VCE Chemistry Question Thread

[bonjour-sarah]

Some questions that I would love some assistance with, as I'm finding the application of the course falling into place, yet have some weak areas that I need to sort out!!!

1. Methods for improving figuring out the compound when given a large variety of specs? I understand the basic bonds from IR and can calculate relative C or H environments, but I always seem to place functional groups at logical places, check for H environments (which seem right) but then they always are more complicated than I assume. I have only got a small number of practice questions for these too, as they are not super common in all the exams I have, but are definitely crucial. Tips!!??

2. Fuel cells: I know this comes up a lot, and I have read up a lot of contradicting things so I'd love last minute advice: what cell equations am I meant to be memorising? Thus-far in VCAA they always give full equation or diagram to work from? So what is it that I should be learning, because there are an infinite mount available to learn? Hall-herault, Downes, etc etc

3. Biochem fats: I seem to feel like this is an issue area but I'm not sure. Is understanding formation and hydrolysis of triglyceride mostly it? I know there are some q's about Br which are very confusing in practice exams?

Sorry if my questions are very vague! But anything in these areas will be super useful!!

[jyce]

Some questions that I would love some assistance with, as I'm finding the application of the course falling into place, yet have some weak areas that I need to sort out!!!

1. Methods for improving figuring out the compound when given a large variety of specs? I understand the basic bonds from IR and can calculate relative C or H environments, but I always seem to place functional groups at logical places, check for H environments (which seem right) but then they always are more complicated than I assume. I have only got a small number of practice questions for these too, as they are not super common in all the exams I have, but are definitely crucial. Tips!!??

2. Fuel cells: I know this comes up a lot, and I have read up a lot of contradicting things so I'd love last minute advice: what cell equations am I meant to be memorising? Thus-far in VCAA they always give full equation or diagram to work from? So what is it that I should be learning, because there are an infinite mount available to learn? Hall-herault, Downes, etc etc

3. Biochem fats: I seem to feel like this is an issue area but I'm not sure. Is understanding formation and hydrolysis of triglyceride mostly it? I know there are some q's about Br which are very confusing in practice exams?

Sorry if my questions are very vague! But anything in these areas will be super useful!!

Hi,

1. Here's some tips regarding each spectrum you might encounter when attempting to identify an organic compound:
- For an IR spectrum, you basically use it to identify functional groups (e.g. a broad band at 2500-3300 cm^-1 and a band at about 1700 cm^-1 would suggest you have a carboxyl group).
- For a ¹H NMR spectrum, this is useful for distinguishing between isomers but more specifically you would use this primarily for determining the number of different hydrogen environments in the compound and for determining the number of hydrogens in adjacent environments using the 'n + 1' rule, but you might also use it for determining the relative number of hydrogens in each environment and for perhaps determining specific hydrogen environments using the chemical shifts (the only really easy one to ascertain is a carboxyl environment, at a chemical shift of 9-13 ppm)
- For a ¹³C NMR spectrum, it's the same as with ¹H NMR but with carbon and there's no peak splitting this time around
- Lastly, for a mass spectrum, you would primarily use this to identify the molar mass of the compound, which = the m/z ratio of the parent molecular ion (note that there may be multiple parent molecular ions, due to different isotopes in the sample, as different isotopes have different masses).
A good and somewhat challenging example of all this is Section B, Question 4 of last year's VCAA exam: you use the IR spectrum to confirm the presence of a particular functional group, you then use the mass spectrum to identify how large the compound is, and you use the ¹H NMR data to determine which of two possible isomers the compound is.
If you look back through this thread a few days ago, you'll find some more specific advice on IR and NMR spectroscopy that I posted for someone else!

2. No, you do not have to know any specific cell reactions. However, one that does often come up and is easy to remember is the cell reaction for a hydrogen-oxygen fuel cell: 2H₂(g) + O₂(g) --> 2H₂O(l). But you could figure this out fairly easily using the electrochemical series if you forget.

3. Yes, understanding how glycerol and three fatty acid molecules will react in a condensation reaction to produce a triglyceride molecule, and how the addition of water will reverse the formation of a triglyceride, is probably the most important aspect of this topic. However, you've touched on something else that gets asked: determining the number of carbon-carbon double bonds in fatty acid molecules. There's two ways this comes up: either they give you the formula/name of a fatty acid and see whether the formula fits the C_nH_{2n + 1}COOH rule, in which case it is saturated (i.e. has no C=C bonds), or you have to calculate the number of carbon-carbon double bonds in a fatty acid for which you are not given the formula. In this second case, an addition reaction has occurred and basically the number of double bonds = the number of times greater the mole of Br₂/H₂/I₂ is than the mole of the fatty acid. You should also be able to relate, via chemical reactions, triglycerides and fatty acids to the production of biodiesel, one of the three main types of biochemical fuels covered in the course.

Hope this helps! If you have any specific questions regarding these topics, just ask!

[sunshine98]

1. By separating the two half-cells, the electrons are forced to travel along conducting wire connecting the half-cells. This flow of electrons in the external circuit is the electricity. Without separating the half-cells, the electrons would simply move through the solution, rather than travelling along wire.
2. Yes, galvanic cells always lose some energy as heat energy. One reason for this is that the wires connecting the half-cells have a resistance.
3. I believe some redox reactions are indeed endothermic, although many, including combustion reactions, are exothermic.
4. We only use catalytic electrodes in fuel cells because reactions in fuel cells tend to be particularly slow, whereas reactions in other types of cells are not so slow. We could use catalysts in other types of cells, but this is expensive and not necessary.
5. You should always use the states that are indicated in the particular question, NOT by the electrochemical series. A good example is the VCAA 2014 exam where magnesium and chloride ions were molten, not aqueous as in the series. States do matter.
6. Particularly high concentrations can change relative oxidising and reducing strengths, but I don't believe '1 M' is a magic number for this.

These are really excellent question, by the way!

^Thank you 

-Is there any way to determine the number of structural isomers an alkane has? I always get this wrong , any tips?
-Do we need to know about secondary and primary structure of DNA and what are they? I haven't seen this anywhere except in two neap trials? I'm assuming that primary = polynucleotide and secondary = double stranded, is this right?
-When an amino acid is placed in either an acidic or basic solution , can it lose all its H+ attached to carboxylic acids (like even if its in the Z group) and likewise can it gain a H+ on both NH2 (given that another one is actually on the Z group). Or does this depend on the extent to which the solution is basic or acidic?
Thanks

[jyce]

^Thank you 

-Is there any way to determine the number of structural isomers an alkane has? I always get this wrong , any tips?
-Do we need to know about secondary and primary structure of DNA and what are they? I haven't seen this anywhere except in two neap trials? I'm assuming that primary = polynucleotide and secondary = double stranded, is this right?
-When an amino acid is placed in either an acidic or basic solution , can it lose all its H+ attached to carboxylic acids (like even if its in the Z group) and likewise can it gain a H+ on both NH2 (given that another one is actually on the Z group). Or does this depend on the extent to which the solution is basic or acidic?
Thanks

1. I always figure out the number of straight-chain isomers I can make, then move on to how many branched isomers I can make. For example, for isomers of butene: while keeping the chain straight, I can only have the C=C bond at an end or towards the middle. Now I move on to branching it: the only way I can branch it is by having a methyl group on the middle carbon, and now in this case there's only one place I can put the C=C (on an end). Therefore, there's three structural isomers of butene. A lot of students tend to over guess the number of isomers, because they draw two structures that look different on paper but they're actually the same if you count the longest carbon chain.
2. Yes, you need to know the primary and secondary structures of DNA, but NOT the tertiary structure (even though some textbooks cover this). And you're correct: the primary structure is the sequence of nitrogenous bases along the sugar-phosphate backbone, and the secondary structure is the complementary base pairing between A and T and C and G on two different strands, such that these strands form a double helix with one another and such that they are anti-parallel with one another.
3. Yes. Note, however, if you are asked to drawn the zwitterion of an amino acid - say, glutamic acid - don't ionise all the carboxyl and amino groups, because although a zwitterion must have both positive and negative charges it simultaneously must not have an overall charge. So in the case of glutamic acid, you would only ionise one carboxyl group (it doesn't matter which), and you would ionise the amino group.

[qwerty123456]

When a reaction involves molten are all the states liquid including solids and gases?

[jyce]

When a reaction involves molten are all the states liquid including solids and gases?

If something is 'molten', it is in the liquid state; hence, solids and gases cannot be molten, because they are not liquids - they are solids and gases. For example, molten magnesium chloride would be MgCl₂(l).

[bonjour-sarah]

Hi,

1. Here's some tips regarding each spectrum you might encounter when attempting to identify an organic compound:
- For an IR spectrum, you basically use it to identify functional groups (e.g. a broad band at 2500-3300 cm^-1 and a band at about 1700 cm^-1 would suggest you have a carboxyl group).
- For a ¹H NMR spectrum, this is useful for distinguishing between isomers but more specifically you would use this primarily  for determining the number of different hydrogen environments in the compound and for determining the number of hydrogens in adjacent environments using the 'n + 1' rule, but you might also use it for determining the relative number of hydrogens in each environment and for perhaps determining specific hydrogen environments using the chemical shifts (the only really easy one to ascertain is a carboxyl environment, at a chemical shift of 9-13 ppm)
- For a ¹³C NMR spectrum, it's the same as with ¹H NMR but with carbon and there's no peak splitting this time around
- Lastly, for a mass spectrum, you would primarily use this to identify the molar mass of the compound, which = the m/z ratio of the parent molecular ion (note that there may be multiple parent molecular ions, due to different isotopes in the sample, as different isotopes have different masses).
A good and somewhat challenging example of all this is Section B, Question 4 of last year's VCAA exam: you use the IR spectrum to confirm the presence of a particular functional group, you then use the mass spectrum to identify how large the compound is, and you use the ¹H NMR data to determine which of two possible isomers the compound is.
If you look back through this thread a few days ago, you'll find some more specific advice on IR and NMR spectroscopy that I posted for someone else!

2. No, you do not have to know any specific cell reactions. However, one that does often come up and is easy to remember is the cell reaction for a hydrogen-oxygen fuel cell: 2H₂(g) + O₂(g) --> 2H₂O(l). But you could figure this out fairly easily using the electrochemical series if you forget.

3. Yes, understanding how glycerol and three fatty acid molecules will react in a condensation reaction to produce a triglyceride molecule, and how the addition of water will reverse the formation of a triglyceride, is probably the most important aspect of this topic. However, you've touched on something else that gets asked: determining the number of carbon-carbon double bonds in fatty acid molecules. There's two ways this comes up: either they give you the formula/name of a fatty acid and see whether the formula fits the C_nH_{2n + 1}COOH rule, in which case it is saturated (i.e. has no C=C bonds), or you have to calculate the number of carbon-carbon double bonds in a fatty acid for which you are not given the formula. In this second case, an addition reaction has occurred and basically the number of double bonds = the number of times greater the mole of Br₂/H₂/I₂ is than the mole of the fatty acid. You should also be able to relate, via chemical reactions, triglycerides and fatty acids to the production of biodiesel, one of the three main types of biochemical fuels covered in the course.

Hope this helps! If you have any specific questions regarding these topics, just ask!

That was absolutely amazing-ly helpful thank you!!!!!

[jyce]

That was absolutely amazing-ly helpful thank you!!!!!

You're very welcome 

[warya]

What is chemical shift?

[Alter]

Anyone know where to find solutions to the VCAA 2013 Sample exam?

[jyce]

What is chemical shift?

Just think of it as a measure of the amount of radio energy needed to change the spin state of hydrogen/carbon nuclei in a particular environment, compared to a reference environment (TMS). The higher the chemical shift, the less energy needed.

[jyce]

Anyone know where to find solutions to the VCAA 2013 Sample exam?

http://www.cea.asn.au/sites/default/files/sample_paper_answers_2013.pdf

Obviously, these are not answers from VCAA itself, and I believe that the worded answers in this document are sometimes a bit too much. 

[cosine]

Exam: http://www.vcaa.vic.edu.au/Documents/exams/chemistry/2008chem2-w.pdf

can someone help me with the following questions:

Q7: No idea
Q8: can you explain why the concentration of hydrogen gas is higher than Iodine? and why is it that the concentration of HI increase more than all the products?
Q12: Still massively struggling with pH. Someone save me please, how can I get better at pH stuff?
Q16: I chose C but idk why its D?
Q18: Chose A, answer is D. Isnt it in galvanic cells, the negative electrode = oxidation and positive electrode = reduction?
Q19: the last three options all produce 2 moles of CO2 per mole of the fuel?

[bonjour-sarah]

I'm really lost with this (Q1 spectra 1 from 2015 Lisachem exam) . I was under the impression that the peak (unless wary of isotopes) that has highest m/z is molecular ion, and hence is molar mass of compound? I thought I remembered being tricked by a tiny peak before, so I was looking at the 76 m/z or even 75. But why is the molecule 74g.mol? Maybe I'm over thinking this, but I'm really not sure.

Unfortunately I am unable to attach anything (nor see anybodies attachments anymore) but to best describe the last few peaks of the spectra- a large one at 74 m/z, quite a small one at 75 and a barely visible one at 76.

When do I assume that the highest m/z are isotope caused? Thanks

[jyce]

Exam: http://www.vcaa.vic.edu.au/Documents/exams/chemistry/2008chem2-w.pdf

can someone help me with the following questions:

Q7: No idea
Q8: can you explain why the concentration of hydrogen gas is higher than Iodine? and why is it that the concentration of HI increase more than all the products?
Q12: Still massively struggling with pH. Someone save me please, how can I get better at pH stuff?
Q16: I chose C but idk why its D?
Q18: Chose A, answer is D. Isnt it in galvanic cells, the negative electrode = oxidation and positive electrode = reduction?
Q19: the last three options all produce 2 moles of CO2 per mole of the fuel?

Question 7
These types of questions, where they sort of combine enthalpy changes from multiple reactions, are always a bit tricky to grasp. Fortunately, we have a diagram to assist us. Looking at the diagram, going from 1Cu₂O(s) and 1/2O₂(g) to 2CuO(s) releases 312 - 170 = 142 kJ mol^-1. Now, I want to go the other direction and I also want to double the moles: 4CuO(s) --> 2Cu₂O(s) + O₂(g). So, the enthalpy change for the new reaction is +142 x 2 = +248 kJ mol^-1. The answer is 'A'.

Question 8
Well, the answer is 'B': the concentrations instantaneously increase when the volume is decreased and there is no further change in concentration as the system cannot oppose this change due to the fact that it has the same number of particles on the reactant and product sides. Your questions here are actually irrelevant to answering the question. The only reason hydrogen is more concentrated than iodine in this particular scenario is because there's more hydrogen in the mixture, and the only reason the concentration of hydrogen iodide increases more is again because it is more concentrated. We haven't been given the initial concentrations, but I can discern this from the correct graph.

Question 12
Only 'II' is true, so the answer is 'A'.
'I' is incorrect because adding a base increases the pH.
'III' is incorrect because although propanoic acid is a weak acid, it will still have ionised to a very small extent.
'II' is a correct statement. Think about it: the more NaOH you add, the more you ionise the propanoic acid. Now, propanoic acid and NaOH would react in a 1:1 ratio, as propanoic acid donates one proton per molecule (i.e. is monoprotic) and sodium hydroxide only has one hydroxide ion per molecule. So, when 100 mL of 0.08 M NaOH is added to 100 mL of 0.16 M propanoic acid, the mole of propanoic acid is double so only half the amount of NaOH needed to neutralise the acid has been added. This means that only 50% of the acid has ionised at this point, so the concentration of propanoic acid = the concentration of its conjugate base.

Question 16
When determining reactions using the electrochemical series, I always encourage my students to circle all the possible reactants.
So, for the electrolysis of molten NaF, and I'm assuming with inert electrodes:
- the potential oxidants are: Na⁺(l), and therefore the cathode/reduction reaction will be Na⁺(l) + e^- --> Na(s)
- the potential reductants are: F^-(l), and therefore the anode/oxidation reaction will be 2F^-(l) --> F₂(g) + 2e^-
For the electrolysis of aqueous NaF:
- the potential oxidants are: Na⁺(aq) and H₂O(l)
- the potential reductants are: F^-(aq) and H₂O(l)
Being higher up on the series than Na⁺(aq), and lower down on the series than F^-(aq), H₂O(l) is now the stronger oxidant and the stronger reductant and therefore both the cathode and anode reactions will have changed.
The answer is 'D'.

Question 18
In a galvanic cell, the reductant is oxidised at the anode, and the anode is negative in a galvanic cell.
- In a cell of P and Cu, Cu is the negative electrode so it is a stronger reductant than P
- In a cell of Cu and Q, Q is the negative electrode and so it is a stronger reductant than Cu
- In a cell of Cu and R, R is the negative electrode and so it is also a stronger reductant than Cu
- In a cell of Q and R, R is the negative electrode and so it is a stronger reductant than Q.
So, the order of weakest to strongest reductant is: P, Cu, Q, R (i.e. the answer is 'D').

Question 19
It's not simply a matter of looking at which equation has the greatest mole of CO₂, as the question has asked for the greatest amount of CO₂ per coulomb of electrical charge. So, the mole of CO₂ in each equation needs to be divided by the mole of electrons!
- For reaction methanol: 1/6
- For reaction ethanol: 2/12 = 1/6
- For reaction ethane: 2/14 = 1/7
- For ethane-1, 2-diol: 2/10 = 1/5
As the mole of CO₂ is only being divided by '5' for ethane-1, 2-diol, this has the greatest mole of CO₂ per coulomb of charge and therefore the answer is 'D'.