Iron is a transition element; a transition element is an element that forms at least one ion with a partially filled d subshell. All transition elements may be found in a variety of oxidation states, for example iron exists as both as Iron(II) and Iron(III). Transition elements can react with both oxidising and reducing agents. An oxidising agent will convert Iron(II) into Iron(III), the iron will lose an electron. A reducing agent will convert Iron(III) into Iron(II), the iron will gain an electron.
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Working out the percentage composition of the iron ions will require two separate titrations. One of the titrations will react with only one of the ions and the other to react with the whole mixture. To be able to carry out a titration in which all of the mixture will react, a preliminary reaction will need to be done.
From previous As practical work I know that Potassium manganate is an oxidising agent and will therefore react with Iron(II) but will not react with Iron(III).
This can therefore be used to work out the percentage of Iron(II) present in the solution. �
To work out the percentage composition of Iron(III) a separate reaction will need to be carried out first. The Iron(III) will first need to be reduced to Iron(II). This can be done by reacting the solution with Zinc. This mixture can then be titrated with Potassium Manganate. The results can then be used to work out the new percentage of Iron(II) present.
From the results of the two titrations the percentage composition can be found.
We are told that the mass of iron present in the solution is between 1.1g and 1.3g. I have therefore decided to use the average mass of 1.2g.
The approximate concentration of the iron solution is:
* Moles of Iron = Mass
Mr
= 1.2_
55.8
= 0.0215
* Concentration of Iron = Moles x 1000
Volume
= 0.0215 x 1000
200
= 0.1075 mol dm-3
I have also decided that for this experiment I wish to use the same concentration of Potassium Manganate for each of the titrations. The concentration of the second titration in which the Iron(III) is reduced to Iron(II) can be easily calculated by using the approximate concentration of the iron solution and the mole ratio found in the equations. For the first titration in which only the Iron(II) is reacting an approximate percentage composition will need to be found which will allow the same concentration of Potassium Manganate to be used
Concentration of MnO4- required for Second Titration:
* 1/5 concentration of Iron solution: 0.1075
(Using mole ratio from equation, pg3) 5
= 0.0215 mol dm-3
From rough calculations I know that a 30 – 70% percentage composition, where 70% is Iron(II) will give rise to a concentration of Potassium Manganate which is similar to the concentration calculated for the second titration.
Concentration of MnO4- required for First Titration:
* 70% of total Iron solution concentration: 0.1075 x 0.7
= 0.07525
* Concentration of MnO4: 0.07525
5
= 0.0151 mol dm-3
Therefore Potassium Manganate with concentration 0.02M can be used for both titrations.
Titration of Iron(II) with Potassium Manganate
MnO4- + 8H+ + 5Fe2+ Mn2- + 5Fe3+ + 4H2O
Apparatus
1 Burette (white markings)
1 25cm3 pipette
1 Clamp stand
1 Conical flask
1 Funnel
1 White tile
1 30cm3 Measuring cylinder
Chemicals
100cm3 of solution containing Iron(II) and Iron(III) ions
100cm3 of Potassium Manganate with concentration 0.02M
80cm3 of Sulphuric acid with concentration 1M
Method
Wash out the burette using a small amount of the Potassium Manganate. Clamp the burette carefully making sure that the tap is closed and fill using a funnel with the Potassium Manganate. Open the tap and let some of the Potassium Manganate run out into a beaker to fill the jet of the burette making sure that there are no air bubbles. Record the initial volume of Potassium Manganate in the burette, reading from the bottom of the meniscus and record to two decimal places.
Carefully place the pipette into the pipette filler and place it in the Iron solution. Turn the thumb dial to suck up the solution. Suck up more solution than what is needed, remove the pipette filler and cover the top of the pipette with your index finger and apply pressure. By decreasing the pressure the level of the solution will fall. Carefully allow some of the solution out so that the bottom of the meniscus is in level with the line marked on the pipette. Transfer the solution into the conical flask. Touch the end of the pipette against the inside of the conical flask to release the remaining solution from the pipette. Add about 20cm3 of dilute sulphuric acid to the solution in the conical flask.
Place the conical flask on the white tile underneath the jet of the burette. Open the tap of the burette and allow about 2 cm3 of the Potassium Manganate out into the conical flask, then swirl the solution. Repeat this until the solution in the conical flask is almost pink. Slow the flow of Potassium Manganate so that it is coming out drop by drop, swirling the solution all the time. As soon as the solution turns pink it has reached the end point. Close the tap and record the volume of Potassium Manganate used. This is the rough titration.
Refill the burette as before recording the initial volume and wash out the conical flask using distilled water and fill as before and repeat the titration. However when within 2cm3 of the volume recorded for the rough titration, slow the flow of Potassium Manganate so that it is coming out drop at a time to make sure that the end point is not crossed. This will ensure the highest level of accuracy. Repeat this process until you obtain results that are within 0.1cm3 of each other.
Titration of Iron(III) with Potassium Manganate
2Fe3+ + Zn 2Fe2+ + Zn2+
MnO4- + 8H+ + 5Fe2+ Mn2- + 5Fe3+ + 4H2O
Apparatus
1 Burette (white markings)
1 25cm3 pipette
1 Clamp stand
2 Funnels
2 Conical Flasks
1 White tile
1 Glass rod
Filter paper
1 Spotting Tile
1 Bunsen burner
1 30cm3 Measuring cylinder
1 10cm3 Measuring cylinder
Chemicals
100cm3 of solution containing Iron(II) and Iron(III) ions
100cm3 of Potassium Manganate with concentration 0.02M
100 cm3 of sulphuric acid with concentration 1M
10 cm3 of Copper(II) sulphate solution with concentration 1M
9 nuggets of zinc
Potassium thiocyanate solution
Method
Pipette 25cm3 of the iron containing solution into a conical flask. Add about 25cm3 of sulphuric acid and about 1cm3 of copper(II) sulphate, also add 1 to 3 nuggets of zinc. Heat over a gentle Bunsen flame until effervescence has almost ceased, this should take about 10 to 15 minutes. When a drop of the solution drawn out on a glass rod no longer gives a red precipitate with potassium thiocyanate solution Iron(III) is no longer present.
Fe3+ + SCN- (FeSCN)2+
Filter the iron solution and rinse the remaining zinc, adding the rinsings to the filtrate. Titrate the iron solution with Potassium Manganate and record the results using the same techniques as in the previous titration.
Results of Titration of Iron(II)
Burette Reading cm3
Rough
Accurate
1
2
3
Final Burette Reading
cm3
Initial Burette Reading
cm3
Volume of solution used
cm3
Average Titre:
Results of Titration of Iron(III)
Burette Reading cm3
Rough
Accurate
1
2
3
Final Burette Reading
cm3
Initial Burette Reading
cm3
Volume of solution used
cm3
Average Titre:
Calculations
Titration of 1(Iron(II) only)
* Moles of MnO4- used: Volume x Concentration
1000
= Volume x 0.02
1000
= A
* Moles of Iron(II) in 25cm3: A x 5
(using mole ratio)
= B
* Moles of Iron(II) in 200 cm3: B x 8
= C
* Mass of Iron(II) in solution: Moles x Mr
= C x 55.8
= D
Titration 2 (Iron(II) and Iron(III)):
* Moles of MnO4- used: Volume x Concentration
1000
= Volume x 0.02
1000
= E
* Moles of Iron(II) in 25cm3: A x 5
(using mole ratio)
= F
* Moles of Iron(II) in 200 cm3: B x 8
= G
* Mass of Iron(II) in solution: Moles x Mr
= C x 55.8
= H
* Mass of Iron(III) present H- D
= X
The Percentage composition by mass for each ion is therefore:
* Iron(II): _D_ x 100
H
* Iron(III): _X_ x 100
H
Health And Safety*
Potassium Manganate: May act as an irritant
Harmful by ingestion
Sulphuric Acid: Corrosive; will burn
Ingestion may be fatal
Toxic
Copper(II) Sulphate: Harmful by ingestion
Potassium Thiocyanate: Harmful by ingestion
Irritant; skin contact may lead to ulceration
Zinc: Harmful by ingestion
May act as an irritant
Due to the health risks listed above safety glasses must be warn at all time when working with the chemicals. Lab coats should also be worn to protect clothing and skin should any chemicals be spilt. Care should be taken when heating the solution as chemicals may spit. Tongs should be used at all times when handling the zinc to avoid skin contact. A safety filler should also be used to fill pipettes. If any chemicals come into contact with the skin was effected area immediately with warm water and if necessary seek medical advice
Due to the amount of glass apparatus being used, care should be taken when moving around the class. All breakages should be reported immediately. Any cuts should be covered before continuing with practical work.
All practical work should be carried out in a well-ventilated area.
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