VCE Chemistry Question Thread

[pugs]

To answer your question about cloud point - If you think about what a higher cloud point means then you would realise that because biodiesel has a higher cloud point than petrodiesel it will be less effective at cold temps. Think about this: at a cold temperature which one is more likely to have already reached its cloud point (biodiesel) - because you would have to go to even lower temp to reach cloud point of petrodiesel (slightly counter intuitive due to lower temp and higher cloud point)

In the past (2017 nht) they did accept denaturation due to heat as occuring due to the breaking of hydrogen bonds in the tertiary strucuture (no mention of the secondary strucutre). However previous multiple choice asnwers have wanted you to know that denaturation can involve breakage of the hydrogen bonds in the secondary strucutre. I personally do mention the breakage of hydrogen bonding in the secondary, tertiary (and quaternary if the protein is quaternary) but it seems VCAA will be happy with you just saying tertiary.
Also, I don't think VCAA wants you to know about the relative strength of hydrogen bonding within the secondary and tertiary strucutres as I haven't seen it come up. As such, I think it would be safe to say that heating that denatures a proteins tertiary strucure will also denature a proteins secondary strucure (atleast at a VCE level). I might be wrong though, so please correct me if this is utter nonsense.
Just to add to what KiNSKi01 has said,
Cloud point is essentialy the temperature at which a fuel begins to solidify. We want our fuels to be liquid so that they can flow through piplines and through engines and what not. Thus if biodiesel has a higher cloud point, it will start to solidify at higher temperatures, meaning it becomes inneffective at a higher temperature than pertrodiesel with a lower cloud point.
For example: if the temperature was aproximately 8 degrees, biodisel may start to solidify (due to its higher cloud point) whereas petrodisel might not solidify (due to its lower cloud point) which makes petrodisel a better fuel option in this circumstance. Note that youdon't need to know the precise temperatures, just that biodisel has a higher cloud point

thank you both so much!!

[turtlebanana]

Anyone mind explaining these two questions?  (VCAA 2002)

For question 19: I just guessed. I knew it couldn't be C because you can't reduce a Br^- to a Br^2-?

For question 20: I did Q = n(e^-) x F

this got me n(e^-) = 1 mol

for n(K): we have the equation: K⁺ + e^- ⇌ K (s)
n(K) : n(e^-) = 1:1 --> therefore, n(K) = 1 mol

for n(Ag): we have the equation: Ag⁺ + e^- ⇌ Ag (s)
n(Ag) : n(e^-) = 1:1 --> therefore, n(Ag) = 1 mol

for n(Cu): we have the equation: Cu²⁺ + 2e^- ⇌ Cu (s)
n(Cu) : n(e^-) = 1:2 --> therefore, n(Cu) = 0.5 mol


This got me the answer C, but anyone know why its wrong?

Thanks

[Bri MT]

what is the difference between uncertainty and errors?

The two terms can be used synonymously.

I prefer uncertainty since it makes the distinction between uncertainty/error and mistake more clear.

For 19 you're supposed to use the electrochemical series in the data book

For 20, don't forget water

[turtlebanana]

Q20:

For 19 you're supposed to use the electrochemical series in the data book

For 20, don't forget water

Q19: thanks! it was actually so simple, didn't see it like that for some reason.
Q20: what about water?? also are the 30 minutes that they give you as info irrelevant? at first i tried doing Q = I x t, but i didn't know what to do with the amps i got (using I = Q / t).

OH, wait. I think i see it now.

[Bri MT]

Q20:
Q19: thanks! it was actually so simple, didn't see it like that for some reason.
Q20: what about water?? also are the 30 minutes that they give you as info irrelevant? at first i tried doing Q = I x t, but i didn't know what to do with the amps i got (using I = Q / t).

The 30 minutes isn't relevant


Before you write the equations down, you need to know what reactions are happening.
In each of these cases there are two potential reactants you need to consider: the metal ion and the water
It turns out that water is preferentially reduced over the potassium ions, so no moles of potassium are deposited at the electrode

Whenever you are doing a galvanic or electrolysis question with an aqueous solution always always check water on both the reductant and oxidant sides.

[anoushka_iyer]

Got another experimental design question:

An investigation was being conducted to examine how changing the pressure affects the volume of a gas.
1) Explain two factors that may result in differences in results between two students:

- differences in temperature: If one student performs the experiment at an excessively higher temperature, this might affect the pressure readings on the barometer (either too high or too low, hence a systematic error), and another student carries it out at a lower temperature which does not affect the instrument, then their readings for pressure will be different for the same volume

- different barometers could be calibrated slightly differently, so one of them could give too high or too low values, again leading to differences in pressure values. 

2) design an experiment to test this hypothesis


IV: pressure of the container (kPa)
DV: volume of the container containing the gas (mL)
CV: same temperature of the gas and same barometer used


Method: errr I'm not quite sure how to go about the method because I don't really know how to measure volume and pressure in this kinda experiment :/


TYSM!!!!

[Stargirl113]

ok so I have a few questions from the NHT 2019 exam, and would appreciate some help - they're mostly quick little queries

MC Q17:  why can dilute solutions of strong bases be titrated more accurately than concentrated solutions? I got this answer C just by elimination, but is that the only way?

Do coenzymes ALWAYS break apart during a reaction?

Question 3 asks for the overall equation for Na and Cl electrolysis, would I lose a mark if I wrote the reactants as Na+ and 2Cl- instead of 2NaCl as the solutions have?

Question 5 about galvanic cells says that any inert electrode can be used as the cathode? Why is this the case - especially since we're usually told that the cathode is the metal that corresponds to the salt in the electrolyte? E.g. I put Ni (s) as cathode for this question, but solution says any other inert electrode or metal higher than Ni on the electrochemical series can be used. Why??

Last question, 6b asks why it is appropriate for average mass to be measured and the answer says that it is because the data is within a narrow range with no outliers -- does this imply that you shouldn't just take the average for any results, and this condition always exists?

Thanks a lot for helping

[anoushka_iyer]

How do I answer this question?
Explain the differences between your line of best fit and your line joining the points. Which is better for this data?

[colline]

2018 NHT Q6 a) i
The numbers on the graph provided had no decimal places. When you do 28-24 to find ΔT you'd get 4 which is 1 sig fig. So shouldn't the calibration factor be left to 1 sig fig?? The answer says 2.13 instead.

(Sorry I know this works better with an attachment but idk how to attach a picture when on mobile)

[Just another student]

ok so I have a few questions from the NHT 2019 exam, and would appreciate some help - they're mostly quick little queries

MC Q17:  why can dilute solutions of strong bases be titrated more accurately than concentrated solutions? I got this answer C just by elimination, but is that the only way?

If you think about it, if you are using a dilute solution of a strong base then you are slowly adding just enough to reach endpoint. If you use a concentrated solution then the last drop may have more base then needed to reach endpoint.

Do coenzymes ALWAYS break apart during a reaction?
From what i heard coenzymes don't actually get consumed, they just sit in the active site and help create a better fit for the substrate. - I'm not 100% sure on this one though.

Question 3 asks for the overall equation for Na and Cl electrolysis, would I lose a mark if I wrote the reactants as Na+ and 2Cl- instead of 2NaCl as the solutions have?
A teacher told me that you should combine the 2 (it's better). VCAA could possibly give the mark but again better to combine.

Question 5 about galvanic cells says that any inert electrode can be used as the cathode? Why is this the case - especially since we're usually told that the cathode is the metal that corresponds to the salt in the electrolyte? E.g. I put Ni (s) as cathode for this question, but solution says any other inert electrode or metal higher than Ni on the electrochemical series can be used. Why??

If the Ni+ ions are getting reduced (turning from ions into solid metal) then the cathode metal will not matter. Since the Ni will just collect on the electrode, in this case the function of the electrode would just be for electrons to flow and for the metal to collect on. The cathode here won't actually participate in the reaction so it doesn't matter which metal is used.

Last question, 6b asks why it is appropriate for average mass to be measured and the answer says that it is because the data is within a narrow range with no outliers -- does this imply that you shouldn't just take the average for any results, and this condition always exists?
It's ok to take an average if the results are concordant of within narrow range, if there are outliers then you would exclude them. In general, you only average results within a narrow range - think of titrations - how you exclude values which are not concordant.

Hope that helps and makes sense!
Also I have not idea how to use the formatting lol

[Erutepa]

Got another experimental design question:

An investigation was being conducted to examine how changing the pressure affects the volume of a gas.
1) Explain two factors that may result in differences in results between two students:

- differences in temperature: If one student performs the experiment at an excessively higher temperature, this might affect the pressure readings on the barometer (either too high or too low, hence a systematic error), and another student carries it out at a lower temperature which does not affect the instrument, then their readings for pressure will be different for the same volume

- different barometers could be calibrated slightly differently, so one of them could give too high or too low values, again leading to differences in pressure values. 

2) design an experiment to test this hypothesis


IV: pressure of the container (kPa)
DV: volume of the container containing the gas (mL)
CV: same temperature of the gas and same barometer used


Method: errr I'm not quite sure how to go about the method because I don't really know how to measure volume and pressure in this kinda experiment :/


TYSM!!!!

1) I think this question depends on what you consider the results on this experiment.

If we were to rearrange PV=nRT for V (our dependent variable) we would get V=(nRT)/P.
Thus, if both students conducted the experiment at different temperatures, while they will observe different volumes for the same pressure, they will both still observe the same relationship between increasing pressure and decreasing volume since in both cases T remains constant. The same goes for systematic errors --> both students will still observe the same relationship, but have differing volumes for given pressures.
Thus, if you were considering the students results as the volume corresponding to a given pressure, the factors you have listed will be valid, however if you were to consider their results as the relationship between pressure and volume observed, these factors would not account for differences. If we assumed the results to be the relationship between pressure and volume, a difference in students results would only be explained by a factor that varied inconsitantly over the different trials (such as an inconsitantly varying temperature across the students trials, or perhaps a inconstantly varying mole amount of gas as a result of a leak in one students aparatus)

Exam questions asking this would have more context that allows you to determine the nature of the 'results' and thus makes answering this question a bit easier.

2) I am also unsure of how to conduct such an experiment. You've listed the correct variables, but I am not sure how one could change the pressure and observe a change in volume as a consequence. Maybe someone else could shed some wisdom though.

[hums_student]

2018 NHT Q6 a) i
The numbers on the graph provided had no decimal places. When you do 28-24 to find ΔT you'd get 4 which is 1 sig fig. So shouldn't the calibration factor be left to 1 sig fig?? The answer says 2.13 instead.

We were discussing the crap out of this question on this thread last year too lol. It's a terrible question.

I don't think VCAA has ever explicitly stated this anywhere, but they would never ask you to leave your answer to one sig fig due to its inaccuracy so you'd always leave it to the next smallest number of SFs if you ever get 1SF from your calculations.

My chem teacher said that if you get 1SF from calculations then it's a sign that VCAA isn't marking on sig figs for that question but just to be safe I generally still used the second lowest SF given in the question. However due to many ambiguities I don't think you'd lose marks if you wrote CF=2

[turtlebanana]

Hey guys, question about enzymes

Does the substrate that is bonding to the active site ALWAYS have two enantiomers? (where only one of them can bind to the active site)

Also what is the difference between ''enantiomers'' and ''optical isomers''? Aren't they the same thing (different 3D arrangement of groups around atoms)? Can the terms be used interchangeably?

[Erutepa]

Hey guys, question about enzymes

Does the substrate that is bonding to the active site ALWAYS have two enantiomers? (where only one of them can bind to the active site)

Also what is the difference between ''enantiomers'' and ''optical isomers''? Aren't they the same thing (different 3D arrangement of groups around atoms)? Can the terms be used interchangeably?

substrates can be achiral (not chiral) and thus may not exist as a pair of enatiomers. I don't think this is particulalrly important to know for VCE chemisty though - you will typically be asked about why a certain enatiomer of a substrate may have one effect while the other enatiomer may have no effect/a differnt effect.

entatiomers is a term used to refer to a pair of optical isomers and they can be used interchangably.

[turtlebanana]

Equilibrium question (VCAA 2017)

If a reaction is exothermic, increasing the temperature would favour the endothermic reverse reaction right? Which means that the rate of the forward reaction would decrease, but this doesn't seem to be the case here?

I read VCAA's explanation and they said ''the temperature increase will cause the rates of both the forward and reverse reactions to increase...''

Is this always the case with an increase in temperature then? That the rates of BOTH the forward and reverse will always increase? As opposed to my (incorrect?) interpretation that if a reaction is exothermic, then an increase in temperature would favour the reverse (endo) reaction and thus decrease the rate of the forward reaction?