Showing posts with label 934. Show all posts
Showing posts with label 934. Show all posts
Wednesday, 2 October 2013
Monday, 22 July 2013
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).
Monday, 25 March 2013
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!.
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.
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
|
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.
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
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)
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
Wednesday, 20 March 2013
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 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.
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