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Author Topic: VCE Chemistry Question Thread  (Read 2313431 times)  Share 

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-_-zzz

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Re: VCE Chemistry Question Thread
« Reply #8130 on: September 19, 2019, 04:47:46 pm »
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Hi Guys,

Just a quick question. Are all amino acids considered soluble in water? Or does it depend on the R group/side chain?

All replies will be much appreciated. Thanks

It should be noted that all amino acids are soluble due to the ability of their acid/base components to ionise. This results in ion-dipole attractive forces with surrounding water molecules.
« Last Edit: September 19, 2019, 04:58:11 pm by -_-zzz »

-_-zzz

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Re: VCE Chemistry Question Thread
« Reply #8131 on: September 19, 2019, 04:52:51 pm »
+2
Hi!

I am confused with the answers to this question. I am pretty sure for % m/m the units should be the same, for eg grams/grams.
But for this question, the answer is in mg/g.
Can someone please provide some clarification?

Thanks a lot!!

The units must be the same otherwise it has no meaning. The 0.15 mg should have been converted to grams first. Hope that helps clarify things.

xxxjss

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Re: VCE Chemistry Question Thread
« Reply #8132 on: September 27, 2019, 11:03:56 am »
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what is the state of water in combustion reactions ? :-[
I understand that due to the nature of combustion, water would be steam hence a gas, H2O(g) but I remember at an ATARnotes lecture at the beginning of the year that liquid state should be used as the data for heat of combustion were acquired at 25 Celsius? and water is a liquid at that temp??? Would this only apply to thermochemical equations then?
 :'( There is nothing worst then losing marks for states  ;-;

sweetcheeks

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Re: VCE Chemistry Question Thread
« Reply #8133 on: September 27, 2019, 01:27:27 pm »
+4
what is the state of water in combustion reactions ? :-[
I understand that due to the nature of combustion, water would be steam hence a gas, H2O(g) but I remember at an ATARnotes lecture at the beginning of the year that liquid state should be used as the data for heat of combustion were acquired at 25 Celsius? and water is a liquid at that temp??? Would this only apply to thermochemical equations then?
 :'( There is nothing worst then losing marks for states  ;-;

Short answer is liquid state.

At the VCE level (and most basic thermodynamic courses), combustion reactions are always measured at standard conditions (1 bar, 298K/25˚C). As a result this means that all species (chemicals) are at their standard state (how they would exist under standard conditions). At 25˚C and 1 bar of pressure, water will be a liquid.

The question will normally tell you the conditions (e.g. at standard conditions). If there is no information like this given, assume standard conditions.

pugs

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Re: VCE Chemistry Question Thread
« Reply #8134 on: September 28, 2019, 07:11:30 pm »
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hi!

for this question: Starch consists mainly of amylose, which is a polymer made from glucose. A particular form of amylose has a molar mass of 3.62 x 10^5 g/mol. How many molecules of glucose are contained in this form of amylose?

after working through the question, I had an answer of 2234.456.. at this point, would you always round up to ensure that you reach the molar mass required?

–

also, for calorimetry questions, do we get penalised if we multiply the volume of water by 0.997 if the temperature isn't 25ēC?/ is it necessary to multiply by 0.997 in the exam even if it is at 25ēC?

thank you! :D
« Last Edit: September 28, 2019, 07:13:25 pm by pugs »


2019 vce journal here

sweetcheeks

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Re: VCE Chemistry Question Thread
« Reply #8135 on: September 28, 2019, 08:20:11 pm »
+3
hi!

for this question: Starch consists mainly of amylose, which is a polymer made from glucose. A particular form of amylose has a molar mass of 3.62 x 10^5 g/mol. How many molecules of glucose are contained in this form of amylose?

after working through the question, I had an answer of 2234.456.. at this point, would you always round up to ensure that you reach the molar mass required?

–

also, for calorimetry questions, do we get penalised if we multiply the volume of water by 0.997 if the temperature isn't 25ēC?/ is it necessary to multiply by 0.997 in the exam even if it is at 25ēC?

thank you! :D

I would stick with the conventional rounding rules. I don't believe that 2234 vs. 2235 is going to matter, as long as you show how you reached your answer. What they would more be looking for is people forgetting that formation of amylose involves condensation polymerisation and dividing by 180 g/mol.

If they don't give you a value such as density or heat capacity in the question, use the one listed in the data booklet. If they say that the solution was at 40˚C but they don't give you the specific gravity (density) at 40˚C you are expected to use 0.997 g/mL.

pugs

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Re: VCE Chemistry Question Thread
« Reply #8136 on: September 29, 2019, 07:03:17 pm »
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I would stick with the conventional rounding rules. I don't believe that 2234 vs. 2235 is going to matter, as long as you show how you reached your answer. What they would more be looking for is people forgetting that formation of amylose involves condensation polymerisation and dividing by 180 g/mol.

If they don't give you a value such as density or heat capacity in the question, use the one listed in the data booklet. If they say that the solution was at 40˚C but they don't give you the specific gravity (density) at 40˚C you are expected to use 0.997 g/mL.
thank you so much for the response!

–––––––––

i also have two other questions (open to everyone)

- why are the products of substitution reactions of haloalkanes with water hydrogen halides? wouldn't they be the acids if the water that was being combined with the haloalkane came into contact with the halogen?

- for questions requiring you to draw the structure of a molecule, is specific branching (eg for chemical shift) required to be shown in the diagram even if it isn't mentioned in the question?

thank you!


2019 vce journal here

jollyboat

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Re: VCE Chemistry Question Thread
« Reply #8137 on: October 01, 2019, 03:59:18 pm »
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I was having trouble understanding a part of my textbook and was wondering if anyone could help me out? It's to do with chiral molecules:

Enantiomers have identical physical properties, apart from the way they rotate plane-polarised light. For example, they have the same melting and boiling points and the same solubility in water.
However, enantiomers usually interact differently with other chiral molecules. The human body contains mainly chiral molecules, so molecules that are chiral have different effects in the body.


I understand that 2 enantiomers of the same chiral molecule would have different chemical properties -- since they have different spacial arrangements. But I don't understand why they would only have different chemical properties when they intact with other chiral molecules, which is what I interpreted the bolded part to mean? Surely they would have different chemical properties when they interact with non chiral molecules too?

Potentially I'm missing something obvious ahah
« Last Edit: October 01, 2019, 04:55:45 pm by jollyboat »

studyingg

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Re: VCE Chemistry Question Thread
« Reply #8138 on: October 01, 2019, 04:39:49 pm »
+2
I was having trouble understanding a part of my textbook and was wondering if anyone could help me out? It's to do with chiral molecules:

Enantiomers have identical physical properties, apart from the way they rotate plane-polarised light. For example, they have the same melting and boiling points and the same solubility in water.
However, enantiomers usually interact differently with other chiral molecules. The human body contains mainly chiral molecules, so molecules that are chiral have different effects in the body.


I understand that 2 enantiomers of the same chiral molecule would have different chemical properties -- since they have different structures. But I don't understand why they would only have different chemical properties when they intact with other chiral molecules, which is what I interpreted the bolded part to mean? Surely they would have different chemical properties when they interact with non chiral molecules too?

Potentially I'm missing something obvious ahah

While you are correct in assuming that different molecular structures lead to different chemical properties, your assumption that optical isomers have different structures is incorrect. Optical isomers are a sub-category of stereoisomers, which unlike structural isomers, are molecules that have the same molecular structure, but they differ in the geometric/spatial arrangement of the atoms within the molecule. As you know, chiral molecules are mirror images of each other, hence they have the same molecular structure, just a different spatial arrangement of the atoms within the structure. This is a bit confusing, so perhaps you could google what this looks like when represented in 3D. For example, your hands have an identical structure they are just mirror images that cannot be superimposed due to the differing positions of your fingers (lol this is a weird example, but in the book, they say that chiral is greek for the hand so its how I like to visualise it.)  The reason that chiral molecules differ chemically when interacting with other chiral molecules is that other chiral molecules also have a distinct shape. Think of the interaction between chiral molecules using the analogy of a lock and key. Only a particularly shaped key can fit into a lock, even if you tried using the 'chiral' counterpart of said key, it wouldn't be able to unlock the lock. hope this makes sense and isn't filled with too many senseless analogies, there's probably a more sophisticated explanation, but this is what I could come up with based on my knowledge of the study design.
« Last Edit: October 01, 2019, 04:41:30 pm by studyingg »

Erutepa

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Re: VCE Chemistry Question Thread
« Reply #8139 on: October 01, 2019, 08:26:46 pm »
+2

- why are the products of substitution reactions of haloalkanes with water hydrogen halides? wouldn't they be the acids if the water that was being combined with the haloalkane came into contact with the halogen?
I think this question falls outside of what would be assessed in VCE. As far as i've seen, you'll only be given substitution reactions involving an alkane and a halogen gas, producing a haloalkane and a hydrogen halide.
- for questions requiring you to draw the structure of a molecule, is specific branching (eg for chemical shift) required to be shown in the diagram even if it isn't mentioned in the question?
I am not sure exactly what you're asking, but if they ask for the structure of a molecule you must show the specific branching and include all bonds.
I've heard from some examiner that the actual application of chemical shift values included in the data booklet have never actually been assessed in identifying the structure of a molecule from NMR spectra. You only ever need to use the number of peaks and (for proton NMR) the splitting and relative peak areas.
Does this answer you question.
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-_-zzz

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Re: VCE Chemistry Question Thread
« Reply #8140 on: October 04, 2019, 11:42:19 pm »
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Hello,

Just have a few experimental design questions which I've had no luck finding answers for :/

For titrations, how does taking the average of concordant titres inherently minimise the effect of random errors? Furthermore let's define reliablity as being 'whether a different person can achieve the same results for the same experiment under the same conditions.' So why exactly does reliabilty increase in tandem with precision?

Any help would be greatly appreciated. Cheers :)

Erutepa

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Re: VCE Chemistry Question Thread
« Reply #8141 on: October 05, 2019, 09:10:56 am »
+2
Hello,

Just have a few experimental design questions which I've had no luck finding answers for :/

For titrations, how does taking the average of concordant titres inherently minimise the effect of random errors? Furthermore let's define reliablity as being 'whether a different person can achieve the same results for the same experiment under the same conditions.' So why exactly does reliabilty increase in tandem with precision?

Any help would be greatly appreciated. Cheers :)
1) If you have done enough trials, non-concordant results should reflect trials affected by random errors, thus by eliminating them from the calculated average, you are minimising their effect on your final results.
To elaborate on this: assume you obtain the titres 11.01ml 11.25ml, 11.05ml, 10.97ml, and 10.80ml.
in VCE chemistry we define concordant titres as grouped within 0.1ml of each other, thus we can say 11.01ml, 11.05ml, and 10.97ml are concordant (as they are all within 0.1ml of each other). The results 10.80ml and 11.25ml are not concordant, as they are not within 0.1ml of the other data, and their significant deviance from the concordant results indicates that errors occurred within the trial such to influence the results. As a result, when calculating the average, we eliminate these nonconcordant results such to attempt to lower the influence of any random errors on our final data.

2) If your experiment has a high reliability, these other people can follow you experiment and obtain the same results consistently, whereas a low reliability will mean these other experimenters will obtain varying results. Since precision refers to the grouping of one's data (consistent results = precise) the higher the reliability of the experiment, the more precise the data that will be obtained.
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dream chaser

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Re: VCE Chemistry Question Thread
« Reply #8142 on: October 05, 2019, 01:47:11 pm »
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Hi Guys,

Just a quick question. When a question on the exam asks you to write an equation for a reaction, do they mean a balanced equation? I thought if they wanted a balanced equation, they would tell that in the question itself?

All replies will be very much appreciated. Thanks  :)

Erutepa

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Re: VCE Chemistry Question Thread
« Reply #8143 on: October 05, 2019, 02:06:38 pm »
+3
Hi Guys,

Just a quick question. When a question on the exam asks you to write an equation for a reaction, do they mean a balanced equation? I thought if they wanted a balanced equation, they would tell that in the question itself?

All replies will be very much appreciated. Thanks  :)
I am not entirely sure off the top of my head, but I always write full balanced equations just to be safe.
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Sine

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Re: VCE Chemistry Question Thread
« Reply #8144 on: October 05, 2019, 03:25:15 pm »
+3
Hi Guys,

Just a quick question. When a question on the exam asks you to write an equation for a reaction, do they mean a balanced equation? I thought if they wanted a balanced equation, they would tell that in the question itself?

All replies will be very much appreciated. Thanks  :)
Yeah in the past if they wanted a balanced equation they will explicitly tell you, but like Erutepa said above it doesn't hurt to balance it and you never know how they will mark it in a partiular year.