Chemistry
PHILIP PONDER
May 03, 2010
A new study design for Chemistry was introduced in 2008, and with it came a far more comprehensive data booklet than had previously been supplied with examinations. Students should therefore expect that many questions will involve the use of this data booklet, and this article will highlight the relevant sections and offer some advice on how to use them.
1. Periodic Table of the Elements
The Chief Assessor's report for last year's June exam suggested that many students seem unfamiliar with the rule for determining the number of significant figures in an answer derived from the addition (or subtraction) of data.
Determining the relative mass (Mr) of potassium hydrogen carbonate (KHCO3) provides an example of the use of this rule. Addition of the relevant relative atomic masses (RAMs) from the table gives a relative mass of 100.1 [39.1 + 1.0 + 12.0 + (3 x 16.0)]. This value is accurate to the first decimal place and so will have 4 significant figures (regardless of the fact that the RAM of H only has 2). This will be important in the likely event that the value is subsequently used in a calculation involving multiplication or division, where the number of significant figures in the answer should be the same as the number of significant figures in the bit of data that has the least number of significant figures (ie. the least accurate bit of data).
Eg. mass (KHCO3) = 4.06 g
Mr (KHCO3) = 100.1 (Molar mass = 100.1 g mol-1)
n(KHCO3) = 4.06 / 100.1 = 0.0406 mol
Note that the first two zeros in the answer are not significant, and that the answer is rounded off to 3 significant figures.
2. The Electrochemical Series
This is mostly relevant for Unit 4, but the writing of balanced half equations for redox reactions is an important skill in Unit 3. A required half equation may be present in the data book, but it is noteworthy that half equations for two important strong oxidants are not provided. You should be able to use your rules to write half equations for acidified MnO4- ions being reduced to Mn2+, and acidified Cr2O72- ions being reduced to Cr3+.
3. Physical Constants
Because the Gas Constant (R) is given in SI units, you need to be careful regarding the units used for the variables in the Universal Gas Equation, pV = nRT. Technically, volume (V) should be in m3 and pressure (p) in Pa (N m-2). However, because R still has the value 8.31 when its unit is kPa L K-1 mol-1, the equation will also 'work' if volume (V) is in litre (L) and pressure (p) in kilopascal (kPa).
The unit of Density (g mL-1) should remind you that d = m/V.
Four of the constants are only relevant for Unit 4 (electron charge, F, Kw, and specific heat capacity of water).
4. SI Prefixes
Make sure you are familiar with the use of these.
5. & 6. NMR Data
This appears daunting, but in many cases chemical shifts will only be used to help you confirm a structure that has already been deduced from other information. This material was not included in previous study designs, so exams that predate 2008 will contain no examples on which to practise.
The best advice is probably to study as many examples as possible of NMR spectra of simple organic molecules in order to 'get a feel' for their interpretation.
7. Infrared Absorption Data
Wave numbers in the 'fingerprint' region (< 1500 cm-1) are unlikely to be helpful. Learn to recognize the characteristic 'trough' caused by the carbonyl group (C=O) and the difference between the trough due to O-H in carboxylic acids compared to that produced by alcohols. The characteristic trough caused by the C-H bond (which may be masked by the O-H trough in carboxylic acids) will be present in the IR spectra of most organic molecules and hence will usually be of little use in classifying the compound.
8. α - amino acids
These are the amino acids that are biologically significant, all having an amino group and a carboxy group attached to the same carbon atom. The structures given are essentially semi-structural in nature; it would be good practice to check that you can turn them into:
i. full structural formulae, showing all bonds
ii. molecular formulae (in particular, the ones containing ring structures).
9. Formulas of some Fatty Acids
Prior to the exam, it would be a good idea to ensure you can classify each of these as one of:
- saturated (fitting the formula CnH2n+1COOH)
- mono-unsaturated (2 H's less than the corresponding saturated fatty acid i.e. CnH2n-1COOH)
- polyunsaturated (4 or more H's less than the corresponding saturated fatty acid)
10. Structural Formulas of some Important Biomolecules
A key skill to practise is to show that you can combine 3 fatty acid molecules with a glycerol molecule in a condensation reaction that produces a triglyceride + H2O, and also the reverse of this process, taking a triglyceride and hydrolyzing it to form glycerol and 3 fatty acid molecules.
You could consolidate this theme by showing how the sucrose structure can be hydrolyzed into glucose (6-sided ring) and fructose (5-sided ring).
An important feature of the biomolecules is their solubility in water, a property you should be able to explain by referring to their ability to form hydrogen bonds with water molecules.
The DNA structure is incredibly complicated, so it is important that you focus on the key aspects of its building blocks, the 2 backbone chains of alternating deoxyribose and phosphate units, with the double helix structure maintained by hydrogen bonding between guanine-cytosine (G-C) and adenine-thymine (A-T) 'base' pairs ('GC3, AT2' is a handy mnemonic). However, beware of referring to 'double' or 'triple' bonds between the nitrogen bases, as this might imply that you think the bonding is covalent.
11. Acid-base Indicators
You should be able to use this table to choose an appropriate indicator for a given acid-base titration. This may involve interpretation of a pH curve for the titration, or consideration of possible hydrolysis reactions of the chemical species present at the equivalence point.
The table can also be used to determine the colour of the indicator at the endpoint.
The Ka values referred to in the last column (and in No.12) will only become relevant in Unit 4.
13. Although molar enthalpy of combustion is a Unit 4 topic, combustion reactions of biofuels are highly relevant to Unit 3.
Beware: It is easy to overlook the oxygen atom in an alkanol when writing a balanced equation for its combustion. When considering the symbol of state for the alkanol, keep in mind that an aqueous solution will not usually burn!
When tackling short answer questions such as those that begin "Describe how ..." or that ask you to "Explain why ...", you are strongly advised to check how many marks are allocated to the question and to give your answer in 'point form'.
A classic application of this approach would be the answer to Q. 2c on the VCAA June 2009 examination, which asked for a description of how the purity of a product could be quantitatively determined by a particular spectroscopic method. Many students gave long-winded, tortuous and often irrelevant explanations, but the examiners report makes it clear that to obtain full marks, a student needed to provide at least 3 points out of a possible 4 that would have been present in a very good answer.
Philip Ponder is a VCE assessor.
http://www.education.theage.com.au/cmspage.php?intid=154&intversion=35
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