9.3.4 HSC Q Feedback

2001

Q21) Key idea – IONS in solution allow charge flow (conductivity) – at start barium and hydroxide ions carry current, but their concentration and thus conductivity decrease as acid is added.. At equivalence point = no free ions = no conductivity. After equivalence point increasing acid concentration increases sulphate and hydrogen ions = increasing conductivity.

2003

Q23a) You would think that I had never tried to trick you by giving you volumes in mL rather than litres. CONCENTRATION CALCULATIONS ARE IN LITRES!

Q24) Always define the key Chemistry term – ie neutralisation. Also give an example chemical and you could possibly also talk about skin spills.

2004

Q22b) As inconvenient as it sometimes is, you have to answer the question asked. If they ask for equations to show why dihydrogen phosphate is amphiprotic you have to do it. You can’t just substitute other chemicals!

2005

Q9) Buffers are a mixture of a weak acid and its conjugate base. Conjugate bases have ONE less proton than the acid (not two, hence dihydrogen phosphate and hydrogen phosphate are buffers but dihydrogen phosphate an phosphate are not).

Q21) Try and link the scientists together specifically noting what the previous model couldn't explain (ie in the scientific method new theories are proposed when old theories can't explain all observations), eg Lavoisier said they contained oxygen based on his experiments but Davy disproved this when.... Arrhenius developed Davy’s theory by noting that acids ionise to produce H+ in aqueous solutions.... But could not explain amphiprotic substances, acidic salts or acids in non-aqueous solutions. BL theory explained acids in all solvents by noting that they were proton donors. It is critical that you note that BL realised the importance of the solvent. Ie a substance would be an acid in a solvent IF it could donate a proton to that solvent. It is vital that you use chemical equations. Eg show how Arrhenius explained the acidity of HCl (by producing hydrogen ions) compared to BL (producing hydronium ions). And this means you have to be careful, Arrhenius said acids ionise to produce hydrogen ions NOT hydronium ions (that is  BL concept).

2006

Q9) Equivalence I when equal # of hydroxide and hydrogen ions have been added. Many strong acid/base neutralisation reactions are not 1:1 in terms of acid to base molecules (eg H₂SO₄:NaOH = 1:2) but are always 1:1 H⁺ to OH^-.

Q11) Arrhenius modelled acids as chemicals that ionised to produce hydrogen ions. The higher the % ionisation the stronger they were. pH was a concept that was developed from his model but was not part of is model.

Q21b) To show a given salt is acidic (or basic) you have to show one of its ions reacting with water to donate (or accept) a proton to produce hydronium (or hydroxide). The parent acid/base argument is useful for titration questions only. BUT NB these salts are weak acids and bases – the equations MUST have an equilibrium sign.

2007

Q25) Show two equations to show it is amphiprotic(ie reacting with OH^- and H₃O⁺) and could also that it is cheap (not absolutely critical but most people overlooked it).

2008

Q8) Arrhenius said that acids ionise to produce hydrogen ions, BL said they were proton donors

Q26) Define key terms ie define buffer

Q27b) See 2006 Q21b above

Q28c) NEVER, NEVER, NEVER use c₁v₁=c₂v₂ unless it is a dilution.

*Find the moles of one species

*Find the moles of the other via a mole ratio

*Do the calculation to find what it is you are asked to find.

2009

Q14) Excellent Q to do – make sure you can do this Q. I have a feeling a similar one is coming in this year’s HSC

Q21) – an excellent Q to do again for revision

a) The more base that has to be added to neutralise the more moles of acid there are. Because they are both in equal volumes you can determine their relative concentrations. The weaker acid will have an equivalence point at higher pH.

d) Answer the Q. The end point ideally happens at the exact equivalence point but as long as the colour change occurs within the ‘steep’ part of the curve, the endpoint (ie amount of base added hen the titration stops) will match the equivalence point.

2010

Q8) Davy said that acids contained replaceable hydrogen. Arrhenius said they IONISED in water to produce hydrogen ions.

Q28) You should show your calculation for the mass of standard. You must say how the conical flask, pipette and burette are cleaned. You have to say which indicator is used and should explain why. Don’t forget that titrations are repeated until 3values within 0.1ml and averaged.

2011

Q15) At equivalence point the moles of H⁺ = moles of OH^-. The number of moles of acid base needed only depends on the mole ratio, not on strength.

Q18) – this is very similar to 2001 Q23

Q26a) You actually need to calculate both concentrations (from solid and from titration) to show how the values are different, not just say that NaOH absorb water and CO₂ from the air.

Q29) If asked a Q about a scientist’s acid theory, you must outline the theory. Ie what ar the basics of BL and Arrhenius theory. In this case re why we still use Arrhenius NB that most everyday acid are aqueous and the Arrhenius model is simple and convenient to use (eg calculating pH, comparing strength etc).

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

Experiment #12

Results Table

Errors in pH measurement lead to big errors when calculating % ionisation. That is because the formula

[H⁺] = 10^-pH  being an exponential gives big differences in [H+] calculations for small differences in pH

In addition because the pH probes accuracy is +/- 0.05 pH units that alone can lead to large errors in %ionisation calculations. I have calculated the range of % ionisation values for each reading so you can see how a big a difference +/- 0.05 pH error can make. In your summary you need to calculate the [H+] and % ionisation using the pH reading given in the table (but you may wish to note the range).

NB - I went back nd remeasured some of the values after I did my calculations as some were very different - one of the probes was not calibrated correctly and I think that the times that students tested the solutions with the probe not fully submerged made a difference too.

CHEM	CONC (mol/L)	pH (+/- 0.05)	HIGH (%I)	LOW (%I)
NH3	0.1	11.6	3.5	4.5
NaOH	0.1	12.9	70.8	89.1
-	-	-	-	-
H2SO4	0.001	2.8	141.3	177.8
HCl	0.001	3	89.1	112.2
H2SO4	0.01	1.9	112.2	141.3
HCl	0.01	2	89.1	112.2
C6H8O7	0.1	2.1	7.1	8.9
CH3COOH	0.1	2.7	1.8	2.2
H2SO4	0.1	0.9	112.2	141.3
HCl	0.1	1.1	70.8	89.1
HNO3	0.1	1	89.1	112.2
HCl	1	0.1	70.8	89.1



Summary

A)     Look at your Focus 3 booklet. Identify that pH is a function of [H⁺]. Identify ALL the factors that can affect pH. Identify specifically how they affect [H⁺] and thus pH (eg increasing concentration increases [H⁺] and thus decreases pH). Make sure you don’t make the mistake of saying that increasing the acidity increases the pH. More acidic means a LOWER pH.

B)      You need to identify the natural source AND a practical application of each acid and base. Eg CaCO₃ being found in limestone is not a ‘use’ of CaCO₃

C)      You need to explain why all factors that can effect pH must be kept constant if using pH to compare acid strength. This comes straight from our notes.

D)     Conclusions are not personal. Conclusions should state the findings – ie what was the order of strength of the four acids?

E)      The diagram must have equal numbers of anions and cations. It must have a key and the cations MUST be H⁺

F)      Your table needed to include [H⁺] with units (which you need to calculate from pH) and the degree of ionisation of each acid (which you also need to calculate).

G)     You need to show all equations used for relating [H⁺] and [OH^-] to pH (4 minimum).

H)     For the USES of acids in food: identify the acid and what it is used for. Saying acetic acid is in vinegar is not a use – instead you need to say the acetic acid in vinegar is used for…

Portfolio #9 Feedback

Generally very good summaries.

Well done

A) Don't forget the risk assessment for both HCl and NaOH and don't forget you need to mention the dangers of the chemical AND how the risks involved in using them will be minimised.

B) For this experiemnt - if you can use colour just write the name of the colour.

C) According to the syllabus "Indicators were identified with the observation that the colour of some flowers depends on soil composition"Thus, it would be wise to use this as a starting point when talking about indicator discovery - ie mention litmus if you want but note that it was a lichen extract - but even still this is not a FLOWER. You should mention hydrangeas... (look up their colour in acidic/basic soil).

D) When assessing their effectiveness you need to specify that to distinguish acid/base/neutral there must be different colours for pH <7, pH 7 & pH >7 AND you should state the colours for each. Eg if the indicator was always red except for pH 8 it would not distinguish between acid base or neutral (as red could mean all three) it could only distinguish between 'pH 8' or 'not pH 8'. If a indicator was green for pH1-6, red for pH 7, blue for pH 8-9, then green for pH 10+ could only be said to distinguish neutral  and weakly basic (as green could mean acid or strong base).
Also - indicators can never determine the exact pH of a sample. Each colour appears over a small range - so indicators can give an approximate pH (or pH range) only. Only probes can give an exact pH.

E) You only need to assess the plant extracts (beetroot, strawberry, blueberry & red cabbage) not UI, LS, BB, MO or PP

F) Re the 'hypothesis':
"Best" is subjective. Hypotheses can't be subjective or non-specific. It would have ben better to say - the indicator with the most distinct colour differences in acidic/neutral/basic solutions will be...
But even that is not really a hypothesis as a scientific hypothesis should relate the independent and dependent variables together based on some scientific basis.
Eg HYPOTHESIS : "If Alkenes are more reactive than alkanes, then alkenes will change the colour of bromine water faster than alkanes" - this is clear and relates the IV (type of bonding) to the DV (rate of colour change) - hence it is a hypothesis.
The one on the sheet is more of a 'Guess' than a hypothesis

G) When writing the conclusion make sure you consider the points in (D) and (F) above  - be careful not to say that your indicator could determine exact pH's etc. You do need to say whether the aim was achieved and whether the hypothesis was supported - but be careful to phrase it on the line of 'most suitable at distinguishing a range of pH's, or 'had colours that distinctly differentiated between acidic basic and neutral' rather than saying 'best'.Remember that a hypothesis cannot be 'proven' it can only be 'supported'.

H) You must assess EACH fruit/vegetable separately. You must specify what each can/can't do with respect to distinguishing acidic/basic/neutral

I) Non destructive testing is when a sample can be analysed without causing damage to the sample OR changing the chemistry of the sample.Eg using gamma radiation to investigate a weld in a pipe is non destructive testing (NDT) as the gamma rays do not damage or change the pipe (they pass through and expose photographic film which Is like an ‘X-ray’ of the pipe) but cutting the pipe open to inspect the weld is destructive testing (DT).
Using an indicator is considered destructive testing as once the indicator chemical is added the chemistry of the sample is changed.
So what can be done? Use indicators on a small sample of the substance being tested – the small sample is ‘changed’ but the remaining substance is unaffected. Use indicator paper which has (effectively) no impact on the substance being tested. Use a pH probe to determine the pH – this is a non-destructive way of measuring pH (NB a pH probe is just a galvanic cell without an electrolyte, once in the solution the acid (or base) acts as the electrolyte, a current flows in the circuit and the pH is determined. It is able measure pH as there is a special glass ‘salt bridge’ around one electrode that only allows H+ ions to pass through – so the current flow in the probe is dependent on the H+ concentration of the solution (and as pH is a measure of H+ concentration) it can calculate the pH for you.

Experiment #11

Results - Decarbonating Soft Drink

Mass Can [Start] (g)	Mass Open Can [5 days] (g)	Mass Empty Can (g)	Mass Control [Start] (g)	Mass Control [5 days] (g)
418.26	414.01	16.28	382.82	379.01
414.21	411.25	16.69	405.42	403.03
416.12	413.42	16.54	405.42	403.03
409.77	406.01	16.21	405.55	402.27
419.36	416.65	16.48	391.2	389.02
413.93	409.88	15.9	380.25	377.93
418.36	414.82	15.91	389.64	386.98

WORKSHEET FEEDBACK – some good work

Q1d) You do not need the same amounts of products and reactants. Equilibrium can occur with any given ratio of products and reactants BUT at equilibrium the concentration of these products and reactants stays constant and the rate of forward and reverse reaction is constant

Q1i) First saying left or right is meaningless without an equation. Secondly if an exothermic reaction is heated the reverse or endothermic reaction is favoured as by LCP that endothermic reaction will act to absorb the heat and thus minimise the change in temperature.

Q1j) Read the whole Q very carefully.

Q2) If you made mistakes here you need to go back and revise LCP – you should be getting all these right now. Check 3c – are there really equal moles of gas on both sides?...

Q3) If the question says explain then you have to give the answer AND GIVE REASONS

Q3) All means P, V and adding/removing chemicals (cant use Temp as no DH data given)

Q5) When graphing the IV goes on the X-axis and the DV on the Y (ie temp on X). You should mark on your graph any interpolated (within your data) and extrapolated (beyond your data) values. If the data points make a clear curve USE A CURVE OF BEST FIT. DRAW THE LINE, SHOW WOKRING ON GRAPH. If we hadn't done it before I would understand but not including this stuff is just sloppy and lazy. I can show you how to succeed in Chemistry, I can encourage you to do your best but if you can't be bothered...

Q6) Bizarre question  - but even still – you have to explain your answers by referring to LCP!

HSC Q - Ranged from outstanding to not

Q9 - Catalysts do not change equilibrium thus LCP is not related to catalysts - see Q24 below

Q10 - Read the equations VERY carefully

Q23b - SIG FIG...

Q9 - LIM EXCESS Q - check before solving

Q15 - This was in your assessment. You need to do those assessment Q again and again until they are second nature. The mole Q in the HSC get harder every year (ER become MC etc) and you need to be ready

Q18 - This equilibrium contains H+ - thus any acidic or basic chemical will affect the position of equilibrium

Q19 - If there is an equation you must use the mole ratio

Q23 - You must get the solubility right and you must relate to LCP. This was the WHOLE point of the experiment. What does it mean if you got it wrong? I don't have an answer. I don't understand.

Q14 - Writ a reaction equation and USE THE MOLE RATIO

Q24 - Catalyts do NOT affect the position of equilibrium. This I what we covered in 9.3.2. The sulfuric acid  in esterification is an exception. You are taught it is an exception in 9.3.5.  This exception I supposed to help you to remember that NORMALLY they DON'T affect equilibrium.

Q25 - You have to illustrate the equilibrium mixture with an equation. Indicators were taught as a specific example of equilibrium in 9.3.2
 

SUMMARY FEEDBACK – NB if you need space only include YOUR results

A)     What visual observation led you to conclude that gas was released

B)      To find the mass of CO₂ we need to take the mass loss of the soft drink can (CO₂ + water loss) and minus the control mass loss (water loss)

C)      Concentration is moles per litre (c=n/V) – ie moles of dissolved CO₂ per litre of solution. Remember that a soft drink can is 375ml!

D)     The control increased the validity of the experiment. But WHY did measuring the water loss make it more valid? – i) you have to define validity ii) you had to note that we are measuring CO₂ content by the mass loss of the can, iii) thus we had to ensure that the value we used for mass loss was for CO₂ only – ie we had to determine and subtract any mass loss due to water else we would get an invalid overestimate.

E)      When assessing you need to introduce the topic (Ie define reliability), address the +ves (eg repetition) address the –ves (ie different drinks, variations in results), and make a judgement. But to assess something is reliable you really need to compare the volumes of CO₂ that each group calculated to see if there was consistency– did you do that? If not why not? What are you waiting for?

F)      If you are discussing the impact of changing conditions on an equilibrium reaction you have to i) give the equation, ii) define LCP, iii) say what factor you are changing iv) Explain via LCP which way that causes the equilibrium to shift

G)     It is called a series of equilibriums because you can write:

CO_{2 (g)} + H₂O _(l) ↔ H₂CO_{3 (aq)} ↔ H⁺_(aq) + HCO₃ ^-_(aq) ↔ 2H⁺ _(aq) + CO₃^2-_(aq) DH -ve

H)     You need to consider the following factors: temperature, pressure, volume, CO₂ concentration, H⁺ concentration, CO₃^2- concentration.

I)        You need to consider all the above when explaining our choice and you need to say what is wrong with each of them – not just that the one we chose (decreasing pressure / increasing volume) was the ‘easiest’ – NB each of the other techniques has a considerable downside. Eg lowering CO₂ concentration of the atmosphere practically impossible. Eg adding H⁺ = safety, adding CO₃^2-  =invalidates the experiment…why?

J)       As for ALL conclusions for quantitative experiments you need to answer the AIM and HYPOTHESIS in a quantitative manner – WHAT DID YOU FIND OUT!?

K)      UNITS!!!!!!

L)       Gas volumes – equal volumes of gas contain equal number of moles – or one mole of a gas occupies a set volume at a given temp and pressure (ie V = nMV) – did we use this concept to calculate the CO₂ volume?

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