Choosing an option

Two major considerations for teachers In selecting the one option (of the five available) for their students to study, are first their students' likely interest in the topic and secondly their ability to cope with the amount and complexity of the material. Other considerations are likely to be how well the material integrates with and reinforces core material and how straight-forward and/or predictable are likely examination questions on the material. The availability of equipment for laboratory work is another consideration.

Judging by the number of pages I needed to cover the material in each of the three options treated in Conquering Chemistry, the Shipwrecks, Corrosion and Conservation option appears to be the shortest, and the material does to a significant extent reinforce and extend core material, though some of it can be difficult to understand for weaker students. In some ways the material in Industrial chemistry may be more straight forward, but apart from chemical equilibrium it does not flow as directly from core material. There appears to be much more material in Forensic chemistry and much of it is quite complex for students whose background in organic chemistry is just the core material. While a good option for students with a strong biological interest, it could be daunting for weaker students.

With the current heavy emphasis on factual recall in exam questions, the quantity of material to be assimilated is probably a key consideration in selecting an option. 

In writing Conquering Chemistry I thought that the other two options, Biochemistry of movement and Chemistry of art, contained too much material of too diverse a nature for the seven weeks available for the study of the option. Of course these options will particularly appeal to certain groups of students – those strongly interested in sport and dance and those with a strong artistic bent – and strong motivation can easily overcome barriers that may seem daunting to others.

 

Some general comments

1. Exam questions

HSC exam questions in the Options section of the paper differ somewhat from questions in the core section in that . 

· there is more emphasis on recall of learned information about the context than on the underlying chemical principles
· there is always at least one question about a compulsory experiments often with a bit about risk assessment
· there is always at least one extended response question (question with five or more marks) and it usually is of the discuss, analyse, assess, evaluate type, and
· apart from the Industrial Chemistry option there is rarely a problem-solving or calculation question.  

Because of this absence of calculations and problems from the options sections of the HSC exam paper, there will be no Further Exercises sections on the Options pages (except for a few interspersed through the text). 
   

2. Replacement of natural products

When I was writing CCHSC it never occurred to me that the one dot point in Syllabus Section 9.5.1 would ever be the subject of a seven mark question and hence the succinct one-paragraph account of each of rubber, soap and wool on pages 313-4. However it sure happened in 2006. The examiners gave a sample answer based on rubber in their Examiners' Report for 2006 (see BOS website) – something they have very rarely, if ever, done before, and that answer included more polymer chemistry than was required in Module 1, While the one paragraph on page 314 may appear to answer the question asked, it is not worth seven marks, so when you get a question like this, you need to pad out your answer with chemistry, even if it is only vaguely related. In my answer ( ) I have included structures of a soap anion and a synthetic detergent anion, and to be on the safe side included the equation for saponification of a fat to form soap. Since you have 12 minutes for this answer that is not wasting time, just making sure of the marks.
   

3. Equilibrium reactions

In the treatment of equilibrium constants in CCHSC only homogeneous gas phase reactions are discussed. However in the 2007 HSC exam paper a heterogeneous reaction (one involving a solid as well as gases) was used. There is an extra 'rule' for writing equilibrium expressions to those given on page 321, namely:

When a pure solid is involved in an equilibrium reaction, there is no term for the solid in the equilibrium expression. For example for the reaction that occurs in a blast furnace for extracting iron from its ore:

Fe2O3(s) + 3CO(g) 2Fe(s) + 3CO2(g)
                      

Exercises
1. Write the equilibrium expression for the reactions
   (a)     C(s) + CO2(g)  2CO(g)
   (b)     2HgO(s)   2Hg(g) + O2(g) 
2. 2.00 mol carbon and 1.00 mol water were placed in a 1.00 L vessel and heated to 800 K and until equilibrium was reached. At equilibrium the concentration of carbon monoxide was 0.20 mol/L. Calculate the equilibrium constant at 800 K for the reaction,
   C(s) + H2O(g) CO(g) + H2(g) 
              

4. Modelling an equilibrium reaction

One of the 'first hand experiences' required for this Option is to model an equilibrium reaction. The example given in Exercise 5 on page 317 of CCHSC is fine on paper. However it may be a little difficult to set up in practice in that it requires the hole B in the container A to be able to produce a flow rate (at an achievable height of water in the container) that matches the pumping speed of the pump (a small one for an indoor water feature?).

An alternative experiment is described in www.chymist.com/equilibrium.pdf (as of 14/2/2009 you do not appear to need name and password; just leave blank and click OK.)
This is a class demonstration that involves two students with beakers moving water in opposite directions from one large container to another (small fish tanks?).

5. Manufacture of sodium hydroxide

The diaphragm cell was the apparatus used in one of the two early methods of manufacturing sodium hydroxide industrially. Very few plants using this technology are still in use. This cell is included in the HSC to help show how advances in science lead to developments of new and better technology. The diagram of this cell in CCHSC, Figure 10.4 on page 353 is a schematic one, drawn to look like an ordinary hydrolysis setup that students are familiar with. Figure 1 is a diagram that more closely resembles what these cells actually looked like. There is an iron mesh container, the cathode, with an asbestos diaphragm lining its insides; in this is placed hot brine solution and the anode which in earlier times was graphite though later on titanium was preferred. This container is suspended inside a larger container. Steam is blown through this to keep the outside of the mesh and diaphragm moist (and to keep the whole cell warm and so speed up the process). On the ion mesh cathode water is reduced to hydrogen gas and hydroxide. Water and sodium ions diffuse through the asbestos diaphragm (along with a small amount of chloride) and sodium hydroxide solution drips off the bottom of the cathode. The chemistry and ion migration is exactly as shown in Figure 10.4. The difference between this old diaphragm cell and the currently used membrane cell is shown in Figure 10.7 on page 356.

Figure 1 The diaphragm cell
  

6. Diaphragm cell ® mercury cell ® membrane cell?

CCHSC claims that the diaphragm cell was the first to be used (p 352) and that the mercury cell was developed to overcome its disadvantages (p 354). The 2007 HSC exam paper implies that the mercury cell was first (and I guess the syllabus document (Section 9.5.4) is implying the same thing). The website on page 357 also says the mercury cell came first. The claim that the diaphragm cell was first is technically true, but my claim that the mercury cell was developed later and in response to problems with the diaphragm cell is not correct.

A history of the chlor-alkali industry is presented in the introduction of Stringer, Ruth and Johnston, Paul, Chlorine and the Environment, an overview of the chlorine industry, Springer, 2001. The first commercial diaphragm cell was used in Frankfurt in 1891, followed by an improved (continuous flow) version (in the US) in 1893. The mercury cell was developed independently of this and the first commercial plant using it was opened in 1897 (in the UK). Both technologies developed and expanded more or less in parallel and independently of each other. Various factors determined which type of cell would be used as different plants were established. Both types had disadvantages (see CCHSC) and different cost considerations. Slowly, despite its higher construction costs, the mercury cell overtook the diaphragm cell in popularity. The problems of contamination of product with chloride and escape of dangerous asbestos were recognised long before it was realised that discharge of small amounts of elemental mercury into oceans was harmful. It was not until the 1960s that it was realised that certain bacteria converted insoluble mercury into soluble mercury dimethyl Hg(CH3)2 that could be absorbed by other organisms and ultimately by humans with serious detrimental effects. Governments put more and more stringent restrictions on the amount of mercury these plants could discharge and the plants spent more and more money trying to meet them. Pressure was on to develop an alternative cell. There was no going back to an asbestos diaphragm because the health dangers of asbestos had been well recognised by the 1970s.  Eventually the membrane cell was developed and now all new plants are of this type, though many mercury cell (and a few diaphragm cell) plants are still in operation. That makes answering the very similar questions in the 2005 and 2007 HSC exam papers a bit awkward. I have now revised my original answer to the 2005 question in the Answers to HSC exam papers section of the website (go back to CCHSC opening page to get there) and have offered an answer to the 2007 question below.
   

7. Saponification

What do you do if the HSC examiners ask you to define saponification? Give the correct chemical definition as given in CCHSC on page 358 or give the less correct definition in the syllabus document Section 9.5.5? The safest procedure would be to give both as in:
Saponification is the reaction between an ester and aqueous hydroxide solution to form an alcohol and the anion of a fatty acid
Ra–COORb  +  OH–  ®   RaCOO–  +  RbOH
Originally saponification referred to the reaction of a fat or oil (an ester) with hydroxide to form glycerol (an alcohol) and anions of fatty acids.
  

8. Detergent solutions

CCHSC unfortunately does not describe the nature of detergent (surfactant) solutions very well. All detergent molecules have a long hydrocarbon tail that is not soluble in water. When such compounds are placed in water, these hydrocarbon tails stick to one another and leave the ionic or polar end of the molecules pointing outwards and into the water which is strongly attracted to these heads; this forms what is called a micelle as shown in Figure 2(a). Micelles are three dimensional and typically contain 50 to 100 molecules.

A 'solution' of a detergent is not strictly a solution at all; it is a dispersion of these small micelles throughout the water. The presence of these micelles instead of just individual molecules dispersed through the water causes detergent solutions to look cloudy. We say that the water attracting head is hydrophilic (water loving) while the water repelling tail is hydrophobic (water hating).

Figure 2 (a) a micelle of detergent molecules in water (b) the way oil or grease gets dispersed through a detergent solution to form an emulsion

When oil is added to such a solution, the hydrocarbon tails 'grab' the oil and dissolve in it and so we get the effect shown in Figure 2(b) and in the right hand drawing of Figure 10.11 (page 365). This disperses the oil through the water or solubilises the dirt as discussed in Figure 10.11.This forms an emulsion.

 

9. Detergents and environmental effects

CCHSC (page 364-5) explains that the word detergent can have two meanings, one for the mixture that people commonly buy as a detergent such as laundry detergent, washing-up liquid or dishwashing powder, while the second is for the actual surfactant, the compound that reduces the surface tension of the water and so sets off the cleaning process. Under the present syllabus HSC examiners have never used the term surfactant and in the contexts in which they have used detergent it has generally been clear that they mean the surfactant compound. 

However on several occasions there has been a question about the environmental effects of detergents. The problem with this is that today surfactants have an almost negligible effect on the environment. Anionic and non-ionic surfactants biodegrade quite rapidly – perhaps not quite as fast as soap, but quickly enough – and they are easily destroyed by good sewage treatment. Cationic detergents can have a small detrimental effect on the environment in that in large concentrations they can kill bacteria before the bacteria can destroy them. Hence some care is needed to avoid excessive use of them.  In the 1960s there was a problem with anionic surfactants (the only type available then), because the original anionic surfactants were branched chain molecules that did not easily biodegrade and so there were often build-ups of them in waterways with detrimental effects on aquatic life forms and unsightly foam on many streams and lakes. However that problem disappeared in the 1970s when the current linear anionic surfactants came into use.

Today the environmental harm caused by detergents comes from another ingredient in the detergent mixtures that are sold, not from the surfactant. Phosphate is added to detergent mixtures to help keep the dirt released during washing in suspension and to stop it settling back on the clothes etc. It is called a builder. This phosphate in these detergent mixtures is of considerable environmental concern because as explained in CCHSC pages 286-8 and 370 it can lead to excessive algal growth.

If there are only one or two marks for the environmental effects part of the question, then mention of the toxicity of cationics is probably enough but if there are three or four marks, you probably need to talk about phosphates as well.

 

10. Solvay process

It is possible to draw a great variety of flow charts for the Solvay process for making sodium carbonate; two are given in CCHSC – on pages  372 and 384. Do not get 'thrown' by seeing a diagram different from what you are used to: just follow it through and see that it works.

The essential features of the Solvay process are (a) a brine solution is saturated with ammonia and carbon dioxide and this produces a precipitate of sodium hydrogen carbonate (b) sodium hydrogen carbonate is heated to form sodium carbonate (c) the reactant carbon dioxide comes from heating calcium carbonate (limestone) and (d) ammonia is recycled (by reacting product ammonium chloride with calcium hydroxide formed from calcium oxide, the other product of heating limestone.

Environmental effects are often included with a question on the Solvay process. The main ones are (a) disposal of the waste product calcium chloride (b) damage caused by extracting and transporting the reactants (limestone and solid sodium chloride or brine solution) and (c) air pollution from the inevitable escape of small quantities of ammonia. The usual effects of heavy industry such as dust and noise are also involved. 

Choice of a suitable location for such a plant has sometimes figures in HSC questions on the Solvay process. Location near the ocean (for easy disposal of calcium chloride solution) is always a high priority.
   
    

Supplementary material
     (not required for the HSC)  

1. Trans fats

We hear a lot today about avoiding trans fats; they are considered worse for our health than saturated fats, so what are they? To answer this we need to explain what is called cis-trans isomerism. In CCPC page 263-5 alkenes were introduced. It was shown there and in Table 9.5 on page 255 that the geometry around a double bond is planar; around each C atom of the double bond the bond angles are 120o. This means that for 2-butene there are two possible isomers:

                                                               
                                                                  (1)                                    (2)

 In (1) both methyl groups are on the same side of the double bond while in (2) they are on opposite sides. When the two groups are on the same side of the double bond, we call the molecule a cis isomer and when they are on opposite sides we call it a trans isomer. So (1) is cis-2-butene while (2) is trans-2-butene. Figure 3 shows molecular models of these compounds. These two structures represent quite distinct compounds; they have different melting and boiling points and densities and have slightly different chemical reactivities.

 

 Figure 3 Space-filling and ball-and-stick models of cis- and trans-2-butene

Double bonds occur in oleic, linoleic and linolenic acids (Table 10.3 on page 361 of CCHSC) and in other naturally occurring fatty acids. In all of these the arrangement around the double bond is cis; for oleic acid:. 

                                      
The triglycerides formed from unsaturated acids are liquid; we call them oils. If oils are hydrogenated (hydrogen added across the double bonds to make them single bonds), they become solid fats. To make these oils less oily but not fully solid in order to make margarines and other cooking products, the oils are only partially hydrogenated, meaning that only some of the double bonds are converted into single ones. Margarines and other spreads made from partially hydrogenated oils are called polyunsaturated.

Unfortunately in the process of hydrogenating only some of the double bonds, some of the remaining ones get converted from cis into trans. So cooking products made in this way can end up containing significant amounts of trans fats. Oleic acid gives rise to the trans compound, elaidic acid:

                                     
Trans fats in the human body raise the level of low density cholesterol (the bad stuff) and lower the levels of high density cholesterol (the good stuff). They are more effective in doing this than are saturated fats and so are considered less desirable in our diets. Trans fats increase the risk of hypertension, cardiovascular disease and diabetes.
  

2. Omega-3 fatty acids

Omega-3 fatty acids are an essential part of the human diet, but what are they? They are unsaturated fatty acids with one double bond starting on the third carbon atom from the hydrocarbon end of the molecule and running to the fourth. Omega is used to denote the end of the acid molecule that is not the carboxylic acid end. Linolenic acid (Table 10.3) is an omega-3 fatty acid. Another one is DHA, docosahexaenoic acid; this is a C22 molecule with six double bonds, one of which starts from the third carbon atom from the omega end (the others start from the 6th, 9th, 12th, 15th and 18th carbons from that end; try drawing the structure).

Some omega-3 fatty acids are called essential fatty acids, meaning that they are essential to the human metabolism. Unfortunately our bodies cannot make them so we must include them in our diet. A good source of these acids is oily fish – tuna, salmon, cod, etc.

 

Answers to selected HSC exam questions

2004 Question 28(b)

(Copied from Board of Studies website)

Comment

Answer
(i) Simple recall of learnt material. But include a diagram. It clarifies your written explanation (i) Anionic detergents clean because the non-polar hydrocarbon 'tail' of the molecule (the hydrophobic end) dissolves in oil and grease while the polar anionic head (the hydrophilic end) dissolves in water. [Include a diagram like Figure 2(b) above] This converts the oil or grease into small droplets with ionic heads sticking out and these readily mix with water and so the grease droplets dissolve and are removed from the surface being cleaned.
(ii) Remember that 'detergent' has two meanings (CCHSC page364-5). In the context of this question the examiners are using it for the surfactant molecules. But 4 marks for assessing the impacts of these molecules seems excessive, since the impacts of current-day detergents are very small. In order to justify 4 marks (7 minutes) you probably should mention the problem with older anionics, and to be safe, mention the problem with phosphate, though of course that refers to detergents, meaning the mixtures sold commercially.

HSC examiners seem to have a 'thing' about the environmental impact of detergents; it was examined in 2003, 04, 06 and 08.

(ii) The original synthetic anionic detergents were branched-chain compounds that did not readily biodegrade. Hence their increased use lead to a significant concentrations of them in rivers and lakes and this lead to banks of froth and foam building up in such water bodies. This was both damaging to aquatic life forms and aesthetically unpleasing. However since the 1970s anionic detergents have had linear hydrocarbon tails and these are biodegradable, so this problem has disappeared. Today anionic detergents have negligible environmental impact.

Non-ionic detergents are non-toxic and biodegradable and their use has negligible environmental impact. Like anionic ones, they are decomposed to water and carbon dioxide in good sewage treatment works.

Cationic detergents are toxic to many organisms; one of their main uses is as a disinfectant. Their discharge into the environment in large concentrations can have detrimental effects by killing off organisms. They can impede the operation of sewage treatment works by killing  many of the bacteria that decompose organic wastes. Greater care is needed in the use and disposal of cationic detergents than of the others. 

The major environmental impact of detergents is not from the actual detergent itself (the surfactant), but from the phosphates that are often present in the cleaning products that are sold commercially. So-called laundry detergents and washing machine detergents etc are actually mixtures that contain several substances in addition to the surfactant. One of these is a phosphate compound. The discharge of phosphates into the environment can lead to eutrophication – the build up of nutrients in water bodies and subsequent excess algal growth., This can be a serious environmental problem.

 

2007 Question 28(b)

Over the past century the production of sodium hydroxide has evolved from the mercury process, to the diaphragm process, to the membrane process.

Analyse the factors that contributed to each of the changes in the production process.
                                                                                                   6 marks

(Transcribed from the Board of Studies website)

 

Comment Sample answer
The opening statement is not correct. Both the diaphragm process and the mercury process were developed more of less independently in the 1890s (see Section 6 above).
       What do you do when the exam paper contains an error? Answer the question as closely as you can without perpetrating the error. 
       By the second half of the 20th century the mercury process had become the predominant method of making sodium hydroxide. As old diaphragm or mercury process plants reach the end of their lifetimes, they are being replaced by the membrane process.
       With 6 marks for the question you should spend nearly 11 minutes on it and make sure you include at least six bits of significant information. Since this is a chemistry exam, you need to demonstrate that you know what the overall process is, so give a brief description of the electrolysis of a concentrated sodium chloride solution. The factors that contributed to the changes in process are (1) contamination of the product by NaCl (2) recognition that asbestos is an environmental hazard (3) lack of recognition for many decades that small discharges of mercury metal into oceans were also dangerous and (4) development of a membrane cell that overcame all of the disadvantages of the earlier cells.		 Industrially sodium hydroxide is produced by the electrolysis of concentrated sodium chloride solution.
At the anode:   2Cl–  ® Cl2 + 2e–
At the cathode:  2H2O + 2e–  ®  H2 + 2OH–
Overall:      2Cl– + 2H2O   ®  Cl2 + H2 + 2OH–
so a NaCl solution becomes a NaOH solution.
     Two processes were developed more or less simultaneously at the end of the 19th century, the diaphragm cell and the mercury cell.
      The diaphragm cell had an asbestos diaphragm separating the anode and cathode compartments. Sodium ions could pass through it from the anode side to the cathode side and so balance the hydroxide ions being produced there. This cell had the disadvantages that
1. some chloride diffused through the diaphragm and so the sodium hydroxide was contaminated with small amounts of sodium chloride and
2. inevitable losses of asbestos into the air and waterways was an environmental hazard (asbestos is highly carcinogenic).
     The mercury cell uses a mercury cathode instead of an iron one. This results in the formation of a Na/Hg amalgam:
Na+ + e–  ®  Na(dissolved in Hg).
Hence these NaOH plants used two vessels, an electrolysis cell with a mercury cathode in which chlorine gas and a Na/Hg amalgam were produced and a second container in which the amalgam was reacted with water to form an NaOH solution:
2Na(in Hg) + 2H2O  ®  H2(g) + 2NaOH(aq)
      This mercury cell method resulted in virtually no chloride contamination of the product and avoided the problems of using asbestos. 
      The factors that led to changes in the production process were first the recognition of the disadvantages of the diaphragm cell along with the failure to recognise the dangers of mercury release from the mercury cell; these lead to the mercury process becoming the favoured method until the dangers of mercury were fully recognised in the 1960s. The second set of factors was the recognition of the dangers of mercury releases to oceans and the difficulty of completely preventing such releases and the development of suitable synthetic ion-exchanging membranes; these led to the membrane process becoming the favoured technology
      On paper in the mercury process all the mercury should be recycled with none escaping. However it was virtually impossible to prevent small losses of mercury into waterways. In the first half of the 20th century the dangers of small quantities of mercury getting into oceans were not recognised, so the mercury process gradually became the favoured one. In the 1960s it became clear that mercury in oceans was passing up the food chain to humans and causing serious health problems. This almost non-preventable release of mercury is the major disadvantage of the mercury process. 
     Advances in chemistry saw the development of synthetic polymers  which incorporated anionic side-chains that could act as cation exchangers. These advances were incorporated into new designs for the old diaphragm cell in which the asbestos diaphragm was replaced with a teflon cation-exchanging polymer membrane. This allowed sodium ions to pass through but not chloride or hydroxide ions. This so-called membrane cell allowed the production of NaOH free of NaCl contamination, free of asbestos problems and without any mercury releases to the environment. All recently built NaOH plants have used membrane cells.
       

 

2007 Question 28(e)


(Copied from Board of Studies website)

 

Comment Answer
(i) This is the experiment described in the website listed above, namely www.chymist.com/equilibrium.pdf

Two marks for describing the procedure of the experiment is fairly mean, but you do not have any choice but to give a complete description of it, even if it takes more than 3.5 minutes. Here you are just required to describe the experiment, not interpret it. A simple diagram may help but is not essential here. Try to make up time in the next two sections.

 (i) We set up two fish tanks on a front bench, each with a ruler taped to a side for measuring the depth of water. One tank was half full with coloured water while the other was empty. Beside each tank was a student with a 500 mL beaker. The students filled their beakers with the contents of their tanks and emptied their beakers into the other student's tank. They filled and emptied their beakers in unison. Eventually the water levels in both tanks became the same and continued transfer of water from one to the other did not alter the levels.

The experiment was then repeated, this time with the students using different sized beakers. Again after a while the water in the two tanks reached constant levels despite continued transfers but this time those levels were different; the level in the tank from which water was being removed with the smaller beaker being higher than the other.
(ii) Even if the risks seem trivial you need to devise one or two. (ii) The main risk associated with this experiment was the possibility of breaking a beaker by banging it against a wall of a tank; we guarded against this by not proceeding too quickly.
(iii) You need to explain how the amount of water transferred varies as the level in the tanks change and how this models the rate of the forward and reverse reactions, and how constant levels of water in the two tanks corresponds to equilibrium.

A second limitation is the difficulty of modelling an increase in temperature. Speeding up the rate of transfer with the beakers, but still staying in unison,  would not alter the equilibrium position because both rates would have been increased by the same proportion. Changing temperature normally does change the position of equilibrium (and the equilibrium constant)

 (iii) Moving beakers of water from one tank to the other models the forward and reverse reactions of an equilibrium reaction; the amount of water in each beaker represents the rate of the reaction. Initially a full beaker of water is transferred from the half full tank, tank A, to the empty tank, tank B, while no water is transferred in the other direction. As the water level falls in tank A and rises in tank B, the amount of water collected by the beaker from tank A decreases while the amount collected from tank B increases. This corresponds to the rate of the forward reaction decreasing while the rate of the reverse reaction increases. Eventually the stage is reached when the amounts of water being transferred by the two beakers is equal and that is when the water levels in the two tanks remain constant. That corresponds to the system being at equilibrium – no observable change in the levels but with both forward and reverse reactions proceeding at the same rate.

Using beakers of the same size leads to equal water levels in the two tanks or an equilibrium constant of 1. When beakers of different sizes are used, equilibrium is reached when both beakers are picking up the same amount of water. For this to happen the water level in tank B must be higher than in tank A so that beaker B can be nearly filled while beaker A can be only partly filled (not enough depth to fill it completely); this is how different sized beakers can transfer the same amount of water. When the two different sized beakers are transferring the same quantity of water, the levels in the two tanks remain constant. This again corresponds to equal rates for the forward and reverse reactions. This time the equilibrium constant would be larger than 1.

A limitation of this model is that we are not able to investigate the effects of changing the concentrations of individual; species in the reaction.

2008 Question 29(c)  

(Copied from Board of Studies website)

Comment

Answer

(i) Note the way this calculation is set out. While the calculation is simple, students often have difficulty explaining the logic of what they are doing, largely because they tend to think that the expression on the left hand side of the equation is the equilibrium constant, instead of being a reaction variable that is equal to the equilibrium constant (a number) when the reaction is at equilibrium. Note carefully the footnote in CCHSC page 319 and avoid saying the expression for the equilibrium constant is ... .

Another point to watch here: you are given quantities, so you need to divide by volume to get concentration.

(ii) This has nothing to do with part (i) of the question!

Probably two of the three changes given here would suffice for full marks, but note the explain in the question, so it is not sufficient just to say increase the temperature and increase the pressure – you need to explain how or why your change causes the reaction to move to the right.

 

Answers to the 2005 and 2006 Industrial Chemistry option questions are given in the complete Answers to HSC exam papers which you can access from the CCHSC home page (Click on it to go there).
  

Answers to exercises

   2.  0.050

 

 

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