40daysofCHEM 9.3.5= Part 20

40daysofCHEM 9.3.1= Part 19

9.3.5 HSC Q Feedback

#1 Thing = ESTERIFICATION IS AN EQUILIBRIUM REACTION!!!!!!

#2 Thing = ESTERIFICATION IS A CONDENSATION REACTION!!!!!

#3 Thing = We made pentyl ethanoate – we used pentan-1-ol and ethanoic acid. The product smelled like banana. Please use this example rather than examples from other peoples notes, other books etc. NB if you choose an ester that ‘cannot’ be made in school it will be marked incorrect.

#4 Thing = Refluxing is heating with a vertical condenser. When justifying the need to reflux you MUST say i) why heating the esterification reactionis needed(to increase rate of reaction and also increase yield) and ii) why the vertical condenser is needed (to prevent escape of volatile ester).

2001

Q22) Justify means give reasons for the steps you took. So don’t just recount a method. Say what each thing was done – ie why heated, why reflux, why a catalyst.

You need a logical flow for esterification Q. 1) eqn and NB slow and low yield, 2) heat – explain why it increases yield (via LC) and rate (via EA), 3) NB problem with evaporation loss of ester (low bpt) 4) = describe refluxing = heat to high temp without loss of ester, 5) explain use of conc. Sulfuric aid (rate and yield!), 6) any other factors – anti-bumping, no naked flame, etc

NB the refluxing DOES NOT STOP evaporation. It just condenses the evaporated chemicals. Ie  It stops loss of ester due to evaporation.

 

2003

Q21b) Make sure your arrows actually touch the thing you are labelling or it will be considered an ‘incorrect guess’

Q21c) You have to say why the esterification needs to be heated, you have to say why the ester would be lost and how the reflux prevents this – but look at the marks(2) – ie simple to the point statements:

*Esterification needs to be heated to…rate (and yield as…)

* However esters are volatile because…and would escape…

*Refluxing (heating with…) allows the reaction to be heated without LOSS of volatile ester (ie the ester condenses back into the reaction flask). NB it is NOT so the ester doesn’t catch alight!

Q25) There are four trends on the graph i) alkanoic aids > alkanols, ii) alkanols > alkanes, iii) all bpt increase as MW increases, iv) bpt differences decrase as MW increases. You need to explain all four trends in terms of intermolecular bonds. Hints: alkanoic acids can form 2 hydrogen bonds per molecule, alkanols only 1, alkanes only form WDB. Increasing size of molecules increases the energy for them to vibrate and increases WDB. As alkanols and alkanoic acids become longer their polar functional groups become less significant.

2005

Q20) Balance your equations correctly. Identify conditions for each reaction – (eg what condition are required for fermentation, why is heating needed for esterification and why does this make refluxing necessary). Esterification is AN EQUILIBRIUM!!. Esterification is NOT an aqueous reaction. All reactants and products are liquids (no water added and sulfuric acid is a dehydrating agent). Actually say what distillation involves (at the very least say heating and separation by boiling point).

2007

Q23) If you are asked to draw products, draw the products only. If you draw the products and reactants 1) you are wasting valuable time, 2) You are telling the examiner that you don’t know the difference between reactants and products, and that’s just silly!

2009

Q16) I can’t believe that after all our work on esterification in class, experiment #17  and this far into the 9.3.5 HSC booklet that people are forgetting to say “reflux’ when describing the esterification procedure. Name the reactants (and the products) – be specific, show that you have actually done the experiment and actually know the names of the reactants and products in your experiment. Name the catalyst, identify the equipment by name (and / or a diagram). Say it is refluxed (ie heated with a vertical condenser). NO FLAMES – I don’t know what source a lot of you are getting this phrase “heat over a steady flame for 30min” from but stop – it’s rubbish. Safety – 18M sulfuric is being used – toxic, highly corrosive (ie goodbye skin and eyes) – so what are the most critical safety precautions?

2010

Q22c) – see 2007 Q23. They wanted see if students were still going to fall for the trap of drawing everything. Also – while there is propan-1-ol and butan-1-ol etc, there is ONLY methanol and ethanol (no 1’s) as there is only one isomer. Also prop is not easy to remember but surely you et that pent means five – so really that should be a clue that prop can’t be five….

2012

Q31c) Hydrogen bonds are responsible for giving alkanols a high boiling point compared to alkanes and esters. But the graph showed bpt vs MW. What makes propanol have a higher bpt than ethanol? It can’t be hydrogen bonding as they both have one hydrogen bond per molecule. It must be because as the MW (chain length) increases the…..increases.

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).

9.3.3 HSC Questions Feedback

This set of questions stood out as something that we can work on. You all knew hat pH was and how it related to strength and concentration but the way that these concepts are expressed is crucial – you have to be very careful with terminology (eg weak & strong) and you have to clearly make the link between strength, concentration [H+] and pH.

WARNING – AVOID THE TRAP we really have to stop falling for the tricks where Q ask about increasing or decreasing pH. HIGHER [H+] = LOWER pH

2001

Q20b)  You need to correctly use the terms strong and weak, You need to link them to %I and thus the final hydrogen ion CONCENTRATION and thus to pH via the formula. ‘Even though both acids are at the same concentration (0.1 mol/L) , HCl is a strong acid with 100% ionisation and thus will have a [H+] of 0.1 mol/L. Citric acid is weak and thus has a %I < 100 &  will have [H+] of less than 0.1mol/L. Thus the pH of the hydrochloric acid is lower than citric acid as pH = -log[H+] (ie pH HCl = -log[0.1] = 1) and thus as [H+] increases, pH decreases’ Note you could figure out the citric and HCl %I form the data – this is a GREAT way to answer these questions and reinforce the concept of strng and weak. But it is also something to watch for because according to the data in this question %I for citric is 25% which means that for THIS QUESTION saying the %I of citric is 8% is technically wrong.

2002

Q22b &c) for b) you needed to know that sulfuric acid was diprotic, thus can produce a higher [H+] then HCl when at the same concentration, so sulfuric acids pH will be lower. For c) ditto Q20b) 2001e above, without the calculations.

2003

Q8) We have been over the diprotic nature of sulfuric acid SO many times…

2004

Q7) WARNING – AVOID THE TRAP we really have to stop falling for the tricks where Q ask about increasing or decreasing pH. HIGHER [H+] = LOWER pH

Q24a) This was dilution so use the dilution equation. In addition the #SF in the ‘normal’ numbers in the data is used to calculate the # decimal places in a pH answer (& the #decimal places in pH data = the #SF you need when calculating the answer)

Q24b)  If you are given two chemicals and asked about their uses you should give a different use for each. Also Citric acid is NOT ascorbic acid (vitamin C). Citrus fruits have both these acids in them but it does not mean that they are the same thing. Just like, Pepsi has water and sugar and caffeine in it but they are different chemicals.

Q24c)  You have to give the pH equation. You have to say that higher [H+] = lower pH then you have to say why the [H+] – ie hydrogen ion CONCENTRATION - is different in each and thus why the pH is different in each. Many fell into the trap of talking about the ‘amount’ or moles of H+ in solution – THAT DOES NOT MATTER. pH depends on hydrogen ion CONCENTRATION. Also – NEVER talk about things being ‘more’ or ‘most’ or ‘less’ acidic. This means nothing in Chemistry and reflects badly on you as a Chemistry student. If you mean ‘stronger’ say ‘stronger’. If you mean ‘more concentrated’ say ‘more concentrated’. Finally you need to ay that HCl was strong a t100% ionised and that citric and acetic were weak but had different [H+] as citric is 8% ionised and acetic 1% ionised(giving citric a higher hydrogen ion concentration and a lower pH than acetic).

2005

Q8)  We have been over the diprotic nature of sulfuric acid SO many times…

2007

Q10)  “...must be added…” I’ve thrown this trick at you a few times. It is cruel but you need to be aware and looking for it in ALL dilution questions.  

Q21a&b)  We have been over the diprotic nature of sulfuric acid SO many times…

Q21c)  The same inexplicable numerical error showed up in approximately half of the papers. Again – work together to discuss solutions to the questions but go and work independently. IF you are just copying answers without thinking: a) it is not helping  b) you are not learning, c) you are not showing the person you are copying from respect (because if you were thinking you might have found their mistake and helped them) & d) it means that I can’t fully diagnose problems you are having.

2008

Q14)  This was basically identical to 2006 Q17b and 2012 Q28 but we did well in those and poorly in this. The main error was using the moles of hydrogen ions rather than the concentration in the pH calculation. A silly, avoidable, mistake.

2010

Q21) You need to distinguish yourself from the crowd so if you are given data USE IT! Just like 2001 Q20b – you have the data to calculate the %I so do so. Basically if you ever get a compare the pH question and you can calculate %I then do it. (remember in our first assessment the rule that if you were provided with a stimulus you had to use it in your answer – this is the same – if provided with numbers and you can do a calculation to support your answer then do the calculation)

2012

Q11)  WARNING – AVOID THE TRAP we really have to stop falling for the tricks where Q ask about increasing or decreasing pH. HIGHER [H+] = LOWER pH. You shoud know the different %ionisation of hydrochloric, acetic an citric acids.

Q18) 1) Weak acids are in EQUILIBRIUM with their conjugate base, water and hydronium. Changing any of these chemicals will shift the equilibrium. Eg CH₃COOH _(aq) + H₂O_(l)  ↔ CH₃COO^-_(aq)  + H₃O⁺_(aq)  Diluting is adding water, thus by LCP the reaction will shift to remove water, thus reducing the acid conc and increasing the conc of conjugate and hydronium – thus raising the %I.

2)  WARNING – AVOID THE TRAP we really have to stop falling for the tricks where Q ask about increasing or decreasing pH. HIGHER [H+] = LOWER pH

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