9.2.5 HSC Questions Feedback

2001 - Q16 – whenever discussing an isotope you MUST say what sort of radiation it emits AND preferably give a decay equation (remember that you are trying to demonstrate how much Chemistry you know in your answers).

2002 - Q19 – you have to say too say unstable n/p ratio AND Z>82 (ie >82 protons - the atomic number is the # protons NOT the number of protons + neutrons). In addition when it says to use examples from the graph USE SPECIFIC EXAMPLES WITH EQUATIONS

2003 - Q18 – You need to revisit this video. The key idea is that a nuclear reactor is a source of neutrons (from fission*) and that a particle accelerator is a source of high velocity charged particles (NB you cannot accelerate neutrons in a PA as they are uncharged – hence the need for nuclear reactors to obtain neutrons). Radioisotopes can be made in TWO ways in a nuclear accelerator (from n-bombardment to make high n/p beta minus emitters AND as a direct result of fission as some fission daughter atoms are radioisotopes). Transuranics up to Z=95 can be made by n-bombardment of U, Np, Pu in a nuclear reactor which makes them beta-minus emitters which transmutate into Np, Pu, Am respectively.
Particle accelerators can be used to bombard protons into nuclei to decrease the n/p ratio and make beta plus emitters (eg O-18 + proton => F-18 + neutron), or can collide small ions into heavy nuclei to make heavy transuranics.
*in the reactor the fuel is a fissionable isotope - usually U-235. When U-235 absorbs neurons it undergoes fission = splits into two smaller nuclei and releases neutrons and gamma rays. Other (non-fissionable) isotopes do not undergo fission. They just absorb neutrons, eg U-238 + n => U-239.

2004 - Q26 -Benefits – must mention NDT for industry = increased productivity, safety, product control etc (NB that testing can be done ‘in situ’ ie while products are being made without stopping the process).

Must mention minimisation of invasive surgery for medical isotopes (lowering surgery risks eg infection) and diagnosing and treating cancer = healthier society

Risks – many – BUT make it clear that it is the RADIATION that the radioisotopes emit that is the danger. Risks include exposure to healthy people and healthy tissue = cellular DNA damage and cancer, terrorism risks, danger of radioactive waste to the environment, problems with long term storage etc

2006 – Q16 - see 2003 Q18 above. But make sure you DEFINE transuranic as Z > 92 (ie beyond uranium)

2007 - Q19 – You have to refer to all the things in 2004 Q26 AND detail how a medical and industrial isotope are used. You must give the decay equation, radiation emitted, you should give the half life and most importantly you have to specify what this isotope does that could not be done in other ways (or could not be done as efficiently/safely etc). ALWAYS say why the radiation it emits is useful - ie gamma is highly penetrating so it can penetrate thick steel AND it can exit the body to be detected with a gamma camera so diagnosis can be made. 
But if you read the question carefully you will see that you have to specify CHEMICAL properties. You need to know that that half life and radiation emitted are nuclear properties so you have to also include things like reactivity, state etc. Eg Co-60 being a relatively inert solid metal can be easily regathered if spilled and is unlikely to contaminate the environment. Tc-99m can be tagged to many bioactive molecules so their path through the body can be traced AND our body had no chemical need for Tc so it is readily excreted. The judgement should evaluate their impact on society ie what can be achieved with these radioisotopes that could not otherwise be achieved and why that is good for society (ie health, economy, industry etc).

KEY misconception to avoid: It is VERY important that there is no doubt in the examiners mind that you understand that the diagnosis, detection, treatment, sterilisation is done BY THE RADIATION NOT THE RADIOISOTOPE. The radioisotope emits the radiation that is detected so don’t fall into the trap of saying ‘the isotope is detected by the gamma camera’ or the isotope kills the cancer cells’ because that is literally what some people think and if you write that it will be assumed that you are one of ‘those’ people.

2011 - Q23b – as for 2006 Q16

2012 - Q27a – it is vital that you emphasise the need for gamma rays to pass through he body for diagnosis (high penetration and low ionising power means they can be detected outside the body and are less likely to cause damage to healthy tissue on their way out). Beta because it is highly ionising can cause local damage to tissues  - ie tumours. But again make sure you emphasis that this radiation is EMITTED by the radioisotopes – the radioisotopes themselves are not the radiation.
Misconception - many seemed to think that the 'ability to pass through biological tissue' referred to the time it took for the radiation/radioisotope to pass through the body - NO!. Beta is ionising so it cannot penetrate very far (ie low ability to pass through biological tissue) once emitted as it collides with molecules, ionises them and loses its energy (good for killing tumours of the isotope is placed in the body at the site of the tumour). Gamma is highly penetrating so it can pass through tissue and exit the body once emitted and CAN be detected outside the body with a gamma camera so diagnosis can be done.

9.2.4 HSC Question Feedback

2001 Q18a – always label your salt bridges as KNO_{3 (aq)} and don’t forget the (aq)!

2001 – Q19 – NEVER discuss your dry cell and silver cell without writing the oxidation and reduction half equations (especially for a 7 MARK questions which asks you TO INCLUDE RELEVANT CHEMICAL EQUATIONS!)

2002 Q5 – oops should not be there

2004 – Q14 – be careful to read the question VERY carefully if they ask you to ‘list in order of…’. In this case they wanted it in order or Increasing ease of oxidation. Ie going from less easily oxidised to most easly oxidised – ie going from least to most reactive metals or from ‘most cathodic’ to ‘most anodic’.

2004 – Q15 – Even though we didn’t study the lead-acid cell the picture did show a dry cell abd you should know where the cathode and anode are (ie reduction and oxidation reactions occur there respectively). In addition, you know the –ve terminal is the node so you should be able to label that too

2004 – Q19 – If you lost marks in this question you really need to do it again. For part b you need to i) realise that #moles Cu = #moles CuSO4, ii) calculate the #moles Cu at the start, iii) calculate the #moles that have been LOST from the solution as the ppt on the cathode, iv) calculate the remaining #moles Cu, v) calculate the final conc of Cu in mol/L.

2005 – Q5 – this is a bit picky but you cannot actually observe electrons moving through the voltmeter (in fact voltmeters are designed to have negligible current flowing through them anyway). Also when metal ions produce a coloured solution(eg Cu= blue, Co = pink, Ni = green, Fe = green/brown) adding more ions intensifies the colour and removing the ions fades the colour.

2006 – Q19d – To measure a correct E^o value all temperatures must be at 25^oC, gases at 100kPa, solutions at 1 mol/L (and  notcontaminated) and the surface of the electrodes must be clean.

2007-Q20 – This Q made me a teeny bit cranky as only half the state studies the lead-acid battery. Anyway = basd on the info provided you should be able to figure out the oxidation reaction (anode reaction and thus the lead anode. For the overall equation you just had to add the two half equations together. Batteries lasting several years is good as lead is a toxic heavy metal and dangerous in the environment. Lasting several years meas less disposal/dumping of lead batteries and less pollution.

2008- Q25d – see tips for 2005 Q5

2009 Q9 – For oxidation number questions – you have to figure out the oxidation #s then put them in a balanced half equation so you can see what is going on. Eg if in the reactants it was Pb⁴⁺ and the products Pb²⁺, I would write Pb⁴⁺ ® Pb²⁺, then I would make the charges balance by writing Pb⁴⁺ +2e^- ® Pb²⁺, so I know it is GAINING electrons and being reduced (or you could just remember that reduction means a decrease in Ox# and reduction means GAIN…)

2009 Q26c & d – Woah – not sure what happened with this Q but for the record if a question has parts a, b, c ,etc then when they refer to a chemical eg ‘the metal’  in part c they usually mean the same metal that they introduced in parts a and b. and again for part d – don’t forget the (aq).

2010 – Q13 – Trick Q – this one got me because I was rushing and not reading the question. Yes connecting it with Fe would give the lowest E^o, but the Q asked for it to be attached to an anode – it the metal had to be more reactive (lower reduction potential) than Zn.

2010 – Q27 – Be careful, the anode is the species being oxidised ie the Zn. The cathode is the species being reduced, ie the HgO

2011 – Q12 – read Q carefully – see 2004 Q14

2011 - Q24 – one of the greatest HSC Q. Most were on the right track but got confused about what was going on. Hint – draw it on the diagram  - draw the Copper leaving solution and depositing on the cathode. Draw the Ni dissolving into solution at the anode –that will remind you when to add and when to subtract mass from the electrodes. Some mentioned that it was the Ni that deposited on the Cu - but– if you drew the picture and looked at the half reactions and what was going on you would see that this is clearly impossible.

2012 – Q13 & 14 – re read the Q and check your data sheet very carefully. Take care when calculating oxidation #s and carefully consider the relationship between oxidation, reduction, oxidant, reductant and change in oxidation #’s

2012 – Q33 – The BOS has not released the marking guidelines but you can find a sample answer I the pdf here and below are their general comments:

Question 33 (from here)

In better responses, candidates provided the correct name for an electrochemical cell and also provided a suitable description of the possible environmental impacts of the cell. Valid reasons for the need for collaboration between chemists were also stated, as were the consequences and effects of a lack of collaboration.

In mid-range responses, candidates often provided the roles of chemists and gave the effect of a correctly name electrochemical cell on the environment. In these responses, candidates provided a valid reason for the need for collaboration.

In low-range responses, candidates identified a role of a chemist and showed a basic understanding of an environmental impact of an electrochemical cell. In these responses, candidates often stated that chemists collaborate, but did not provide reasons for the collaboration.

9.3.1 HSC Question Feedback

2002 - Q20b - Make sure you are being specific with pH’s when you are assessing whether something will be an acid or a base. Make sure you say what pH (range) each solution is eg you can say that A is greater then 12.4 because it is purple BUT for C you will need to say what pH range it covers AND then assess whether it is acid, base or neutral…

2006 – Q23a – see 2002 Q20b above

2012 – Q7 – be careful that the indicators have a a distinct color in the transitions and that for a mixture to appear yellow – all the indicators must be yellow or clear

Experiment #14

Oh dear - I cannot find the Excel sheet. The computer has no record of me even using Excel (not just today but not record of me ever using Excel from the L2 computer?...I'm sure I have - please back me up here because I'm feeling a bit creeped out).

Anyway I redid it from 'memory' and astonishingly the results exactly match the theory. How lucky is that!.

Formula	Name	UI colour	UI pH	Probe pH	pH
Na2CO3		purple	10	11.2	11
Na2SO4		green	7	6.8	7
NaCl		green	7	6.6	7
KCl		green	7	7.2	7
K2CO3		purple	10	11.1	11
KNO3		green	7	7.1	7
NaNO3		green	7	7.1	7
NaCH3COO		blue-green	8	8.3	8
Na3C6H5O7		blue-green	8	8.2	8
NH4Cl		yellow	6	5.7	6
(NH4)2SO4		yellow	6	5.6	6
NaHCO3		green	7	7.5	7.5

Summary Feedback
Generally the reports were very good. Suggestions

(A) You should use 2 equations per non-neutral salt. Some people were very thorough and explained the reason for the salts being non-neutral from 'first principles' - ie BL theory. But this Q asked you to'write equations to ACCOUNT  for' the salts being non neutral. It did not ask for an explanation for each. You explain in general the reasons for salts being non-neutral in the next question. All you need to do is write equations to show why they are non-neutral. Eg
NH4Cl was acidic. This is because

NH4Cl (aq) => NH4+ (aq) + Cl- (aq) (this show the ions produced when the salt is in solution)

NH4+ (aq) + H2O (l) => NH3 (aq) + H3O+ (aq)

(this equation shows that the NH4+ it is a proton donor – ie an acid, thus NH4Cl is an acidic salt) You don’t need to write the stuff in italics.
(B) You need more detail about the indicators used in titrations. Specifically define endpoint and equivalence point and give the indicator colour ranges (you need to practise) I’ll do one in full here. The others are up to you.
“When titrating a strong acid and strong base the salt produced will be neutral (NB you may have already said why in the previous Q of the report, but if not you need to put it here like I do next). This is because the salt will consist of the very weak conjugate base of the strong acid and the very weak conjugate acid of the strong base. Thus the solution will be neutral at equivalence point. Thus and indicator with END point at pH 7 must be chosen. Thus Bromothymol Blue is used as it changes from yellow to green to blue at pH 6 – 7.6. NB the use of the terms equivalence point and endpoint. NB the specific colour changes matches to the specific pH’s

(C) The conclusion needs to address all the aims. Thus, it needs to say what equipment was chosen  and that the pH’s were measured and some were A, B & N

(D) I accidentally used C and D for the same thing. So, see (C) above

(E) The citrate ion is what is produced when citric acid has donated all it protons ie C6H5O7 3-. In addition this means that sodium citrate is Na3C6H5O7.

(F) The bicarbonate ion is slightly basic. This ion is in high concentration is sea water. Hence sea water is slightly basic.

(G) For this question you need to introduce Bronsted Lowry theory, ie define what a acid and base are and what a conjugate pair is (eg a conjugate base is formed when an acid has donated its proton - ie it has one less H+ than the parent acid). You need to give examples of what the pH will be for salts made from SA-SB, SA-WB, WA-SB with reference to strength of conjugates.

(H) Your table needs to be complete & correct.

(I) You can put the table on a second page to save space (some of the explanations for the last two questions can be quite long.

(J) A GREAT way to introduce indicator choice questions is : "In neutralisation Acid + base =>salt +water. Thus the pH at the equivalence point (define) depends on the acidic/neutral/basic behaviour of the salt. The endpoint (define...) of the indicator needs to match the pH of the equivalence point (define...) and thus its colour change must correspond to the salt's pH".

 
Finally – have a play with these animations. Most are about acidic environment. However, depending how your skills at balancing equations and figuring out molecule polarity (make sure you choose the 'real molecules' tab) are you might find some of the non acid/base ones useful too.

Click to Run

Click to Run

Click to Run

Click to Run

Click to Run

Experiment #13

'Results' shown here in glorious colour.

Sulfuric Acid

Hydrochloric Acid

Acetic Acid

(systematically known as Ethanoic acid))

Citric acid

(systematically known as 2-hydroxypropane-1,2,3-tricarboxylic acid)

Worksheet

Q2) & Q3)

Unless the acid/base is strong (ie NaOH, KOH, HCl, HNO3, H2SO4) you MUST use the equilibrium arrow. AND you must include states!

Q2) & Q3) & Q4) & Q5)

When an acid donates a proton it is losing one unit of positive charge. Thus the conjugate base must have one less positive charge. Eg H2SO4 (aq) + H2O(l) => HSO4-(aq) + H3O+(aq) The conjugate base (HSO4-) has one less H and one less positive charge than the parent acid (H2SO4)

When a base accepts a proton it is gaining one unit of positive charge. Thus the conjugate acid must have one more positive charge. Eg NH3(aq) + H2O(l) => NH4+(aq) + OH-(aq) The conjugate acid (NH4+) has more H and one more positive charge than the parent base (NH3)

Simply put - when writing chemical reactions the atoms AND charge must balance.

Always show states

Q6 & Q7)
It is good practise to show the H+ ion transfer in acid/base reactions - this is especially tru if you are having trouble identifying the conjugate pairs. The acid is what has donated the proton. The conjugate base is the resulting species that has one less H+. Similarly the base is the substance that accepts the proton. The conjugate acis is that new species with one extra H+.

Q8)
For stepwise dissociation, there can only be as many ionisations as there are 'donatable' protons. Citric acid is triprotic thus there will be three ionisations. Don't forget that the conjugate bases produced by these ionisations will have one less H  and one less + charge each time. Finally - the questions specified that these were all weak acids so use the appropriate arrows.

Q9)
NEVER show an amphiprotic substance donating AND accepting protons to water. Water is either more acidic or more basic than the amphiprotic substance not both. Eg bicarbonate ions will always accept protons from water (making the solution slightly basic), but will not donate them to water.  SO when showing how a species is amphiprotic write one reaction where it is accepting protons from H3O+ (strong acid) and one where it is donating protons to OH- (strong base).

Summary

The expression/language/communication in these summaries was generally very poor and many did not answer the questions that were asked. I can't stress enough how important it is to express yourself clearly in Chemistry. You need to know your definitions and terms thoroughly and  need to use correct definitions and explanations. You notes are full of examples of explanations and definitions designed specifically for the HSC course. Please use them. But, as disappointed as I am, I now have an excuse to post my new Chemistry pic!

(A) You MUST know where the double bonds are in citric acid and you must know which H is removed from acetic and which H's from citric acid. You must remember to i) include and ii) draw correctly the hydroxyl group in citric acid. You were asked to draw the acetic acid and its conjugate so you need to do this and label them as such. In addition, the conjugate base should have a – sign next to where the H was removed. Finally - you needed to draw 'ball-and-stick' models, structural diagram are not sufficient (and it using coloured circles you need to include a key).

(B) Don’t even think about copying and pasting anything into a experiment summary ever again.

(C) This is the most important concept. It is vital you use the correct wording. Poor selection of words conveys that you don’t understand the difference between strong and weak, because IF you completely understood the difference THEN you would not use poor wording. Strong means that EACH acid molecule is ionized. It does NOT mean that ‘every hydrogen is removed’, or ‘all hydrogen’s ionized’. It is important that you understand this distinction. Weak acids have only SOME of their molecules ionized (or some of their molecules donate a proton). This is very different to the incorrect ‘ only some of their hydrogen’s are ionised’, or 'only some of the hydrogens came off the molecule; or ‘some of their hydrogen’s are removed’.

(D) Give the % ionisation equation! and make sure you define it correctly (see above). Also you need to give examples - ie you should specify the exact % ionisation for hydrochloric, acetic and citric acids.

(E) You need to say WHY a BASE is formed when an acid donates a proton (and vice versa) and you must define what a CONJUGATE PAIR is – i.e. answer the question

(F) You must explain WHY strong acids/bases produce very weak conjugates and why weak acids/bases produce weak conjugates – not just state that they do – ie again, answer the question

(G) The conclusion must answer the aim. SO -1) What did you use to model acids?, 2) How did you use those models to model strong and weak?

(H) Don't forget that you have to explain why structural formulae (diagrams) are more useful than molecular formulae for organic compounds. i) It allows you to see what the functional group is and where its is ii) it helps you to predict the properties of the compound. iii) there are many isomers of organic compounds and the structural diagram allows you to clearly determine the isomer. Eg see here for all the isomers of this simple molecular formula C4H8O2

Some Videos - Titrations and Haloalkanes

I decided to make some spur-of-the-moment videos in Period 6 today whole I had some free time in L2.

The first was going to be an in-class PowerPoint presentation in response to the BOS and the HSC 2012 Haloalkane question.

The other two I thought might be useful to watch before we started titrations. I'm rethinking the second titration one - while making the video I figured out a better way to explain one idea. Perhaps I'll use that idea when explaining it in class.

The video belowgoes with the notes in our 9.3.4 booklet - the colour scheme was inspired by an awesome badge that I saw recently.

 

Experiment #10

Results

CHEMICAL	Type	UI colour	UI pH	pH
ammonia	cleaning	blue	9	9.5
drain cleaner	cleaning	purple	10	11
bicarb soda	cleaning	grey-green	8	8.2
stain remover	cleaning	pale green/yellow	6-8	8.1
oven cleaner	cleaning	purple	10	10.5
deep heat	cosmetic	orange	5	6.5
Listerine	cosmetic	red	3	4.9
mascara	cosmetic	yellow	6	7.6
deodorant	cosmetic	orange	5	6.4
perfume	cosmetic	green	7	6.8
sunscreen	cosmetic	orange	4	5.7
tea	food	red	3	5.7
cola	food	red	3	4.2
lemonade	food	red	3	3.5
milk	food	yellow	6	7.2
orange fizz	food	red	3	4.3
kiwi	food	red/orange	3	3.3
OJ	food	red	3	4.1
coffee	food	yellow brown	6	6.3

Worksheet Feedback

MC questions – Read the question carefully and answer the questions that is asked

Q11 – you actually need to write a sentence or two to explain the difference. Eg for d) saying 10⁴ difference means absolutely nothing. However, saying ‘pH 5 is an acidic solution with a concentration of hydronium ions 10⁴ times greater than a basic pH 9 solution” explains the difference AND more importantly demonstrates your understanding of Chemistry as you have brought in the connection between pH and acidity and linked the numerical pH value to its physical meaning. Sometimes explaining the difference can be as simple as defining each term. Eg for g) ‘A pH meter is an electronic device for measuring pH while an indicator solution is a chemical mixture that changes colour at specific pH’.

Q13) -  We are now in yr 12 Chemistry and the BrØnsted Lowry model is OUR model of acids and bases. Thus when asked to show ionisation of an acid in water you must show the acid donating protons to the water. In addition, only STRONG acids have the complete reaction arrow ®. Weak acids (being incompletely ionised) are in equilibrium with their conjugate base and thus need the ↔ arrow when ionising.

Q14) - Don’t fall for the trick in b) and e). If you are given grams per mL – you need to convert that into moles per mL and then moles per litre before you can calculate pH (or pOH). In Part e many used the moles of OH in the pOH equation rather then the CONCENTRATION of OH. Also remember that the # decimal places in a pH answer must be the SF of the question. For a) there are 2 sig figs (in scientific notion EVERY figure before the x10 is significant) do the pH answer needs 2 decimal places.

Q15) - As above but in reverse – the # decimal places in a pH needs to be the # Sig Fig in the final answer. So for a) there are 2 decimal places thus the answer should be to 2 sig fig. DON’T FORGET UNITS. pH is one of the only times you can leave off units (as there are no units!).

Q17) - We have finished 9.3.1 so we should have mastered these questions by now. IF the solution is yellow then that means each indicator is yellow (or clear). Thus you have to figure out the smallest possible pH range. In addition if an indicator goes from yellow to blue at pH 6.2 – 7.6 that means it is yellow BELOW 6.2, green between 6.2 and 7.6 and blue ABOVE 7.6.

Eg Indicator X changes from orange to blue at pH 7.4 to 8.3.  Indicator Y changes from green to orange at 4.4 – 6.4. Indicator Z is changes from blue – clear at 6.0 – 7.1. If a mixture of all three turns a solution orange, is the sample acidic, basic or neutral. Think then highlight below for the answer.

If the solution is orange, solution X must be orange so the pH < 7.4. This means it could still be acidic basic or neutral.

Indictor Y also has to be orange so that means the pH must be > 6.4. So now we know the possible pH range is 6.4-7.4.

However, indicator Z has to be clear else the sample would be ‘blueish’ colour. This that tells the pH > 7.1. Thus, the possible pH range is 7.1 – 7.4 and the sample is slightly basic.

Summary

A)     You must use the pH data to determine if each substance is acidic basic or neutral and this must be a column in the table. Do not include your predicted results, just the actual results.

B)      Do not say ‘universal’ indicator when you mean ‘all’ indicators. Call them ‘chemical’ or ‘liquid’ pH indicators – or ‘pH indicator solutions’.

C)      There are three main ways to test for pH: liquid indicators, paper indicator strips and pH probes. Each is useful in different situations eg one is very good for precise numerical pH readings, one is good for testing non-aqueous soil, one is good If you just need to test the A/B/ N nature of small sample, some are good for testing pH ranges of household liquids, one is good for automated pH reading, some are good for coloured solutions, one is good if destructive testing is permissible, some are good if it needs to be non-desructive testing, some are good if cost is a factor.

D)     You need to start using our notes for your summaries and when answering questions – ie take definitions etc from our notes. Do not take definitions from the Internet. Always include an equation to explain a relationship where possible.

E)      Don’t forget to mention the general A/B/N nature of cleaning products AND food.

F)      Remember you have to say what advantages the pH probe had (over indicator solutions) in this experiment (testing household chemicals) – eg effectiveness in coloured solutions…