Post by: Happy Physics Land on November 01, 2017, 12:42:56 am
That's it from me! How do you guys think you went? Pop your thoughts below and share the thread with your friends so that they can have a rough prediction of their marks!
Multiple Choice:
1. A (Fixed now!)
2. D
3. A
4. D (A is wrong because putting a stopper will cause pressure to build up -> explosion. B is most definitely wrong because concentrated H2SO4 is not safe, it only catalyses the reaction. C would not affect safety.)
5. A (amphiprotic: can gain or lose a H+, therefore HSO4-. B and D can only give away H+, C can only take H+)
6. B (collision of neutrons into the Np-237 nucleus to synthesise Np-239)
7. C (It should be noted that hexane does not react with bromine whereas hexane does. Because the Br-Br bond is what gives bromine water its colour, once the bond reacts with hexane then it is decolourised. It should also be noted that bromine being a non-polar substance dissolves well in hexane and hexane. What forms the immiscible layer is actually the water which is polar and does not dissolve in non-polar substances)
8. C (Check solubility rules. A is wrong because sulphate would precipitate with both barium and lead. B is wrong because nitrate is soluble for everything. D is wrong because carbonate would precipitate with both barium and lead)
9. D (incomplete combustion, B is wrong because the black thing left behind is carbon and in B only carbon monoxide is produced (which is a gas), C is wrong because hydrogen gas is not a product of combustion, hence D)
10. B (Zinc is the site of oxidation (high reactivity) whereas the carbon rod is the site of reduction. Pb is the site of oxidation, and PbO2 is the site of reduction.)
11. D (Work out the corresponding charge on each of the elements in both the reactant and product side of the equation. It is found that Cr went from +6 to +3 whilst S went from 0 to +2. Since oxidation is the losing of electrons, the element should be more positive. Therefore Sulfur is the one being oxidised.)
12. D
13. B (Citric can give off most amount of H+ ions per mole, meaning that more OH- ions are required to neutralise. Equivalence point is when moles of a standard solution (titrant) equal the moles of a solution of unknown concentration (analyte))
14. A
15. B (A, C and D are incorrect because N in the middle is bonded with 10 electrons (or it could just be incorrect representations that HSC markers made up))
16. B (A is incorrect because adding CO2 will increase the amount of gas molecules in a fixed volume despite the equilibrium will shift right. C is incorrect because decreasing volume = increase in pressure. D is incorrect because increase temperature will cause equilibrium to shift left, causing more CO2 gas to be produced)
17. C (molar mass / molar volume under room temperature and pressure = 1.936)
18. D (The sharp decline in concentration for all three substances means that there has been an increase in volume. Because X decreases after the disturbance whilst Y and Z increase, therefore Y and Z must be on the same side, having more moles of gas than X (LCP states that equilibrium will favour the side with most moles of gas when pressure decreases/volume increases). C would be a close answer in this case, but because Y and Z increase at the same rate, they must have the same molar ratio and this makes C a wrong answer.)
19. C (0.48 x 1.2 = 0.576 g, moles of SO4(2-) = 0.576/96.07 = 0.006 mol, 0.006 x (137.3 + 96.07) = 1.4g)
20. C (moles of Ba(OH)2 = 0.02 x 0.02 = 0.0004 moles, moles of HCl = 0.05 x 0.04 = 0.002 moles. When you write out the balanced chemical equation, the ratio of Ba(OH)2 to (HCl) is 1:2. Hence HCl is in excess by 0.002 - (0.0004 x 2) = 0.0012 moles. Concentration = 0.0012/0.07 = 0.017 mol/L. pH = -log(0.017) = 1.76 = 1.8 )
Short Answers:
Q21. a) Ozone in the troposphere (lower atmosphere) has a negative effect and is harmful to human health by affecting lung functions, respiratory tracts and can potentially result in asthma especially when other air pollutants are present as well. Ozone in the stratosphere (upper atmosphere) is beneficial for humans because it is able to sacrificially absorb most of the UV radiation from the Sun, preventing humans from over-exposure and therefore decrease our chances of contracting skin cancer.
b) Oxygen properties: lower boiling point, odourless
Ozone properties: higher boiling point (higher dispersion force), pungent odour
Both are similar in that they all exist in a gaseous state.
c) Chlorine radical + Ozone (g) --> ClO radical + O2(g)
22. a) Beware when sketching the graph:
1. Line of best fit should go through as many points as it can, with equal number of data points below and above the line.
2. Descriptive title is a must!
3. Label all your axes, make sure your units are correct (Note: absorption does not have a unit)
4. Extrapolated lines must be shown in dashed lines
b) Find the point on the line of best fit that matches absorbance = 0.58, and if the corresponding zinc concentration value is less than 2.80 ppm, then the water is not overly polluted with heavy metals and therefore is safe for drinking.
23. a) Salt bridge is required for maintaining electrical neutrality in the galvanic cell. It delivers anion to the cathode to compensate for the zinc metal that has been oxidised into Zn2+ (aq) to ensure that the electrolyte is still neutral. In a similar way, the salt bridge delivers cations to the anions to compensate for the loss of Ag+ in the solution and react with NO3-(aq) to ensure that the electrolyte is neutral.
b) Zn(s) + 2AgNO3(aq) --> Zn(NO3)2(aq) + 2Ag(s)
moles of zinc solid lost = 1/63.55 = 0.0157 moles
moles of silver solid gained = 0.0157 x 2 = 0.0314 moles
mass of silver solid gained = 0.0314 x 107.9 = 3.388g
new mass of silver electrode = 10 + 3.388 = 13.388g = 13.4g (3.sig.fig)
24. a) NaOH(aq) + CH3COOH(aq) --> NaCH3COO(aq) + H2O(l)
moles of CH3COOH = 0.025 x 0.502 = 0.01255 moles
moles of NaOH = 0.01255 moles
Concentration of NaOH = 0.01255/0.0193 = 0.6503 mol/L
b) The pH level of the resulting salt solution is not 7, but was instead slightly below 7 because we are reacting a strong base with a weak acid. The final product NaCH3COO(aq) --> Na+(aq) + CH3COO-(aq). CH3COO-(aq) is a weak base, therefore the endpoint is slightly basic.
25.a) Cellulose --> (enzymatic hydrolysis, enzyme = cellulase) --> Dissarcharide Sugar (C12H22O11) --> (enzymatic hydrolysis) --> Glucose (C6H12O6) -->
(fermentation reaction, anaerobic condition, 25-30 degrees environment, yeast catalyst) --> ethanol (C2H5OH) --> (dehydration, concentrated sulphuric acid) --> Ethylene
Expand on the above process to obtain full marks
26. a) Release of sulphur dioxide ---->
1. form acidic rain (SO2 + H2O --> H2SO3), acidifying soils and lakes to make the conditions inhospitable for plantations and marine species, affecting wildlife habitats and marine ecosystem
2. toxic gas with pungent odour that seriously affect human respiratory system
3. Degradation of buildings and households
Expand on the above dot points to obtain full marks
b) The areas of high sulphur dioxide concentration coincides with metal smelters and coal-fired power plants. This is because most metal ores and fossil fuels contain Sulphur as their constituent. When smelted or combusted, the sulphur reacts with oxygen, forming sulphur dioxides that will gradually rise up in the atmosphere.
Smelting of copper ore: 2CuFeS2(s) + 4O2(g) → 2FeO(s) + Cu2S(s) + 3SO2(g)
During the combustion of sulfur-containing fossil fuel, sulphur is oxidised (S(s) + O2(g) → SO2(g)), resulting in sulphur dioxide.
27. Their boiling points are results of the different types of intermolecular forces within each compound. Acetic Acid (CH3COOH) is bonded together through hydrogen bonding between the hydrogen and the lone pair on the oxygen. Butan-1-ol is also bonded together through hydrogen bonding which involves the hydroxyl group in the compound. The reason why CH3COOH despite having less molar mass (i.e. less dispersion force) but still have similar boiling point is because two hydrogen bonds can be formed between two single CH3COOH compounds. Butyl acetate only contains dipole-dipole interaction, which is less powerful than hydrogen bonds and is easier to be overcome. Therefore CH3COOH has similar boiling points to the other two despite having a much higher molar mass (i.e. higher dispersion force).
28. Advantages of Ethanol:
1. Burns cleanly, releasing less CO2 per mole combusted (due to the addition of -OH in ethanols) (i.e. lower emissions)
2. Acts as a gas pipe anti-freeze
Disadvantages of Ethanol:
1. Does not supply as much energy as petroleum for each mole combusted
2. Over 15% ethanol in fuel can cause damage to automobile engine system
29. Water source Y would be more preferable.
Water source X is neutral in pH (pH = 7.3) and has low turbidity (2 NTU), which makes it more desirable than water source than Y.
Water source Y is less hard (5 ppm) and contain much less phosphate (0.0001ppm) than source X, which makes it more desirable than X.
However, the water purification system does not have the facility to solve the issues of high calcium content (i.e. hard water) and high phosphate content (i.e. possible pollution by fertilisers and pesticides which can harm human health). However the water purification system can effectively address the issue of turbidity through sedimentation tank and sand filter, and neutralise the pH level of water source Y through pH control. Therefore water source Y is more desirable for usage.
30. Firstly provide an equation of Haber process: N2(g) + 3H2(g) (Equilibrium symbol) 2NH3(g)
1. High pressure is preferred for yield, because that forces equilibrium to shift to the right, favouring the side with less moles of gas. However this has to be compromised (200 atm) because high pressure within a sealed container is very dangerous.
2. Low temperature is preferred for yield, because that forces equilibrium to shift to the exothermic forward reaction, favouring the production of NH3. However this reduces the rate of production, therefore also has to be compromised at 450 degrees celsius.
3. Liquefaction of NH3 as it is produced to remove product, forcing equilibrium to shift to the product side, increasing yield.
4. Utilisation of a magnetite (Fe3O4) catalyst in order to reduce the activation energy required and improve the rate of production.
Industrial Chemistry:
31. Industrial Chemistry
a) ii) To carry our saponification:
1. Mix 250 ml of water with 100 g of NaOH in a beaker
2. Add coconut and olive oil into a container
3. Mix until thick
4. Let the mixture cure for a while before testing
Emulsion Testing:
1. Place 1g of the synthesised soap in one tube,. 1g of commercial detergent in another
2. Pour 20 ml of water in each test tube
3. Pour 10 ml of oil in each test tube
4. Stopper the test tubes and shake violently for a minute
5. See if both form stable emulsion (that is, water and oil separate).
b) i) When temperature increases, the equilibrium shifts to the reactant side, favouring the production of reactants, therefore decreasing the yield of products. The new curve should thus be below the original curve with the starting point still at the origin.
b) ii) Set up a MICE table in order to answer this question (Molar ratio, initial concentration, change in concentration, equilibrium constant)
c) i) X = NaHCO3, Y = CaO
c) ii) Brine is purified at the start of the Solvay Process by adding CO3(2-) and OH- to precipitate out the Ca2+, Mg2+ and Fe3+ impurities (e.g. 3NaOH(aq) + Fe(NO3)3 (aq) --> Fe(OH)3 + 3NaNO3(aq)).
c) iii) The brine is bubbled with ammonia to facilitate the production of HCO3-, which is then used to produce NaHCO3(s) - this is a very integral material that enables the production of sodium carbonate. At the same time, the carbonation of ammoniated brine also produces NH4+(aq) which would allow NH3 to be recycled, hence decreasing the overall expense of the Solvay process. Evidently, ammonia is a very important part of the process as it enables the most crucial reactant of sodium carbonate to be produced.
d) Speak about mercury cell, diaphragm cell and membrane cell in the order of their development.
In Mercury cell, the ineffective treatment of mercury is a concerning environmental issue. It is a neurotoxin, and if it leaches into waterways, it can disturb the marine ecosystem and cause unrecoverable mental damage to consumers of these marine species. The high amount of voltage and current required to carry out the process is a technology barrier that makes this form of sodium hydroxide production expensive.
Improvements in technology has led to the invention of diaphragm cell which utilises graphite anode and steel mesh cathode instead of mercury. Its low voltage requirement effectively addresses the technological issue that exists in the mercury cell. In terms of environmental issues, diaphragm cells has prevented the possibility of major heavy metal mercury leakage, however proposes another environmental problem associated with the use of asbestos in its diaphragms. The asbestos fibres can pollute the atmosphere, causing lung cancers and severe respiratory conditions in the long term, proposing health threat to humans. A technological issue that rises from this is the impurity of product (only 60% pure!).
Membrane cell is the most recent cell to be developed, effectively resolving the high power need through requiring even lower voltage supply. It utilises the new teflon technology to replace the asbestos diaphragm, hence addressing the issue of environmental pollution. The ion-selective membrane also resolves the issue of product impurity, making the final NaOH almost 99% pure.
Expand slightly along these lines in order to obtain full marks.
If there's any errors here, please point them out and I will change them. :)
Post by: elysepopplewell on November 01, 2017, 01:07:57 pm
Firstly, Happy Physics Land is a LEGEND for giving up his time to do this paper when he finishes his uni class today. He's super clever and dedicated and kind so please join in the chat by posting your thoughts and show Jacky some chem loooooove.
Feel free to tell us what you thought about the paper as we wait for answers. A few people I've spoken to have said this paper was really quite kind - what do you think?
Post by: Arib on November 01, 2017, 01:14:34 pm
Post by: Annie657 on November 01, 2017, 01:20:25 pm
Post by: MisterNeo on November 01, 2017, 01:21:47 pm
I predicted the question lol
Post by: angelahchan on November 01, 2017, 01:21:56 pm
Post by: Invat on November 01, 2017, 01:23:39 pm
Post by: MisterNeo on November 01, 2017, 01:24:11 pm
does anyone know the answer for q4? (the one about how to increase safety for esterification)It was the electric hotplate because a direct flame is dangerous with the flammable volatile alkanol.
Post by: angelahchan on November 01, 2017, 01:24:31 pm
It was to use the other heat source instead of bunsenthank god lol (i changed my answer at the last minute to that one)
Post by: Invat on November 01, 2017, 01:25:41 pm
Post by: angelahchan on November 01, 2017, 01:29:32 pm
Post by: AnneZ on November 01, 2017, 01:35:12 pm
Did all of you got a lot of Ds? :-X
Post by: Arib on November 01, 2017, 01:36:07 pm
Post by: AnneZ on November 01, 2017, 01:38:58 pm
there was actually quite a lot of tricky multiple choice, what did u guys put for the first question?!
A, a lot of As
Post by: kauac on November 01, 2017, 01:42:00 pm
I thought the short answer section was beautiful with easily understandable questions.
Multiple choice was average.
Out of the probably 15 other people in the state who did the biochemistry of movement option, I found it pretty good, except for one nasty calculation question.
Overall it was fairly nice!
Post by: kauac on November 01, 2017, 01:44:17 pm
Just wondering, a lot of friends saying that they got a lot of Ds, and I got a lot of As as I didn't have time to do, just randomly chose.
Did all of you got a lot of Ds? :-X
Hey...For the first few questions, I had alternating As and Ds, so I think you may have been on the right track!
Post by: AnneZ on November 01, 2017, 01:47:26 pm
Post by: Arib on November 01, 2017, 01:55:21 pm
Hey...For the first few questions, I had alternating As and Ds, so I think you may have been on the right track!OMG same i had a lot of As and Ds in the start too!!
Post by: NinjaDatHSC1 on November 01, 2017, 02:16:56 pm
Did you guys pick site x or y? (I picked y cause i said it would be hard for them to filter out the Ca cause it's in ppm which is very small units)
And for industrial chemistry, did you guys times the equilibrium constant question equation by 2? Would it give the same answer if you had a square root for one of them instead?
Post by: MisterNeo on November 01, 2017, 02:20:58 pm
2 things:I picked Site Y and said that you can flocculate the suspended particles that make the water turbid, then use the sediment tank and sand filters. (idk if that's correct lol).
Did you guys pick site x or y? (I picked y cause i said it would be hard for them to filter out the Ca cause it's in ppm which is very small units)
And for industrial chemistry, did you guys types the equilibrium constant equation by 2? Would it give the same answer if you had a square root for one of them instead?
I used the square root for O2 when calculating Q, since they asked for that specific equation so I didn't want to risk it by using the x2 equation.
Post by: kauac on November 01, 2017, 02:22:22 pm
Did you guys pick site x or y? (I picked y cause i said it would be hard for them to filter out the Ca cause it's in ppm which is very small units)
Yes I picked site Y too... The turbidity and pH were the two inadequate aspects of the sample, though they could be improved with the designated purification processes listed. I wrote a similar justification about the calcium ions as well.
Post by: NinjaDatHSC1 on November 01, 2017, 02:22:37 pm
I picked Site Y and said that you can flocculate the suspended particles that make the water turbid, then use the sediment tank and sand filters. (idk if that's correct lol).
I used the square root for O2 when calculating Q, since they asked for that specific equation so I didn't want to risk it by using the x2 equation.
Yeah i pretty much said what you did, except i didn't write flocculation cause i didn't know if they had a flocculant (I didn't see it in the equipment that was given) - i just said sedimentation and then sand filter it
Post by: liya1234 on November 01, 2017, 02:24:56 pm
Post by: MisterNeo on November 01, 2017, 02:32:00 pm
I found it okay but some of the multiple choice and the first half of industrial chem threw me off a bit. What were we supposed to do for that first industrial chem q bc they gave us an ester not a triglyceride??I said this for ethyl propanoate, but not entirely sure if it's correct.
It does produce an alcohol and a carboxylate salt like the other saponification reactions, so maybe.
Post by: toasted on November 01, 2017, 02:38:04 pm
Post by: NinjaDatHSC1 on November 01, 2017, 02:54:26 pm
Yes you are quite right! Thank you for notifying me about my error! I wasn't reading the question properly and was thinking more of a volumetric flask!
Post by: Em444 on November 01, 2017, 03:12:50 pm
Hi...
I thought the short answer section was beautiful with easily understandable questions.
Multiple choice was average.
Out of the probably 15 other people in the state who did the biochemistry of movement option, I found it pretty good, except for one nasty calculation question.
Overall it was fairly nice!
Hi!
Is biochemistry of movement interesting? I looked at the syllabus for it and think I would have liked it. My teacher is a bio/chem teacher as well so she would have done it except there are more resources for shipwrecks
I found the exam ok, looking at the multiple choice I think I got a lot wrong, I tried to get through them quickly because I'm a slow writer and I'm thinking I shouldn't have
The haber process question was great! I only talked about temperature, pressure and the catalyst and linked them all to reaction energy, reaction rate and yield, do you think that was enough?
Post by: angelahchan on November 01, 2017, 03:15:50 pm
The haber process question was great! I only talked about temperature, pressure and the catalyst and linked them all to reaction energy, reaction rate and yield, do you think that was enough?
I think you should mention liquefying ammonia as well as correct mole ratio of nitrogen to hydrogen
Post by: ellybubble on November 01, 2017, 03:22:52 pm
A, a lot of As
me 2! one of our chem teachers said the answer to Q1 was also A (i changed it in the last 30 seconds)
Post by: browniesandpuppies on November 01, 2017, 03:30:43 pm
Post by: Zainbow on November 01, 2017, 03:37:19 pm
For 13, isn't it the strongest acid that would require the most amount of NaOH? I chose H2SO4 because it is diprotic and hence would require the most amount of NaOH
Hey!
With titration, it doesn't matter what's strong or weak, it's the amount of hydrogens that get reacted. Citric acid has the most amount of hydrogens that will react, being triprotic, so a higher volume of NaOH is needed
Post by: Happy Physics Land on November 01, 2017, 03:39:04 pm
Hey!
With titration, it doesn't matter what's strong or weak, it's the amount of hydrogens that get reacted. Citric acid has the most amount of hydrogens that will react so a higher volume of NaOH is needed
Definitely agree with you Zainbow! Good explanation!
Post by: angelahchan on November 01, 2017, 03:42:19 pm
for 20 I don't remember what answer I picked, but don't you have to divide the moles by the total volume or something to find concentration? (before putting it into the -log calculation I mean)
20. D (moles of Ba(OH)2 = 0.02 x 0.02 = 0.0004 moles, moles of HCl = 0.05 x 0.04 = 0.002 moles. When you write out the balanced chemical equation, the ratio of Ba(OH)2 to (HCl) is 1:2. Hence HCl is in excess by 0.002 - (0.0004 x 2) = 0.0012 moles. pH = -log(0.0012) = 2.9)
Post by: mbdtHSC on November 01, 2017, 03:42:59 pm
for 20 I don't remember what answer I picked, but don't you have to divide the moles by the total volume or something to find concentration?
Yeah you need to do c=n/v1 +v2 to get the final concentration, then pH from that
Post by: maria19990 on November 01, 2017, 03:44:11 pm
Hey!
With titration, it doesn't matter what's strong or weak, it's the amount of hydrogens that get reacted. Citric acid has the most amount of hydrogens that will react, being triprotic, so a higher volume of NaOH is needed
but citric acid is weak, so despite it being triprotic, it doesn't ionise completely in solution so there are less H+ ions dissolved and hence less OH- required to neutralise. Shouldn't the answer be sulfuric acid??
Hey there! NaOH being a strong base will drive one way reaction and will force the citric acid to ionise completely.
Post by: tripat23 on November 01, 2017, 03:44:58 pm
Post by: maria19990 on November 01, 2017, 03:45:13 pm
Hey!
With titration, it doesn't matter what's strong or weak, it's the amount of hydrogens that get reacted. Citric acid has the most amount of hydrogens that will react, being triprotic, so a higher volume of NaOH is needed
but citric acid is weak, so despite it being triprotic, it doesn't ionise completely in solution so there are less H+ ions dissolved and hence less OH- required to neutralise. Shouldn't the answer be sulfuric acid??
Hey there! Because NaOH is a strong base, it is going to drive one-way reaction and make the citric acid ionise completely even thought it is a weak acid.
Post by: angelahchan on November 01, 2017, 03:45:45 pm
Yeah you need to do c=n/v1 +v2 to get the final concentration, then pH from thatdo you remember what answer you picked? because i forgot if I remembered to divide the moles by 2
Post by: tripat23 on November 01, 2017, 03:46:13 pm
Post by: Paul.I on November 01, 2017, 03:46:54 pm
Post by: mbdtHSC on November 01, 2017, 03:47:17 pm
do you remember what answer you picked? because i forgot if I remembered to divide the moles by 2
Not sure from memory, 1.8 or something maybe?
Post by: Zainbow on November 01, 2017, 03:49:58 pm
but citric acid is weak, so despite it being triprotic, it doesn't ionise completely in solution so there are less H+ ions dissolved and hence less OH- required to neutralise. Shouldn't the answer be sulfuric acid??
Yes citric is weak. But the titration is a reaction and not simply just a dissociation, so the OH ions force all able protons to react
Post by: Paul.I on November 01, 2017, 03:52:08 pm
Post by: angelahchan on November 01, 2017, 03:53:29 pm
Not sure from memory, 1.8 or something maybe?thanks, I put it in my calculator and that seems like the right answer... do you remember which option that was?
Post by: mbdtHSC on November 01, 2017, 04:06:24 pm
thanks, I put it in my calculator and that seems like the right answer... do you remember which option that was?
C I think?
Post by: angelahchan on November 01, 2017, 04:10:03 pm
C I think?whoops, i got that one wrong then cuz I forgot to multiply
Post by: mbdtHSC on November 01, 2017, 04:18:12 pm
whoops, i got that one wrong then cuz I forgot to multiply
not sure though HAHA I never remember the actual letter I put
Post by: Happy Physics Land on November 01, 2017, 04:23:52 pm
but citric acid is weak, so despite it being triprotic, it doesn't ionise completely in solution so there are less H+ ions dissolved and hence less OH- required to neutralise. Shouldn't the answer be sulfuric acid??
Hey there! Because NaOH is a strong base, it is going to drive one-way reaction and make the citric acid ionise completely even thought it is a weak acid.
Post by: Happy Physics Land on November 01, 2017, 04:25:26 pm
for 20 I don't remember what answer I picked, but don't you have to divide the moles by the total volume or something to find concentration? (before putting it into the -log calculation I mean)
Absolutely Angela! Thank you for reminding me! Haven't done this for a while and things have been a little rusty! I apologise about the mistake!
Post by: winstondarmawan on November 01, 2017, 04:27:33 pm
Absolutely Angela! Thank you for reminding me! Haven't done this for a while and things have been a little rusty! I apologise about the mistake!
So, would the answer be C?
Post by: Dylpickle01 on November 01, 2017, 04:36:38 pm
Post by: MahmoudHassan2017 on November 01, 2017, 04:45:00 pm
Post by: lakwas7 on November 01, 2017, 04:47:48 pm
When you say, "It delivers anion to the cathode to compensate for the zinc metal that has been oxidised into Zn2+", I thought anions went to the anode and cations went to the cathode (or is this something in physics that I got mixed up with xD).
Take this diagram for example
(http://images.tutorvista.com/content/electrochemistry/salt-bridge.jpeg)
Any clarification would be appreciated :)
Post by: mbdtHSC on November 01, 2017, 04:53:44 pm
Post by: mbdtHSC on November 01, 2017, 04:57:32 pm
Hey,
When you say, "It delivers anion to the cathode to compensate for the zinc metal that has been oxidised into Zn2+", I thought anions went to the anode and cations went to the cathode (or is this something in physics that I got mixed up with xD). Any clarification would be appreciated :)
Think he meant to say that for the salt bridge (say KNO3 or whatever), the K+ cation will migrate to the cathode (through the salt bridge) to mitigate negative build up of charge from reduction and the NO3- will migrate to the anode to mitigate build-up of positive charge from oxidation
Post by: kylesara on November 01, 2017, 05:10:21 pm
Post by: liya1234 on November 01, 2017, 05:25:10 pm
Post by: angelahchan on November 01, 2017, 05:28:35 pm
25.a) Cellulose --> (enzymatic hydrolysis, enzyme = cellulase) --> Dissarcharide Sugar (C12H22O11) --> (enzymatic hydrolysis) --> Glucose (C6H12O6) -->
(fermentation reaction, anaerobic condition, 25-30 degrees environment, yeast catalyst) --> ethanol (C2H5OH) --> (dehydration, concentrated sulphuric acid) --> Ethylene
Not entirely sure, but wasn't this q25a on polyethylene? also can you please explain q 1 and 3? thank you!
Post by: kauac on November 01, 2017, 05:46:18 pm
Hi!
Is biochemistry of movement interesting? I looked at the syllabus for it and think I would have liked it. My teacher is a bio/chem teacher as well so she would have done it except there are more resources for shipwrecks
Hey... Yes, the lack of resources in the main downside :-\. But other than that, I really loved the topic! It's mainly about the chemistry of nutrition and food metabolism, energy production and muscle contractions. It would be very helpful for anyone considering studying biochemistry, nutrition and dietetics, sport and exercise sciences, medical or health sciences at uni. Pretty much gives you a good head start on first year uni topics. Would highly recommend!
Post by: Zainbow on November 01, 2017, 05:55:43 pm
Wouldnt question 1 be A - Burette? Burette is more accurate than measuring cylinder and theres no dot point in the syllabus that requires you to use a measuring cylinder (i.e. Measuring cylinder is out of the syllabus and hence cant be the answer)
Hey!
Technically measuring cylinder IS part of the syllabus just as much as a Bunsen burner is, both not explicitly referenced but an integral component of chemistry nonetheless.
And I disagree with you when you say burettes are more accurate, because with a burette you can accidentally overpour and not have the miniscus sit where you want it to. With a measuring cylinder, however, you can use droppers to gain the most accurate measurement. I'm not saying that burettes aren't accurate, but they're less accurate than measuring cylinders bc too much can go wrong
Post by: kauac on November 01, 2017, 06:00:25 pm
And I disagree with you when you say burettes are more accurate, because with a burette you can accidentally overpour and not have the miniscus sit where you want it to. With a measuring cylinder, however, you can use droppers to gain the most accurate measurement. I'm not saying that burettes aren't accurate, but they're less accurate than measuring cylinders bc too much can go wrong.
I think that perhaps the hint in the question was the conical flask - typical for a titration. Yes, measuring cylinders can be more practical and accurate to administer small quantities... but when it comes to the context of the question, in a titration, a burette is probably a better option. :)
Post by: Merin Thomas on November 01, 2017, 06:04:17 pm
Post by: Zainbow on November 01, 2017, 06:04:43 pm
I think that perhaps the hint in the question was the conical flask - typical for a titration. Yes, measuring cylinders can be more practical and accurate to administer small quantities... but when it comes to the context of the question, in a titration, a burette is probably a better option. :)
You can't assume it's a titration though, and you can use conical flasks for more than just titrations anyway. And even if it was a titration, out of the options given, measuring cylinder remains the most accurate which is what the question is looking for. I think they only said conical flask to trick students into picking burette
Post by: Em444 on November 01, 2017, 06:07:53 pm
But that's a good point about the tap on the burette potentially letting too much out
Post by: Zainbow on November 01, 2017, 06:17:17 pm
I think the burette/measuring cylinder one is burette, because burettes that hold 30mL are accurate to plus or minus 0.05mL, whereas a measuring cylinder large enough to hold 30mL probably only has 1mL graduations, and doing multiple pours from one with smaller graduations would have a greater chance of error.
But that's a good point about the tap on the burette potentially letting too much out
That's an interesting point, and it's true, some measuring cylinders have graduations that are too close but then again so do some burettes. I don't want to get too deep about this question lol, it remains only question 1, but if you have a burette and a measuring cylinder with equally spaced graduations which will be the most accurate in that case?
Post by: Dante1091 on November 01, 2017, 06:24:10 pm
I forgot which question it was for but what was the answer to the battery mc question?
I remember that I knew the answer to the dry cell but not the answer to the lead-acid one.
Also would it be ok if I used two equation for the ozone one instead of one?
Post by: Merin Thomas on November 01, 2017, 06:25:08 pm
Post by: Janu383 on November 01, 2017, 06:31:20 pm
Post by: Merin Thomas on November 01, 2017, 06:36:05 pm
Post by: angelahchan on November 01, 2017, 06:40:23 pm
was it in multiple choice or short answer o_oshort answer, 3 marks
Post by: Zainbow on November 01, 2017, 06:40:30 pm
What was the answer to the "how much energy is produced if 1kg of CO2 is combusted" question with the ethanol heat of combustion?
I can't remember the exact amount but I'm pretty sure I had a 5 digit answer (or maybe 4?) and kJ as my units. Don't think that helps lol
was it in multiple choice or short answer o_o
it was in short answer
Post by: tripat23 on November 01, 2017, 06:43:24 pm
Post by: lakwas7 on November 01, 2017, 06:44:39 pm
was it about 15 450 kJ ??
Your answer sounds very familiar! :)
Post by: Merin Thomas on November 01, 2017, 06:45:03 pm
c2h5oh + 3o2 -> 2co2 + 3h2o
But what i did was coverting 1kg into gm and calculating moles of co2. so
1000/12.01+(2x16) = 22.722
so the moles of C2H5OH = 22.722/2 = 11.36 moles
I think (from my memory) it was 1360 KJ/mol.
So H = 1360 x 11.36 = 15451.03 KJ
Post by: Janu383 on November 01, 2017, 06:53:11 pm
Post by: GodlyOne on November 01, 2017, 06:57:47 pm
Post by: browniesandpuppies on November 01, 2017, 06:59:35 pm
Post by: jiaruilu on November 01, 2017, 07:00:44 pm
Post by: cindy_smilez on November 01, 2017, 07:10:54 pm
Post by: tripat23 on November 01, 2017, 07:13:33 pm
Post by: Merin Thomas on November 01, 2017, 07:21:02 pm
Post by: jenna20354 on November 01, 2017, 07:35:43 pm
When you say "but was instead slightly below seven" did you mean "but instead was slightly above seven?" as bases have a pH higher then 7?
Post by: lakwas7 on November 01, 2017, 07:40:07 pm
"The pH level of the resulting salt solution is not 7, but was instead slightly below 7 because we are reacting a strong base with a weak acid. The final product NaCH3COO(aq) --> Na+(aq) + CH3COO-(aq). CH3COO-(aq) is a weak base, therefore the endpoint is slightly basic."
When you say "but was instead slightly below seven" did you mean "but instead was slightly above seven?" as bases have a pH higher then 7?
I believe he meant to say pH is slightly higher than 7 but he had to rush the questions since he was at uni
Also did anyone get a pH of 1.6 for Q20. I looked at solutions for a similar question and it is similar to my working. Instead of multiplying two, I would have thought since they react in a 2:1 ratio you divide it by two in order to find the excess moles. If anyone could pinpoint what I'm missing out on that'd be great :)
Post by: Invat on November 01, 2017, 07:51:39 pm
I believe he meant to say pH is slightly higher than 7 but he had to rush the questions since he was at uni
Also did anyone get a pH of 1.6 for Q20. I looked at solutions for a similar question and it is similar to my working. Instead of multiplying two, I would have thought since they react in a 2:1 ratio you divide it by two in order to find the excess moles. If anyone could pinpoint what I'm missing out on that'd be great :)
A lot people got 1.6, i think there is a mistake with the question
Post by: Invat on November 01, 2017, 07:52:53 pm
Post by: Nick Seb on November 01, 2017, 08:04:44 pm
Also i think you may have missed the second part of question 28 which i believe was a ethanol combustion question.Yeah I was looking for that one too, I think I had 11.3*1360kJ or so, something like 15500kJ per kg of CO2 produced.
~Nick
Post by: Nick Seb on November 01, 2017, 08:09:20 pm
I believe he meant to say pH is slightly higher than 7 but he had to rush the questions since he was at uni
Also did anyone get a pH of 1.6 for Q20. I looked at solutions for a similar question and it is similar to my working. Instead of multiplying two, I would have thought since they react in a 2:1 ratio you divide it by two in order to find the excess moles. If anyone could pinpoint what I'm missing out on that'd be great :)
The 2:1 ratio only applies to the amount of barium hydroxide reacting. 0.0004moles of Ba(OH)2 requires twice the moles of HCl to completely react, i.e. 0.0008 moles. Thus, you subtract 0.0008 from the initial 0.002 (because that's what reacted and got neutralised), leaving 0.0012moles. Find concentration, find pH, which gave the same answer as OP.
Hope this helped,
~Nick
Post by: Happy Physics Land on November 01, 2017, 08:10:54 pm
I believe he meant to say pH is slightly higher than 7 but he had to rush the questions since he was at uni
Also did anyone get a pH of 1.6 for Q20. I looked at solutions for a similar question and it is similar to my working. Instead of multiplying two, I would have thought since they react in a 2:1 ratio you divide it by two in order to find the excess moles. If anyone could pinpoint what I'm missing out on that'd be great :)
Thank you for understanding! With Q20, because of the 2:1 ratio between HCl and the Ba(OH)2, you need to minus twice the Ba(OH)2 from moles of HCl (as Ba(OH)2 is the limiting reagent!)
Post by: Happy Physics Land on November 01, 2017, 08:12:59 pm
"The pH level of the resulting salt solution is not 7, but was instead slightly below 7 because we are reacting a strong base with a weak acid. The final product NaCH3COO(aq) --> Na+(aq) + CH3COO-(aq). CH3COO-(aq) is a weak base, therefore the endpoint is slightly basic."
When you say "but was instead slightly below seven" did you mean "but instead was slightly above seven?" as bases have a pH higher then 7?
Yes thats exactly what I meant! Thank you for pointing that out!
Post by: angelahchan on November 01, 2017, 08:27:58 pm
Post by: Invat on November 01, 2017, 08:36:37 pm
for q28 for the advantages/disadvantages of ethanol is it acceptable to talk about more general disadvantages e.g. opportunity cost of land to grow crops which are instead used to make fuel, or did it specifically have to link to its function in cars?
I think it is fine as the the only reason why we would replace normal fuel (such as octane) with ethanol was if it was produced in a renewable way. Hence the use of arable land over food to grow sugar cane that is used to produce ethanol would be a fine disadvantage of replacing current fuels with ethanol
Post by: J.B on November 01, 2017, 08:49:57 pm
Post by: Zainbow on November 01, 2017, 09:07:04 pm
For Industrial Chem were we had to name x and y, I put sodium hydrogen carbonate and calcium oxide, since it said name. Is this right?
I think that's alright yeah :)
Post by: Happy Physics Land on November 01, 2017, 09:24:12 pm
for q28 for the advantages/disadvantages of ethanol is it acceptable to talk about more general disadvantages e.g. opportunity cost of land to grow crops which are instead used to make fuel, or did it specifically have to link to its function in cars?
The question did speak about the usage of ethanols so that's why I was going a bit practical. But I feel like renewable energy and opportunity cost are also very valid and very acceptable points to make!
Post by: Happy Physics Land on November 01, 2017, 09:25:05 pm
For Industrial Chem were we had to name x and y, I put sodium hydrogen carbonate and calcium oxide, since it said name. Is this right?
Yep! You are definitely right! All industrial chemistry solutions are out now so have a look for yourself!
Post by: RuiAce on November 01, 2017, 09:32:52 pm
Yep! You are definitely right! All industrial chemistry solutions are out now so have a look for yourself!累了吗你？ :P
Post by: Happy Physics Land on November 01, 2017, 09:35:07 pm
累了吗你？ :P
Nah not really
No Chinese here Rui!
谢谢你！
;) ;) ;)
Post by: RuiAce on November 01, 2017, 09:36:58 pm
Nah not really;) Was bored.
No Chinese here Rui!
谢谢你！
;) ;) ;)
Post by: phopo on November 01, 2017, 09:42:23 pm
Post by: hanaacdr on November 01, 2017, 10:04:52 pm
Post by: Nick Seb on November 01, 2017, 10:09:29 pm
For the Question about Sulfur released into atmosphere at Coal burning fuel site and Mining site, I reckon you had to talk about how the amount of So2 gas reduced as you got futher away from the darker, shaded in regions because the released SO2 will dissolve in water vapour in atmosphere to form acid rain.Or that x quantity of SO2 is diffusing into an increasing volume of air so its relative concentration continually drops as it radiates out from the source, lol. You'd likely also have to refer to the white area and briefly outline why it's like that (I wrote about natural sources contributing to the low concentrations).
~Nick
Post by: Nick Seb on November 01, 2017, 10:13:38 pm
~Nick
Post by: Henry Kocatekin on November 01, 2017, 10:21:48 pm
Also, what do you think a raw of 92 will equate to?
Many thanks,
Post by: Happy Physics Land on November 01, 2017, 10:45:40 pm
For 31bi, given that the question was 2 marks, shouldn't the new curve also reach equilibrium more rapidly, as temperature was increased which increases reaction rate?
~Nick
Good Observation! Thank you for that advice! Sorry I was trying to finish my uni assignment at the same time as I am doing this. But yes, definitely on the ball!
Post by: Happy Physics Land on November 01, 2017, 10:47:25 pm
Hi, In your opinion, what will equate to a band 6?
Also, what do you think a raw of 92 will equate to?
Many thanks,
Hey Henry!
Congratulations for such an amazing raw mark! If you do get this kind of mark, chances are you can receive 95 or 96 scaled mark, or even higher depending on how everyone else in the state did.
Post by: adelaidecruz on November 02, 2017, 07:39:33 am
Post by: cchhiillyy on November 02, 2017, 11:26:05 am
Post by: tripat23 on November 02, 2017, 01:21:24 pm
Post by: liya1234 on November 02, 2017, 02:37:34 pm
Does anyone know where a copy of the exam can be found
usually nesa puts it up two days after the exam so it should be up with the other past papers tomorrow
Post by: SpanishPear on November 27, 2017, 09:33:14 am
Post by: liya1234 on November 28, 2017, 05:15:34 pm
Do they put the answers up when the ATARs get released?
the answers are up now!