Tuesday, 5 September 2017

Transition metals: Some chemistry of iron (2)

Some chemistry of Iron (2):

Edexcel
24. be able to record observations and write suitable equations for the reactions of Cr3+(aq), Fe2+(aq), Fe3+(aq), Co2+(aq) and Cu2+(aq) with aqueous sodium hydroxide and aqueous ammonia, including in excess.
25. be able to write ionic equations to show the difference between ligand exchange and amphoteric behaviour for the reactions in (24) above.
34. understand the role of Fe2+ ions in catalysing the reaction between Iand S2O82— ions.
Manganate(VII) with iron (II) titration self-indicating

catalyst in Haber process


AQA
Exchange of the ligand H2O by Clcan involve a change of co-ordination number (e.g. Fe3+(aq), Co2+(aq) and Cu2+(aq).
Haem is an iron(II) complex with a multidentate ligand.
Oxygen forms a co-ordinate bond to Fe(II) in haemoglobin, enabling oxygen to be transported in the blood.
Carbon monoxide is toxic because it replaces oxygen co-ordinately bonded to Fe(II) in haemoglobin.
The redox titrations of Fe2+(aq) and C2O42– with MnO4
Students should be able to perform calculations for these titrations and similar redox reactions.
Examples include, finding:
   the mass of iron in an iron tablet 

   the percentage of iron in steel 

   the Mr of hydrated ammonium iron(II) sulfate 

       Fe is used as a heterogeneous catalyst in the Haber process.

In aqueous solution, the following metal-aqua ions are formed:
[M(H2O)6]2+, limited to M = Fe and Cu
[M(H2O)6]3+, limited to M = Al and Fe
The acidity of [M(H2O)6]3+ is greater than that of [M(H2O)6]2+
Students should be able to:
   explain, in terms of the charge/size ratio of the metal ion, why the acidity of [M(H2O)6]3+ is greater than that of [M(H2O)6]2+
   describe and explain the simple test-tube reactions of M2+(aq) ions, limited to M = Fe and Cu, and of M3+(aq) ions, limited to M = Al and Fe, with the bases OH, NH3 and CO32—
       Students could carry out test-tube reactions of metal-aqua ions with NaOH, NH3 and Na2CO3
Halogen carrier in arene organic chemistry

OCR
Redox reactions

(k) redox reactions and accompanying colour changes for:
(i) interconversions between Fe2+ and Fe3+
Fe2+ can be oxidised with H+/MnO4and Fe3+ reduced with I


7. Iron as a catalyst

Haber Process
a) The Haber process is used to form ammonia from atmospheric nitrogen and hydrogen extracted from methane (natural gas). 

The industrial process uses iron as a catalyst in a process in which the pressure is around 200atm and the working temperature around 450oC. 

N2(g)    +     3H2(g)          2NH3(g)

You can find out more about the Haber Process if you follow this link:


Iron here acts as a heterogenous catalyst because it is in a different state to the gaseous reactants.

b) Redox catalysis of the peroxodisulphate/iodide reaction

The reaction between peroxodisulphate ions and iodide ions is typically used to illustrate the action of iron ions and their catalytic effect in this reaction. 

This is the equation for the redox reaction between peroxodisulphate ions and iodide ions. 

2I      +      S2O82—                  I2      +      2SO42—

As you can see both ions are anions negatively charged and so will repel each other.

The result is that this reaction has a very high activation energy. 

Iron ions have a catalytic effect because they lower the activation energy of the reaction. 

When a small amount of iron(II) ions (Fe2+) are added to the iodide -peroxodisulphate mixture, the peroxodisulphate ions oxidise them to iron(III) ions. 

2Fe2+        +      S2O82—                  2Fe3+      +      2SO42—

Then the iron(III) ions formed oxidise the iodide ions to iodine and they are reduced back to iron(II)

2I      +      2Fe3+                   I2      +      2Fe2+       

These iron(II) ions can now start the redox process all over again.

The reaction is much faster because the two reactions catalysed with iron ions have much lower activation energies. 

In this example the iron(II) ions act as a homogenous catalyst since they are in the same state as the iodide and peroxodisulphate ions. 


8. Titration of iron(II) with manganate(VII) ions

Example 1:
Finding the mass of iron in an iron tablet: 


A typical multivitamin with iron tablet weighs 0.364g and is said to contain 14mg of iron. 

Is this the case?

To determine the iron content of this tablet first weigh two tablets and then grind them up and dissolve them in dilute (0.5M) sulphuric acid and make up the solution to 250ml in a volumetric flask. 

The role of the sulphuric acid is there to prevent oxidation of the iron (II) ions to iron(III) ions.  I’m here assuming that the iron in the tablet is in the form of iron(II)sulphate.   

Next titrate a 25ml aliquot of the solution from the volumetric flask with potassium manganate(VII) solution (0.0005M). 

Manganate(VII) ions oxidise the iron(II) ions according to the equation below.

5Fe2+      +      MnO4    +        8H+               5Fe3+      +      Mn2+      +     4H2O

The titration results show that it requires 20ml of the manganate(VII) solution to completely oxidise the iron(II) ions.

The end point of the titration is when the iron(II) solution turns a very pale purple because manganate(VII) is the purple and it is therefore self-indicating.

Calculation:

a)    Calculate moles of manganate(VII) ions from the titration. 

Use n=cV

Therefore n=  0.0005  ×   20/1000  =   0.00001 moles manganate(VII) ions.

b)    Determine the number of moles iron(II) ions in the aliquot from the equation since 5 moles iron(II) ions react with 1 moles manganate(VII) ions.

5Fe2+         +      MnO4   
0.00005         0.00001

c)    if there are 0.00005 moles iron(II) ions in the aliquot then there are 10× this number of moles in the volumetric flask i.e. 0.0005 moles.

d)    Calculate the mass of iron that this number of moles represents.

Use m  =   n  ×  M  therefore mass Fe2+ ions =  0.0005  ×  55.9  =  0.02795g  or  27.95mg.

This mass of iron corresponds closely to the mass of iron reportedly present in 2 of these tablets 28mg. 


Example 2:
Finding the percentage of iron in a piece of steel weighing 0.034g.

First thing to do is dissolve the steel in the minimum volume of 2M sulphuric acid.  You may need to heat the flask in which you carry out this part of the experiment and do so in fume cupboard because of the hydrogen fumes that will be evolved. 

The resultant solution should look pale green in colour showing the presence of iron(II) ions. 

Next allow the solution to cool and then transfer it all to a 250ml volumetric flask and make up to the mark with distilled water. 

Next prepare a solution of potassium manganate(VII) 0.0005M and titrate a 25ml aliquot of the iron(II) ions solution with the purple manganate(VII) solution.

If the titration results show that 23.7ml of the manganate(VII) solution completely oxidise the iron(II) ions, calculate the % of iron in the sample of steel. 

a)    Calculate moles of manganate(VII) ions from the titration. 

Use n=cV

Therefore n=  0.0005  ×   23.7/1000  =   0.00001185 moles manganate(VII) ions.

b)    Determine the number of moles iron(II) ions in the aliquot from the equation since 5 moles iron(II) ions react with 1 moles manganate(VII) ions.

5Fe2+                +            MnO4   
0.00005925              0.00001185

c)    if there are 0.00005925 moles iron(II) ions in the aliquot then there are 10× this number of moles in the volumetric flask i.e. 0.0005925 moles.

d)    Calculate the mass of iron that this number of moles represents.

Use m  =   n  ×  M  therefore mass Fe2+ ions =  0.0005925  ×  55.9  =  0.03312g.

e)    Therefore the percentage of iron = 0.03312/0.034  ×  100  =  97.4



Example 3:
Titration of iron(II) with ethandioate ions

Follow this link here to details about this titration.



9. Iron and Haemoglobin

Haem is an iron(II) complex with a multidentate ligand.

Oxygen forms a co-ordinate bond to Fe(II) in haemoglobin, enabling oxygen to be transported in the blood.

Carbon monoxide is toxic because it replaces oxygen co-ordinately bonded to Fe(II) in haemoglobin.


You can find out more about this area by following this link: Haemoglobin.

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