Chemistry Teaching Technician (Term Time Only)
Ref: N105R
Further information for applicants:
Typical work activities:
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liaising with academic staff to discuss timetables, equipment requirements and work
plans;
setting out equipment and preparing chemicals and stock solutions before lessons;
ensuring that equipment is properly cleaned and that chemicals, glassware and other
materials are appropriately stored;
ordering supplies and managing the stock control of chemicals and equipment;
maintaining and repairing equipment and laboratory apparatus;
supporting the work of lecturers during laboratory teaching sessions and giving
technical advice;
preparing student attendance sheets;
ensuring that all health and safety procedures are understood and followed
correctly;
Applicants are expected to have an understanding of organic and inorganic chemistry and
have the relevant experience in providing technical support in science laboratories.
Typical examples of practical activities to be supported by the role holder:
1) Introduction to Organic Chemistry
Formation of an alcohol by reduction of a ketone:…………………………………
Reduction of benzophenone (diphenylmethanone) with sodium borohydride
Sodium borohydride is a mild reducing agent, capable of converting aldehydes and ketones
into the corresponding alcohols but it is compatible with hydroxylic solvents such as
alcohols and even water, unlike other, more powerful reagents. In this experiment a ketone
is reduced to an alcohol and the progress of the reaction is monitored by thin layer
chromatography (TLC) to ensure that enough time has been allowed for complete reaction.
Materials
Benzophenone ....................................................................... irritant
Sodium borohydride ............................................................. harmful
Ethanol .............................................. flammable liquid, intoxicating
Hydrochloric acid ................................................................corrosive
Petroleum ether ..................................................... flammable liquid
Prepare a 5 cm wide piece of foil-backed TLC plate and mark it out (with a soft pencil) to
take at least 5 samples to run in parallel. The first ‘lane’ will be for a solution of the starting
ketone just prior to NaBH4 addition, and remaining ‘lanes’ will be for samples taken 2, 6, 10
and 16 minutes after the borohydride addition. A finely drawn out glass capillary is used to
spot samples on to the TLC plate (see below).
Reaction
Weigh out about 120 mg benzophenone (a ketone that absorbs UV and is a component of
many suntan creams) in a small specimen tube and 30-35 mg sodium borohydride (NaBH4)
in a different 6 ml specimen tube. Add 0.3 ml ethanol to the ketone and dissolve it, with
heating if necessary, and then cool it. Spot a sample on the first lane of your TLC plate with a
finely drawn out glass capillary. Add 0.5 ml ethanol to the borohydride and as soon as it has
largely dissolved add to it the ketone solution. Spot samples of the reaction mixture onto
the TLC plate after 2, 6, 10, and 16 minutes. Develop the plate in a closed jar containing a
mixture of 10 parts of 60-80 petroleum ether to 1 part of ethyl acetate. When the plate is
developed, remove it and shake it dry then examine it under a UV lamp to check that the
reaction has gone or is proceeding to completion. Mark the spots you see under the UV light
with a soft pencil. Don’t discard you TLC plate – when dry use adhesive tape to stick it into
your practical book
Work-up, isolation and purification of the product
When you have evidence from your TLC that the starting ketone has reacted, add 3 ml of
2M hydrochloric acid to the reaction mixture, initially dropwise until the fizzing stops, and
then add the remainder. Cap the tube and cool it in an icebath. Filter the solid product on a
Hirsch funnel under suction, dry it and recrystallise the crude solid by adding about 1 ml 6080 petroleum ether, warming to dissolve it (don’t forget to use an anti-bumping granule)
and then allowing the solution to cool and crystallise. Filter the recrystallised solid, suck it
dry, weigh it and determine its melting point (guideline 45-75 . Calculate your % yield.
Label your product and place it in the appropriate box.
2) Transition Metal Chemistry
THE INFLUENCE OF LIGAND FIELD STRENGTH UPON THE SPECTRA OF Cu(II) COMPLEXES
The visible spectrum of an aqueous solution containing Cu 2+ ions consists of a single
unsymmetrical broad band. This is due to the electronic transition
t2g6eg3  t2g5eg4 in the ‘octahedral’ complex ion *Cu(H2O)6]2+. As ammonia is added to
this solution water molecules are replaced by ammonia and the whole family of complex
ions [Cu(H2O)n(NH3)6-n]2+ n=1-6 can be prepared in solution.
This exercise is to study what effect ammonia replacement has upon the spectrum.
Procedure (you will work in groups for this experiment)
The following solutions are necessary for this experiment.
(A)
100 ml 1M Cu(NO3)2.3H2O in water
(B)
100 ml 2M NH4NO3
(C)
Standardised solutions of NH4OH 1M 2M and 3M respectively.
Using these solutions prepare aqueous solutions of the ions
[Cu(H2O)n(NH3)6-n]2+ as follows:
(1)
To 5 ml copper(II) solution (A), add solid ammonium nitrate until saturated. Then
add slowly 5 ml 1M ammonium hydroxide. Add further solid ammonium nitrate to
resaturate. Dilute 1 ml of this solution to 25 ml with 2M ammonium nitrate. The
resulting solution contains [Cu(H2O)5(NH3)]2+.
(2)
Repeat (1) twice but use 2M and 3M ammonium hydroxide instead of the 1M
solution and thereby prepare solutions of [Cu(H2O)4(NH3)2]2+ and
[Cu(H2O)3(NH3)3]2+.
(3)
Add 1 ml of conc. ‘0.880’ ammonia to 1 ml copper(II) solution (A). Dilute this
solution to 50 ml with water giving [Cu(H2O)2(NH3)4]2+.
(4)
Dilute 0.5 ml copper(II) solution (A) to 25 ml with conc. ‘0.880’ ammonia to give
[Cu(H2O)(NH3)5]2+.
(5)
Dilute 0.5 ml copper(II) solution (A) to 25 ml with water. [Cu(H2O)6]2+ remains in
solution
Record and print the UV/Visible spectrum of each of the above solutions over the
wavelength range (x-axis) 400-900 nm and absorbance range (y-axis) 0.0-2.0 on the Helios
spectrophotometer. The spectra should be recorded using 1 cm pathlength cells. [For the
most weakly coloured solutions, however, a 4 cm pathlength cell may be necessary]. Label
each spectrum and note clearly on it the value of the wavelength (in nm) of the maximum
absorption peak (
max).
Compare the
values for each solution and correlate the max values with the number
max
of molecules of ammonia in the complex. Comment upon the variations in max - to what
extent is the observed trend in accord with the relative positions of water and ammonia in
the spectrochemical series? What do you note about the position of max for
[Cu(H2O)(NH3)5]2+ and how can this be explained? Comment on the shapes of the
absorption bands – why are they so broad? Suggest a method for preparing [Cu(NH3)6]2+.