Section II (options)

Question 32 – Forensic Chemistry (25 marks)
Chemistry Contexts 2 defines organic compounds as compounds that contain mainly carbon and hydrogen .... By this definition compounds such as CFCs, halons and tetrachloromethane would not be organic compounds (no H) which is not true. However the HSC examiners would probably have accepted this incorrect definition because it was in a widely used textbook.
 (page 439)		 (a) (i) Organic compounds are compounds of carbon (except for CO, CO2 carbonates and bicarbonates, hydrocyanic acid and metallic cyanides).
The test for carboxylic acids is the most clear-cut one of those you are expected to know, so it would be a good one to give. The bromine test for alkenes is probably acceptable (though alkynes and some alkanols give the same result). The sodium test for alcohols is also probably acceptable though carboxylic acids also give the same result.

Remember alkanoic acids and alkanols are subsets of carboxylic acids and alcohols respectively so you could use either term.

(pages 439 and 441-2)

 (ii) For carboxylic acids (or alkanoic acid): add a solution of sodium carbonate to a sample of the substance to be tested; if bubbles of colourless gas (CO2) form, the substance is a carboxylic acid (or alkanoic acid).
OR
For alkenes: add drops of a solution of bromine in hexane (or other suitable solvent) to a sample of the substance to be tested; if the sample decolorises the bromine solution, the substance is an aklene (though it could also be an alkyne or an alcohol).
OR
For alcohols (or alkanols): carefully dry a sample of the substance to be tested by adding granules of anhydrous calcium chloride to it and allowing it to stand for ten minutes or so, then add a small piece of sodium; if bubbles of colourless gas (H2) form the substance is an alcohol (alkanol). (carboxylic acids give the same result)
 
Make sure you look at all the results: using only the first two could have led to the wrong answer.
 		 (b) (i) tallow
Three facts are required: (1) structure of a fatty acid (2) polarity of the acid group and the hydrocarbon chain (3) the dominance of the long hydrocarbon chain and so insolubility.

(pages 351-2 and 458-9)

 (ii) Fatty acids consist of a hydrocarbon chain attached to a carboxylic acid (functional) group: for example,
CH3(CH2)14COOH (palmitic acid).
The carboxylic acid group is quite polar (because of polar C–O, O–H and C=O bonds) and therefore would tend to make the molecule soluble in water; however the hydrocarbon chain is non-polar (non-polar C–C and C–H bonds) and so would tend to make the molecule insoluble in water. If the hydrocarbon chain is long (say > six Cs), its non-polarity dominates the molecule and so the compound becomes insoluble in water. The fatty acids all have long hydrocarbon chains (>12 Cs) so they are insoluble in water.
 
This is another extended response question – 5 marks and not much instruction about what is required.
This is an exam so you had better explain (briefly) what mass spectrometry is – allow say 1 mark for this, Then you need at least four bits of information in your assessment. Include the main use – identifying compounds from cracking patterns– and advantages – quick and small samples – probably three marks. Other uses – molecular weights and isotopic composition – should ensure full marks.

(pages 498-503)

 (c) Mass spectrometry is a technique that identifies substances on the basis of the masses of the positive ions that are formed when the substances are bombarded with electrons.
The most useful forensic evidence that it can provide is the identity of an organic or biological substance based upon its cracking pattern, meaning the masses and intensities of the many peaks that are formed when a complex molecule is bombarded with electrons. A cracking pattern is generally unique to the particular compound and so quite complex compounds can be identified by comparing the cracking pattern of an unknown with those of known compounds.
Mass spectrometry is particularly useful because it can identify substances using quite small samples (a few micrograms). The analysis is quite rapid which is always an advantage for forensic investigations. (The disadvantage is that mass spectrometers are very expensive and require skilled technicians to operate and maintain them.)
Mass spectrometry can also provide the molecular weight of a substance (from the heaviest peak in the cracking pattern) which can also be useful for forensic purposes. In addition it can determine the isotopic composition of an element which in some cases can determine the location from which the sample (such as a soil) came.
Overall mass spectrometry is  very useful in providing a wide range of forensic evidence.
 
These are the two techniques the examiners were looking for. Fractional distillation may separate organic compounds but it requires samples larger than are usually available to forensic scientists.
 		 (d) (i) Chromatography (or paper chromatography, gas chromatography, column chromatography etc)
OR
electrophoresis
There probably is not much choice here; most schools do not have the equipment to perform electrophoresis, so chromatography is the most likely experiment. Briefly describe an experiment you performed. You cannot use the 2 marks, 2 bits of information rule here: you have to describe the actual experiment. However the two marks means you can be very brief – not too much detail. (ii)

We used paper chromatography to separate a mixture of food dyes. A small drop of the mixture was spotted near the bottom of a piece of filter paper, then after it had dried the piece of paper was suspended in a beaker with the bottom of the paper just dipping into the solvent as shown in diagram (a). The solvent crept up the paper by capillary attraction and washed the dyes up the paper at different rates. When the solvent had nearly reached the top of the paper we took it out of the beaker and dried it. It looked as in diagram (b) below.

 

You have to make it clear in your answer that you know the difference between accuracy and reliability. The safest way to do this is to explain the difference at the start. One mark for pointing out that difference leaves only two for the improvements. Probably just one way of improving each would suffice. Improving the accuracy is the hard bit but try to give one way. Giving several for improving the reliability won't compensate for nothing about accuracy. (iii) Reliability means how reproducible the experiment is: if it is repeated several times how closely will the results of the experiments agree with each other. Accuracy means how close is the result of the experiment to the true value or situation: in this case is the number of spots (compounds) detected the actual number present in the sample.
The reliability (reproducibility) of the experiment can be increased by ensuring that (1) the sample spot size is exactly the same in each experiment (2) the height of the start line above the solvent level is the same in each experiment (3) loss of solvent by evaporation is restricted by inverting a bigger beaker over the experimental one (4) the temperature is constant from one experiment to another (5) the solvent is allowed creep up to the same level in each experiment.
The accuracy of the experiment can be increased by changing the solvent until the one that gives the cleanest separation and the maximum number of spots is found. For a given mixture, depending on the polarity of the compounds, some solvents are better than others for producing a good separation.
 
With 7 marks (and 13 minutes) for this question, it needs careful thought before rushing in. Work out what you think the examiners are expecting then plan your answer on that basis.
In an exam before you discuss something you really need to describe briefly what it is (convince the examiner you know what you are talking about).
You should briefly explain what DNA analysis is and how it is used in forensic chemistry, and then discuss its advantages and limitations and perhaps compare it to other techniques. You might guess that there would be 2 or 3 marks for describing the technique, 2 or 3 for its advantages, perhaps 2 for its limitations, and maybe 1 for comparing it to some other technique. This would suggest 2 or 3 bits of information about advantages and 2 bits about limitations and/or comparisons. You would need more than 2 or 3 facts to describe the technique but do not go overboard: the description would not get more than 3 marks maximum.
The temptation here, if you have studied DNA analysis carefully might be to give a lot of information about what it is and how it is done, but that would not get you full marks, no matter how detailed and accurate your description was – the question was discuss so advantages and limitations and comparisons are essential.
It is important to mention the high sensitivity of DNA analysis and to point out that this arises because of the PCR amplification that is used.
(pages 489-93)
 		 (e) DNA (deoxyribonucleic acid) is the substance that carries the genetic code in all living matter. Each person's or organism's DNA is unique. In DNA analysis for forensic purposes (which is different from DNA analysis to study genetic makeup or defects) DNA is extracted from the sample, selected segments (10 in the procedure currently used in Australia) along  the chain are 'grown' (using PCR, polymerase chain reaction) to produce about 100 million copies of each of the selected segments. Electrophoresis is then used to separate and measure the lengths of these segments. If the lengths of the segments in the crime-scene sample are the same as the lengths in a sample from a suspect (or from a data base), then the identity of the person from whom the crime-scene sample came is identified with an extremely high degree of certainty (better than 1 in ten billion).
The advantages of DNA analysis are first that it is extremely accurate in identifying the source of the sample (though that may not necessarily be the culprit of the crime) and secondly that it is extremely sensitive – only a minute sample is required (because of the PCR amplification). Another advantage of DNA analysis is that it can be used to identify the father of a child in disputed paternity cases (Half the analysed fragments in the child's DNA will match fragments from the mother while the other half will match fragments from the father.)
The disadvantages are that (1) it is quite time-consuming to perform, (2) it requires a specialised laboratory containing expensive equipment with well-trained technical staff and (3) it can only be applied to certain biological samples such as blood, semen, saliva or hair follicles (not the hair itself).
Some other techniques such as mass spectrometry and infrared spectroscopy can be applied to a wider range of forensic samples (i.e. not restricted to biological samples), but where DNA analysis is  applicable it is much more sensitive than those other techniques.