Sept/Oct 2002 Syllabus changes

The revised syllabus and/or a version of the original syllabus with the changes marked in it can be downloaded from the Board of studies web site (www.boardofstudies.nsw.edu.au). 

Conquering Chemistry is still a very close match to the revised syllabus though certain sections and exercises, detailed below, can now be omitted. The first of the two sub-sections below is for students: it tells you what you can omit from CCHSC. The second sub-section is primarily for teachers: it gives more detail about what the changes are and how they affect CCHSC.

In both sub-sections LC, MC and RC mean left, middle and right column respectively and DP means dot point. The references are to the original syllabus (1999 or March/April 2001)

1. For students

As a result of these syllabus changes you should omit or change the following:

2. For teachers

The changes to the syllabus and their consequences for Conquering Chemistry are set out in the following table.

3. Modified Exercise 22 in Chapter 13 (page 457)

4. Chromatography

Gas-liquid chromatography and high performance liquid chromatography have been added to the syllabus to replace u.v and i.r spectroscopy, electron spectroscopy for chemical analysis, scanning tunnelling microscopy and atomic force microscopy. The following account should provide adequate information for HSC purposes.

Introduction

Chromatography was first developed by the Russian botanist, Tswett in 1903. He wanted to separate mixtures of colouring matter he had extracted from leaves, particularly chlorophylls. Finely divided calcium carbonate was packed into a column and the mixture of substances in a petroleum ether solution was poured on to the top of the column. By continuously washing the column with solvent Tswett found that the mixture slowly passed down the column and separated into its various coloured components.

	Fig 1 Column chromatography: (a) an acetone extract of colouring matter from leaves is added to the top of a column of alumina (shown as green), (b) the column is washed with a suitable solvent: separation starts (c) the different compounds are fully separated by continued washing.

Figure 1 shows a modern version of Tswett's pioneering experiment. Washing the column continues until the components elute (wash out) one at a time from the bottom of the column. They can be collected in separate beakers. The separation in this form of column chromatography is based upon different strengths of adsorption on the solid of the substances to be separated. Passage of solutes down the column results from repeated adsorption from, and desorption back into, the solvent. Substances which adsorb on to the solid only weakly desorb back into the solvent readily and therefore pass through the column rapidly. Substances which adsorb strongly pass through the column only slowly. The solid is the stationary phase and the liquid or solvent is the mobile phase. When the separation is based upon adsorption, the technique is called liquid-solid chromatography.

The name, chromatography, (literally colour writing) arises because the technique was originally developed for the separation of coloured substances ("chrome" means "colour"). Today as stated on page 476 of CCHSC chromatography is the separation of substances based upon their differential distribution between two phases, one stationary, the other mobile.

Although Tswett invented column chromatography in 1903, it did not gain widespread use until the 1930s when it was adopted by organic chemists. This form of column chromatography is still widely used for preparative and purification purposes.

Between 1938 and 1941 the Americans, Martin and Synge, developed an important variation which became known as liquid-liquid chromatography or partition chromatography. In this technique the stationary phase is a liquid film adsorbed on the surface of a finely divided solid (still in a column as in Figure 1). The substances to be separated are dissolved in a solvent (which must be immiscible with the stationary phase) and this solution is again poured on to the top of the column and washed down with pure solvent (the mobile phase). Separation is based upon different solubilities of the substances in the stationary and mobile phases. Substances which are very soluble in the mobile phase but only slightly soluble in the stationary phase pass down the column rapidly; substances which are very soluble in the stationary phase but only slightly soluble in the mobile phase move through the column very slowly. In this way a separation is effected.

Martin and Synge devised the technique to separate mixtures of amino acids (as acetates). Their stationary phase was water adsorbed on silica gel; the mobile phase was chloroform. They received a Nobel prize for this work in 1952.

If a suitable detector is fitted to the outlet of the column, such as a refractive index monitor or uv absorption cell, colourless compounds can be separated and analysed. Use of such detectors can provide quantitative as well as qualitative analysis.

Paper chromatography, as described at the start of this section (i.e. on page 476 of CCHSC, is a form of partition chromatography.

Thin layer chromatography, TLC, is a variation of column chromatography. Here the stationary phase is a solid spread on the surface of a glass plate or stiff sheet of aluminium foil. The sample to be analysed is spotted on to the plate which is then stood in a shallow pool of solvent in an enclosed container as shown in Figure 14.1 on page 477 of CCHSC. By capillary attraction the solvent creeps up the plate and elutes the sample. Different compounds travel up the plate at different speeds. Colourless compounds can be located by fluorescence (holding the plate under a uv light) or by exposing the plate to a suitable colouring agent.

TLC is a cheap, quick and easy way of qualitatively determining what compounds are present in a mixture. Spots in an unknown are identified by comparison with spots from known compounds treated in exactly the same way. Exercise 1 below shows how TLC can be used to identify the compounds in mixtures of analgesics (painkillers).

It is the next two sub-sections that are really important for the HSC.

A major development in liquid-solid and liquid-liquid chromatography was the introduction of high pressure pumps in the 1970s. These could speed up the passage of the mobile phase through columns packed with very small solid particles (which lead to better separations). The technique was originally called high pressure liquid chromatography (HPLC) but the name got transformed by the commercial marketeers into high performance liquid chromatography. 

	Fig 2 A typical high performance liquid chromatograph

A typical instrument is shown in Figure 2. The column is usually a metal tube 15 to 30 cm long and about 3 to 10 mm internal diameter. A solution of the sample to be analysed is admitted to the top of the column then a pump pushes solvent through the column at high pressure to wash the components of the sample down the solid packing (solid or liquid coated on a solid). A suitable detector such as a refractive index monitor or an ultraviolet light absorption cell detects the various components of the mixture as they come out the bottom of the column. It does this by feeding its output to a computer which displays the signal as a function of time after the start of the analysis. Typical HPLC traces are shown in Figure 3. Substances are identified by the time taken to wash through the column (called the retention time) under controlled conditions such as temperature and solvent flow rate or by comparison with standard known samples run under identical conditions. From the size (height or area) of the peak the actual amount of the substance can be estimated

	Fig 3 Two typical HPLC chromatograms: (a) analysis of sugars in apple juice (b) analysis of a mixture of alkanoic acids

HPLC is a sensitive and accurate method of identifying the compounds present in a sample (mixture) and for determining their relative amounts. For preparative work, that is for purifying and collecting samples, column chromatography is used (because big columns with large amounts of stationary phase are needed to handle big samples).

In 1952 Martin and James (the same Martin as above) developed another form of chromatography in which the mobile phase was a gas. The substances to be separated (or analysed) are vaporised into a flowing stream of a gas such as nitrogen or helium. This is called gas chromatography of which there are two types. In gas-solid chromatography separation is based upon selective adsorption of the compounds from the gas phase (mobile phase) on to the solid (stationary phase). In gas-liquid chromatography (GLC) separation is based upon different solubilities of the compounds from the gas phase into a liquid adsorbed on a finely divided solid (stationary phase). In the last two decades capillary GC columns have been introduced. These consist of very fine silica capillary tubes about 0.1 to 0.5 mm in diameter, often tens of metres long, with the stationary liquid adsorbed on the inside surface of the tube.

Gas chromatography developed very rapidly in the decade after its first introduction, because it proved extremely useful to the petroleum industry for analysing the complex mixtures of organic compounds which make up crude oil and its refined products.

Gas chromatography is described briefly on page 189 of CCHSC. 

In a typical gas chromatograph the sample to be analysed is introduced by a syringe into the hot flowing carrier gas as shown in the figure at the top of page 189 of CCHSC. The sample vaporises immediately and the analysis proceeds by repeated transfer of the components to the stationary phase and back to the gas again. The more strongly a substance is adsorbed or the more soluble it is in the liquid film, the more slowly it moves through the column. At the end of the column is a detector which signals when a component is leaving the column (ie., passing through the detector). It also provides a measure of the amount of the compound present. The diagram on page 189 shows a chart recorder for displaying the chromatogram; today a computer is generally used  to show the trace and to estimate the relative amount of each component in the mixture (from the relative areas of the peaks). As with HPLC compounds are generally identified by the time (after injection) that they take to reach the detector (the retention time) when conditions are carefully controlled. This means that pure samples of all likely compounds in the mixture must be available.

As with HPLC, GLC is a very sensitive method of analysing mixtures both qualitatively and quantitatively. Some typical chromatograms are shown in Figure 4.

	Fig 4 Gas chromatograms of (a) simple hydrocarbons (b) simple alkanols  (c) some common insecticides (d) some commonly abused drugs

Finally

Chromatographic techniques were originally developed to separate mixtures. The original gravity-fed columns required quite large samples, of the order of milligrams or more. TLC and paper chromatography could handle samples down to micrograms. Modern-day GC and HPLC can analyse samples in the nanogram range and lower.

Hence HPLC and GLC have become major methods for analysing trace constituents and pollutants. Typical analyses include detection of pesticide residues in beef and other foods, illegal drugs in athletes, and trace pollutants in air and water (aromatic hydrocarbons, CFCs, dioxins, vinyl chloride and so on).

In summary the main chromatographic techniques are thin layer and paper chromatography for the cheap and rapid qualitative identification of substances, column chromatography for separation and purification of moderately large quantities of materials, HPLC for analysis of small quantities of non-volatile substances, gas chromatography (GC and GLC) for analysis of small quantities of substances which vaporise at 100 to 250° C. Another form of chromatography is ion exchange chromatography which separates mixtures on the basis of the charge they carry: it is widely used for analysis of amino acids.

Exercises on chromatography

Choosing an option

Some aspects of choosing the option to study were discussed in the Shipwrecks and Salvage option (click on it to go to). Those comments were made before the Sept/Oct syllabus changes so may not be entirely appropriate now.

Options questions in the 2001 HSC exam

(Comments on the 2002 exam later)

Emphasis on recall of information
Although there was a strong emphasis upon regurgitating memorised information in the whole of the 2001 examination paper, this emphasis was strongest in the options questions: out of the 25 marks, between 21 (Industrial chemistry) and 25 (Shipwrecks and salvage and Biochemistry of movement) marks were for recall of information type questions. In the specimen HSC paper all 25 marks came from factual recall questions for four options (with 16 for such questions in Industrial chemistry). It is therefore important that students do everything in the student activity column and prepare themselves to describe and discuss what they did. I think it is regrettable that a chemistry exam has become a test of students ability and/or willingness to memorise great slabs of information.

Importance of experiments
In the 2001 paper each option question contained a significant segment on a compulsory experiment – not just describe the experiment performed but assess its accuracy and reliability (in three options) and discuss safety aspects (in the other two). This is a clear indication of the continued importance that examiners place on laboratory work; students should prepare themselves to answer similar questions on all the compulsory experiments.

Extended response questions
An innovation in the 2001 exam (and in the specimen paper) were questions that carried a large number of marks (5 to 7) but which do not give specific instructions about what was required – so-called extended response questions. These are the hardest questions on the paper because students have to work out what the question really wants. Fortunately the questions of this type in the 2001 paper came from the student activity column of the syllabus.
Some advice about planning and answering such questions is given in Answers and Comments for the 2001 HSC exam paper.

Accuracy and reliability
The meaning of these two terms is explained in  Answers and Comments for the 2001 HSC exam paper.

Answers to the HSC exam question
A complete working of the 2001 HSC exam question along with some advice about planning answers to the segments of the question are given in  Answers and Comments for the 2001 HSC exam paper.
 

Molecular models and computer simulations

Students are required to perform first-hand investigations using molecular model kits, computer simulations or other multimedia resources 

Some molecules for which space-filling and ball-and-stick models can be made using a fairly basic molecular model kit are listed below. Photographs of these models (using a Molymod model kit) along with their structural formulae are shown in the Molecular Model Photo Album (click on the name to open it).

The HSC examiners seem intent upon examining material in the student activity (right) column of the syllabus document. Examining this activity could present a challenge but perhaps one possibility is:

Name and draw the structure of two compounds relevant to the Forensic Chemistry option for which you made molecular models or for which you carried out computer simulations or viewed other multimedia presentations. (2 marks)
What benefits did you derive from this activity or alternatively why did you consider this activity to be an inefficient use of valuable time? (3 marks)

The benefits of model building for moderately complex molecules such as those listed above are (a) it gives you some idea of what actual molecules look like, particularly how crowded together atoms can become in big molecules (b) it allows you to see molecules in three dimensions instead of trying to visualise them from two dimensional structures (c) it gives you experience in relating chemical structures to the actual appearance of molecules (d) it shows you how 'floppy' some big molecules can become and how many different arrangements of the molecule are possible without changing the geometry around any particular atom. 

(At a later date I'll try to provide some guidance in finding suitable computer simulations of model building on the internet.)

(Will add more material to this option at a later date)