These suggested practical activities relate closely to the learning outcomes for OCR AS-level Chemistry, and may be used to develop students’ experimental skills in preparation for practical assessment. However they are not intended to form a complete practical course. It is hoped that they will provide a starting point for centres setting up an AS-level chemistry course, and also provide some ideas for centres with courses that are already running.
Safety
Although great care has been taken in checking the accuracy of the information provided,
Cambridge University Press shall not be responsible for any errors, omissions or inaccuracies.
Teachers and technicians should always follow their school and departmental safety policies.
You must ensure that you consult your employer’s model risk assessments and modify them as
appropriate to meet local circumstances before starting any practical work. Risk assessments will
depend on your own skills and experience, and the facilities available to you. Everyone has a
responsibility for his or her own safety and for the safety of others.
The practicals should be carried out by teachers themselves before they are presented to students. Additional notes relating to each activity in this chapter are given below. These include, where
appropriate, references to the CLEAPSS Hazcards and Laboratory Handbook, both of which
provide model risk assessments and are available to CLEAPSS members.
The notes given below also contain some background information and advice, but in no way should
they be regarded as risk assessments. Teachers and technicians are advised to read the relevant
sections of the CLEAPSS Laboratory Handbook for further advice and guidance. The Handbook
includes a great deal of useful background information and is an excellent source of references.
Practical:Identifying unknown compounds using ‘wet tests’ and spectroscopy
Hazcards 32, 34, 38B, 78, 84A, 84B, 98A
A model risk assessment for the test-tube oxidation of propan-1-ol may be found on Hazcard 84A.
CLEAPSS guide L202 Spectra contains the IR and mass spectra of some 40 common organic compounds; these may be used to supplement or modify this activity.
This sheet can be used as a revision and discussion exercise. The reaction of the carboxylic acid is not strictly on the specifications, but the means of identifying it is quite straightforward and it is a product of the complete oxidation of a primary alcohol. The students are also required to use infrared spectra to identify a carboxylic acid.
The three compounds are:
A2-methylpropan-2-ol
Bpropanoic acid
Cpropan-1-ol
Deductions from wet tests
B is the only compound that turns universal indicator red, so it is the carboxylic acid.
The acidified dichromate will be unchanged by A, so A is the tertiary alcohol.
Only C is left, so that must be the primary alcohol.
COAS Chemistry 1 Teacher ResourcesOriginal material © Cambridge University Press 2005, 20081
15 Practical guidance
Deductions from spectra
Compound A
The infrared spectrum of A shows the typical broad absorption at 3200–3550 cm–1, supporting the identification of an alcohol.
Using the value for the molecular ion peak, the relative molecular mass of A is 74.
There must be a C–OH group, which this corresponds to a relative mass of 29, leaving 45 for the mass of the remainder of the molecule. This corresponds to three CH3– groups; therefore A must be 2-methylpropan-2-ol.
Compound B
The infrared spectrum for B confirms that it is a carboxylic acid because it has a sharp absorption between 1640–1750 cm–1 and a very broad absorption in the range 2500–3300 cm–1.
Using the value for the molecular ion peak, B has a relative molecular mass of 74.
Of this mass, 45 must be due to the –COOH group; therefore the remainder of the molecule has a relative mass of 29. This corresponds to two carbons and five hydrogen atoms; i.e. CH3CH2–. Therefore B is propanoic acid.
Compound C
The infrared spectrum of C shows the typical broad absorption at 3200–3550 cm–1, supporting the identification of an alcohol.
Using the value for the molecular ion peak, C has a relative molecular mass of 60.
Since C is a primary alcohol, part of its molecule is the –CH2OH group, which corresponds to a relative mass of 31. This leaves 29 for the remainder of the molecule, and this corresponds to two carbons and five hydrogen atoms; i.e. C2H5–. Therefore C is propan-1-ol.
COAS Chemistry 1 Teacher ResourcesOriginal material © Cambridge University Press 2005, 20081