Finding the empirical formula of aluminium chloride.

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                        A-Level Chemistry Coursework Investigation:-

        Finding the Empirical Formula Of Aluminium Chloride

                                                        

                                                                      By Yasir Al-Wakeel

        Introduction:

        

   The purpose of this experiment is to find the empirical formula of a sample of anhydrous

aluminium chloride by titrimetric analysis.

   

   The empirical formula of a compound, as opposed to the molecular compound, is the

simplest formula which represents its composition.  This shows the elements present and the

ratio of these elements.  The molecular formula, on the other hand, is a simple multiple of the

empirical formula and shows the actual number of atoms of the different elements in one

molecule of the compound.  Thus ethene, a homologue of the alkenes, has a molecular

formula of C2H4 , whilst the empirical formula of ethene is CH2.  To find an empirical formula

the ratio of the amounts (in moles) of the elements is required, which in this particular

experiment is to be found by means of titration.

   Titration is a type of volumetric analysis used to find the amount of a substance in solution.  

A ‘standard’ solution, or solution of known concentration, is reacted with a certain volume

of the solution under investigation and the volume of the standard required to complete the

reaction is recorded.  In some cases the end-point of the reaction is self-indicating, yet in

most cases, including this experiment, an indicator is required.  Indicators change colour to

show the reaction is complete.   The presence of colour and colour change are usually due

to the presence of transition metals, as ions, in the indicator. Transition elements are those in

the d-block of the periodic table and can be defined as an element with at least one ion with

a partially filled d-subshell  The ‘phenomenon’ of colour is related to the incompletely filled

d-orbitals in the transition metal ion.  In short, light photons falling on transition element

compounds interacts with the d-electrons, exciting them and causing electron transitions.  

Some of the wavelengths in the light are absorbed leaving the complementary colours to be

seen. In our case we are using potassium chromate indicator- here the chromium is the

transition metal.    The equipment necessary for a titration are illustrated below:

                                A: Conical flask- this is the reaction vessel

B: Burette- a graduated tube with a tap and is used to    

     deliver the standard solution into the reaction vessel

C: pipette-filled by suction, it is used to transfer a known  

     volume of the solution to be analysed into the flask

D: pipette safety filler and is the suction device used to  

     fill the pipette

   

        Although the purpose of this investigation is to find the empirical formula of

aluminium chloride by experimentation, we can also determine the molecular and hence

empirical formula of aluminium chloride hypothetically.  Indeed this shall prove of great use

as it will help in explaining the various reactions and their stoichiometry as well as allowing us

to verify the accuracy of our results.

        Aluminium, together with the other elements in group III, are predominantly trivalent

and showing an oxidation state of  +3.  Aluminium forms ions with a charge of 3+ by losing

its three outermost electrons (3s23p1):

                Al                        Al3+    +     3e

        Chlorine, together with other elements in group VII, gain an electron forming:

        Cl2  +    2e                         2Cl-

        Thus combining the two we get:

         2Al  + 3Cl2                          2AlCl3

However, the atypically high charge density of the Al3+ ion results in the polarisation, or

distortion of the chloride electron cloud.  Hence, according to Fajan’s rule of bond type,

aluminium chloride exhibits covalent bonding (to a certain extent).  The aluminium chloride

may be represented by the following dot and cross diagram:

        Yet, as can be seen above, aluminium only has six electrons in its outer shell and has

not yet attained the stability of its nearest noble gas element argon.  In order to complete the

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outer octet, aluminium chloride molecules dimerise forming Al2Cl6 .  This being an example

of dative covalent bonding, where a bond is formed by the sharing of a pair of electrons

both of which are provided by a chlorine atom of another aluminium chloride molecule.  The

diagram below illustrates the structure, where an arrow denotes a dative covalent bond

pointing from donor to acceptor:

        Hence the molecular formula for the anhydrous aluminium chloride is Al2Cl6 , which

written in its empirical form is AlCl3.

Method and ...

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