Section II (options)
Question 29 Industrial Chemistry (25 marks) Simple description of a process you need to know about.
(p 331-2)
(a) (i) Pipe 1 compressed air
Pipe 2 superheated water
Pipe 3 liquid sulfur and waterIf you do not remember the melting point of sulfur, you can get around by writing:
Water is heated under pressure to a temperature that is significantly above the melting point of sulfur ...(ii) Sulfur has a melting point of 113oC. Superheated water (water at high pressure and temperature) has a temperature well above this, so it can melt the sulfur which then disperses as droplets in the water; the compressed air forces this mixture to the surface. When the mixture cools solid sulfur separates out, as sulfur is not soluble in water.
.For two marks at least two issues are needed, but give more if you can. The five given here are not all necessary. Even if you have not memorised a set of specific environmental issues, use your imagination to think up some based on what you know is happening during the process. While this answer mentions sulfur dioxide, under some conditions hydrogen sulfide could be formed and this is also a serious air pollutant.
(p. 332)
(iii) The high temperature of the water and compressed air with its high concentration of oxygen can cause some oxidation of sulfur to sulfur dioxide, a dangerous air pollutant, which may escape to the atmosphere. The compressed air may pick up other volatile impurities from the sulfur deposit and these could be released into the atmosphere. The hot water may dissolve impurities from the sulfur deposit and these could be discharged into natural waterways. There could be thermal pollution if the process water is not fully cooled before discharge into natural waterways. There could be significant damage to flora and fauna around the mine site. There could be future land subsidence because of the empty cavities left underground by the removal of the sulfur.
Your procedure should be sufficiently detailed to show that you actually did the experiment. It should also include your observations. A diagram is desirable though probably not essential in view of the few marks allotted.
(p.349-51)(b) (i)
A pair of graphite electrodes was suspended in an aqueous sodium chloride solution containing a few drops of phenolphthalein indicator. A voltage supply was connected to the electrodes and the voltage increased from zero until bubbles of gas started to appear at both electrodes. A pink colour developed at the cathode (negative electrode) showing that the solution was becoming alkaline.
We performed the experiment with two different concentrations of sodium chloride solution. With the concentrated solution (> 1 mol/L) the gas formed at the anode (positive electrode) has a distinctive smell (chlorine) whereas with the dilute solution (< 0.1 mol/L) the gas was virtually odourless (oxygen).
Identify the reactions that occur ... and give equations ... implies that some verbal description is required in addition to the equations. 2H+(aq) + 2e– ® H2(g)
is acceptable for the cathode reaction as is
2OH–(aq) ® O2(g) + 4H+(aq) +4e–
for the anode reaction.
(p.349-51)
(ii) At the cathode water is reduced to hydrogen gas:
2H2O(l) + 2e– ® H2(g) + 2OH–(aq)
At the anode with concentrated solution chlorine gas is produced by oxidation of chloride:
2Cl–(aq) ® Cl2(g) + 2e–
At the anode with dilute solution water is oxidised to oxygen gas:
2H2O(l) ® O2(g) + 4H+(aq) + 4e–
(p. 349-51) (iii) Molten sodium chloride would need to be used (instead of a solution).
Although the term, reaction quotient, does not appear in the HSC syllabus document, it is a useful term, particularly for exercises such as this one. You can avoid using it by setting out this exercise as
(p. 318-25, particularly Example 3)
(c) (i) To determine whether a reaction is at equilibrium or not we calculate the reaction quotient Q for the conditions given and compare its value to the equilibrium constant.
Explain, do not just state how to do it. Temperature and pressure are sufficient. The last sentence is not necessary but put it in if you have time.
(p. 328-30, 117-9)(ii) Lower the total pressure and increase the temperature.
Because this reaction involves 2 moles of gas ® 4 moles of gas, by Le Chatelier's principle, if we lower the pressure the reaction will move in the direction that increases pressure, that is to the right. Because the reaction absorbs heat as it goes from left to right, if we raise the temperature, the reaction will move to the right to take in heat and so minimise the temperature increase.
We could also use an excess of the cheap reactant, steam, to force a greater proportion of the expensive reactant, methane, to products.
With 7 marks for this question, probably there will be 3 or 4 for chemical composition, 2 for uses and 2 for environmental impacts. While the marking schemes in the examiners' report are expressed in more general terms than this, dividing up the marks gives you an indication of what depth is required for each part of the question.
Note that you are asked to compare these types of detergents, so point out similarities and differences.
Even if you are not able to give the structures of specific detergents, give a schematic drawing of the structures (similar to the three used in the answer opposite).
Note that there is an error in CCHSC p.367 in the structure of the sulfonate in two places (an extra O between the R tail and the SO2 group; it is listed on the errors page of the website).
An alternative structure for dodecyl alcohol ethoxylate is:
Uses are just something you have to know.
For environmental effects note the have had in the question, not just currently have, so you need to mention that the early anionic detergents caused problems for lakes and rivers in the 1960s and early 70s before the present-day linear-chain anionic detergents were developed. Since the word detergent is being used here to mean surfactant as opposed to cleaning mixture (p.364), there is no need to include the phosphate problem but there is no harm in mentioning it.
The answer given here is a bit long for eleven minutes, but does give an indication of what you should be aiming at.
(p. 367-70)
(d) Chemical composition All three classes of synthetic detergent have a long hydrocarbon chain attached to a polar or ionic head:
Their detergent properties arise because the hydrophobic (water hating) hydrocarbon chain dissolves in fats and oils while the hydrophilic (water loving) polar or ionic head dissolves in water.
Anionic synthetic detergents are alkylbenzene sulfonates. an example is the sodium salt of laurylbenzene sulfonate:
Cationic synthetic detergents are alkyl ammonium cations – compounds in which the 4 Hs of the ammonium ion have been replaced by alkyl groups at least one of which is along-chain group. Schematically:
An example is cetyl trimethyl ammonium bromide:
In non-ionic synthetic detergents the long hydrocarbon tail is attached to a polar head containing several O atoms and ending in an alcohol group. Schematically
These compounds are called ethoxylates or polyethylene ethers. An example is dodecyl alcohol ethoxylate:
Uses. Anionic detergents are used for general cleaning both domestically and in industry, particularly in laundry detergent mixtures and as soap substitutes. Non-ionic detergents are used in low-foam applications such as in dish-washing powders and for front-loading washing machines. They are the usual detergents in paints, pesticides and cosmetics. Cationic detergents are used in hair conditioners, fabric softeners, disinfectants and antiseptics; they are also particularly good for cleaning plastics.
Environmental impacts. Non-ionic and present-day anionic detergents have minimal impact on the environment. They are easily biodegraded and produce no harmful end-products. However the earliest synthetic anionic detergents were not easily biodegraded – they were branched-chain molecules – and they caused serious damage to streams and lakes and aquatic organisms in the 1960s and early 70s. Cationic synthetic detergents are biocides (they kill many organisms) and so their indiscriminate use and discharge into the environment can be harmful. Too much cationic detergent in sewage can kill the bacteria used in treatment works to break down organic wastes. Fortunately the rather specialised uses of cationic detergents mean that only relatively small amounts of them are used.
The phosphates that are present in many commercial cleaning products are far more harmful to the environment that the detergents themselves; they cause eutrophication.
Question 30 Shipwrecks, Corrosion and Conservation (25 marks) Zinc is an acceptable alternative to magnesium.
(p. 423-4)
(a) (i) Magnesium. Magnesium being a more reactive metal has a greater tendency to oxidise than iron does and so it corrodes preferentially:
Mg(s) ® Mg2+(aq) + 2e–
The released electrons flow to the iron via the conducting wire and by making the iron negatively charged prevent it oxidising to Fe2+. If any Fe2+ did form it would be converted back to Fe:
Fe2+(aq) + 2e– ® Fe(s)
For this method of protection to work the soil between the pipe and the Mg rod must be kept moist – to allow a flow of ions to balance charge.
Simply describe what you have learnt.
(p. 420-4)(ii) Any two of the following three methods would suffice.
1. The pipe could be painted inside and out, preferably with a special paint designed to adhere tightly to the metal. This would provide a barrier that would prevent water and oxygen coming into contact with the iron and so prevent rust formation. The disadvantage of this method is that if the coating is scratched, rust could form.
2. The pipe could be galvanised (coated with zinc) inside and out. The zinc and the impervious layer of zinc oxide that it forms would provide an effective barrier between the metal and oxygen and water and so prevent rust formation. This method has the advantage that even if the coating is scratched, it still protects the iron because if any iron oxidises to Fe2+, the Zn would convert it back to Fe and the Zn2+ so formed would re-form an impervious ZnO coating over the scratch.
Fe2+(aq) + Zn(s) ® Fe(s) + Zn2+(aq)
3. The pipe could be made electrically negative with respect to earth by locating an inert electrode in the ground beside the pipe and applying a voltage between it and the pipe:
Again the ground needs to be kept moist.
Your procedure should be sufficiently detailed to show that you actually did the experiment. It should also include your observations. A diagram would not contribute much to this answer. (b) (i) A small piece of iron sheet and a similarly sized piece of stainless steel were each half immersed in salt water in separate test tubes and observed daily for two weeks to determine the relative amounts of brown discoloration and/or particle formation that occurred. The iron corroded much more quickly in that significant amounts of discoloration appeared on and around the sheet, particularly near the water, air interface. There was negligible corrosion on the stainless steel. Salt water, rather than tap water, was used to speed up corrosion.
The three equations given here would seem to be essential. Giving the actual formula of rust as hydrated iron(III) oxide is probably not required.
(p.416-7)(ii) The brown discoloration/particles on the iron is rust. It forms as follows:
At certain sites on the iron Fe(s) ® Fe2+(aq) + 2e–
These electrons travel along the iron to other sites (often at a stress point or impurity site) and there
O2(g) + 2H2O(l) + 4e– ® 4OH–(aq)
The Fe2+ and OH– ions migrate towards each other and form iron(II) hydroxide
Fe2+(aq) + 2OH–(aq) ® Fe(OH)2(s)
This hydroxide reacts further with O2 and water to form hydrated iron(III) oxide which is rust, Fe2O3.xH2O.
No rust formed on the stainless steel.
(iii) By making it into an alloy containing between 10% and 30% chromium and up to 10% nickel (called stainless steel). This electrolysis can be performed in a beaker as shown here. While the U tube shown in the examiners' report works fine, the U tube is not necessary.
(p. 407-11)(c) (i) Identify the reactions ... and give equations ... seems to imply some verbal description as well as the equations, despite the examiners' sample answer.
(p.407-11)(ii) At the anode, oxidation of water to oxygen gas:
2H20(l) ® O2(g) + 4H+(aq) + 4e–
At the cathode, reduction of silver ions to metallic silver:
Ag+(aq) + e– ® Ag(s)
(p. 411) (iii) 1. area of the electrodes immersed in the solution
2. distance between the electrodes
3. concentration of the silver nitrate solution
4. the magnitude of the voltage applied.
This question seems to be asking you to discuss two maritime archaeological projects that are going on in Australia at the moment. (Several ... projects exist ...) However the projects described in CCHSC and the other HSC texts are all completed projects. You probably have little choice but to treat those completed projects: the examiners in their outline sample answer refer to completed projects. This question could have been worded better.
The projects chosen here have been selected because they involve considerable chemistry and they allow some comparisons and contrasts.
You could have described the restoration of portion of the wooden ship Batavia (p. 439-40) but the problem with this is that there is very little chemistry involved and comparisons would be difficult.
The answer here may seem long for eleven minutes, but you do have to treat the chemistry (with some equations) and make some comparisons. You could have included a diagram for each process but it would not have added a great deal and you really are pressed for time on this question.
(p. 433-40)
(d) Restoration of silver coins recovered from the Dutch ship Batavia that sank off the West Australian coast in the seventeenth century. A salvaged bunch of silver coins was heavily encrusted with calcium carbonate (limestone) formed by marine organisms. This was removed by treatment with dilute acid:
CaCO3(s) + 2H+(aq) ® Ca2+(aq) + H2O(l) + CO2(g)
The coins were covered with a layer of black silver sulfide. This was removed by electrolysis so as to cause minimal damage to the markings (embossings) on the coins. To do this the coins were made the cathode with a platinum rod as the anode using an alkaline solution as electrolyte.
At the cathode: Ag2S(s) + 2e– ® 2Ag(s) + S2–(aq)
At the anode: 4OH–(aq) ® 2H2O(l) + O2(g) + 4e–
In this way the coins were restored with minimal damage and finally coated with a colourless lacquer to prevent further corrosion.
Restoration of cannons from Cook's Endeavour. Several cannons had been thrown overboard off what is now Cooktown when the Endeavour hit a reef in 1770. These were recovered in the 1970s and have been restored. When found they were heavily encrusted with a mixture of rust, coral and limestone. Because of the size and solidity of the cannons these concretions were removed mechanically – by hitting with a hammer. This removed most of the rust The cannons were then treated with electrolysis to remove the last of the rust and the chloride from the fine pores of the metal. If chloride is not completely removed from such salvaged objects, it slowly forms hydrochloric acid that can further corrode the object. For this electrolysis the cannon was made the cathode with an inert metal mesh as the anode and with a dilute sodium hydroxide solution as the electrolyte.
At the cathode: Fe(OH)Cl(s) + 2e– ® Fe(s) + OH–(aq) + Cl–(aq)
At the anode: 4OH–(aq) ® 2H2O(l) + O2(g) + 4e–
This converts iron ions back to metal and more importantly transfers chloride from the fine pores of the cannon into the solution which is periodically replaced. Eventually, after several weeks, all chloride is removed from the cannon. Finally the cannon is washed and coated with a suitable wax to protect it from further corrosion.
Similarities and differences in these two projects are
1.The method of removal of encrustations is quite different. Chemical dissolution is used for the delicate objects (coins) while mechanical force is used on the large solid objects (cannons).
2. Electrolysis is used in both projects. For the silver coins the sole purpose was to restore the silver sulfide back to metallic silver. For the cannons where complete recovery of the metal is less important (because of their size and lack of fine detail) and less practical, some of the metal was restored but more importantly all chloride was removed from the object.
In both cases the restored objects were covered with a protective coating to prevent further corrosion.
Question 33 Forensic Chemistry (25 marks) In listing four pieces of evidence make sure you include two that can be analysed with the instrumental techniques used in part (ii). In other words take a quick look over all the question before answering each part in detail.
(a) (i) fingerprints, blood, oil, dirt (mud) from tyre tracks, paint flakes from the broken glass, threads of cloth, fragments of headlight glass Given the nature of the pieces of evidence available, HPLC and atomic emission spectroscopy are the best techniques to choose. Gas-liquid chromatography could be used for the oil, though engine oil is not very volatile.
The examiners' report includes atomic absorption spectroscopy(AAS). This is not a very useful technique here in that a separate lamp (and so a separate analysis) is required for each element thought to be in the mixture. AAS is better suited to the quantitative analysis of specified elements (metals) rather than the qualitative determination of the elements present in a sample.
Maybe electrophoresis could be used as part of the DNA analysis of the blood, but it is not the best of choices. Gas chromatography could be used on the paint flakes by heating the paint quite strongly to vaporise components into the carrier gas. Mass spectrometry, the other major instrumental technique in the course, is not particularly useful here apart from identifying the peaks as they emerge from chromatographs.
Strictly speaking the question does not call for any account of the principle of the techniques used – just explain how each could be used to analyse the evidence collected. However the marking guidelines require you to relate the principle of these techniques to their application for each piece of evidence. This seems to imply that the paragraphs in italics in the answer opposite are required, though personally I did not think they were! This highlights an important factor in any exam: this is a chemistry exam so the emphasis must always be on the chemistry even when the question seems to imply otherwise.
(p.512-29)
(ii) High performance liquid chromatography (HPLC)
Atomic emission spectroscopy
HPLC could be used to gain information about the oil. The oil would be a complex mixture of hydrocarbons contaminated with some decomposition products. The chromatogram would consist of many peaks each corresponding to one compound present in the oil. By comparing this chromatogram with ones obtained under similar conditions from the engine oil of vehicles suspected of being involved in the crime, the actual crime vehicle could be identified. The comparison would be in terms of both peak position (retention time) and peak area (relative amount of compound present).
HPLC separates compounds based upon their different solubilities in the stationary liquid phase that is adsorbed on the support material in the column. The more soluble compounds travel through the column more slowly and so a separation is effected.
Atomic emission spectroscopy could be used to determine what elements were present in the soil sample (mud from the tyre tracks). The emission spectrum would consist of many sharp lines at different wavelengths. By comparing this spectrum with spectra of known elements (in terms of wavelengths) the elements present could be identified. Similar analyses could be done on soil samples collected from different locations or from suspected vehicles or shoes of suspected persons. By concentrating on the unusual elements present in the crime-scene sample, it may be possible to identify the location the soil came from or to identify the crime vehicle or to establish that a particular suspect had been in the same location as the vehicle.
In atomic emission spectroscopy the sample is heated to a high temperature to vaporise it, break it into atoms and to excite electrons in the atoms into excited states. As these electrons fall back to their ground states, they emit light of wavelengths characteristic of their energy states. This light is analysed to produce a spectrum which shows the wavelengths at which light was emitted.
Describe the experiment fully and give the results you obtained.
(p. 476-7)(b) (i) A portion of an aqueous solution of the sugar to be tested was mixed with some Benedict's solution (copper sulfate in alkaline citrate solution) and warmed in a beaker of boiling water for a few minutes with gentle shaking. If a brown precipitate forms, the sugar is a reducing one; if there is no change it is a non-reducing sugar. We obtained positive tests with glucose, fructose and maltose but a negative one with sucrose. For two marks you probably do not need equations but if you have time you could include them (p.476) or simplified forms of them.
(p. 474-6)
(ii) A reducing sugar is one that in its open-chain form is either an aldehyde, R–CHO, or a ketone next to a terminal alcohol group, R–CO–CH2OH. These compounds are easily oxidised to carboxylate ions by very mild oxidising agents such as alkaline Cu2+. Because these sugars reduce Cu2+ to copper(I) oxide, Cu2O, they are called reducing sugars.
Glucose and maltose have a terminal aldehyde group and fructose has the terminal CO–CH2OH group, whereas sucrose has neither (in fact it cannot form an open-chain isomer).
(p. 514-6)
(iii) High performance liquid chromatography (HPLC) (p. 512-6) (c) (i) Stanozolol (ii) If the compound was very non-volatile (high boiling point), because gas chromatography requires that the compound be vaporised into the carrier gas.
If the compound decomposed when heated, so that it could not be vaporised into the carrier gas.A diagram is essential here, but try to keep it simple to illustrate the principle without becoming too cluttered.
(p. 517-9)(iii) In a mass spectrometer the sample is vaporised in a vacuum and bombarded with electrons which produces positive ions:
M + e– ® M+ + 2e–
These positive ions are accelerated by an electric field into a magnetic field. The ions then follow a circular path with the radius dependent upon the mass to charge ratio of the ion, the accelerating voltage and the strength of the magnetic field used. By varying the accelerating voltage, ions with different mass to charge ratios can be made fall on the collector and so a graph of peak intensity versus mass to charge ratio can be prepared; this is called a mass spectrum.
You are asked to explain the theory, so a diagram of the layout of an emission spectrometer is not required. However a diagram of energy levels and transitions would be desirable. Note however that your answer must be in terms of a specific example and how the spectra can be useful in determining the origins of a mixture, so do not ignore this aspect..
(p 526-9)
(d) Emission spectra can be used to analyse a sample of car paint at a crime scene and used to determine either which car the sample came from or what type and brand of paint it was.
Emission spectra are produced by heating samples in a flame, discharge or plasma to vaporise them, break compounds into atoms and excite electrons in atoms into excited states. As the electrons fall back to lower (or ground) states the energy is released as visible or u.v. radiation. The emitted radiation is broken into its various wavelengths by passing it through a monochromator (such as a prism) and detected by some electronic device, such as a photomultiplier or CCD, or on a photographic plate. A graph of the intensity of the emitted lines versus their wavelengths, called an emission spectrum is produced.
The emission spectrum of sodium or copper, for example, can be obtained by heating suitable compounds in a flame and recording the emitted lines. These compounds produce lines in the visible region as well as in the u.v.
The excited states for electrons are at different energies for different elements and so each element has a distinct set of sharp emission lines (at different wavelengths). Hence the elements present in a mixture can be identified by assigning the emission lines to their respective elements. All elements have at least some lines that are unique to them.
The emission spectrum from a chip of car paint from a crime scene would allow determination of the elements (particularly the metals in the pigments) in the sample and this could identify the type and brand of paint. Alternatively by comparing spectra from the crime scene sample with spectra from cars suspected of being involved in the crime, the actual car used could be identified.
