potassium iodide. The iodine that is produces will react with the potassium iodide in excess forming the triiiodide ion I3-

Question 44

Explain why the use of the distilled water instead of deionised water throught this expeirment would be more likely to ensure a more acctuate result.

Describe the procedure to dertime the % w/v of sodium hyprochlorite in bleach

Answer 44

Deionised water has all ions removed but could still contain chlorine- chlorine is an deoxidising agent.
Distilled water is the most pure form of water