VCE Chemistry Question Thread

[QueenSmarty]

Can we specifically be asked a question on the carbon monoxide poisoning equilibria? If so, will we be given the equations or do we need to memorise them?

[Gogo14]

Dont understand how and why a compound can react with itself when the anode and cathod reactinos are th same. E.g. Fe turns to Fe2+ ions at te anode,and Fe2+ ions turn into Fe at th cathode.(Specifically VCAA 2014 q9d)

[VanillaRice]

Can we specifically be asked a question on the carbon monoxide poisoning equilibria? If so, will we be given the equations or do we need to memorise them?

Carbon monoxide poisoning and its competing equilibrium appears in the study design under AoS 2 Outcome 2:
Le Chatelier’s principle: identification of factors that favour the yield of a chemical reaction, representation of
equilibrium system changes using concentration-time graphs and applications, including competing equilibria
involved in the occurrence and treatment of carbon monoxide poisoning resulting from incomplete combustion
of fuels.

So it is fair game for VCAA to ask you questions about this.
Not sure on whether they will supply you with the equations, but they aren't too complex, so they shouldn't be too hard to memorise.

Dont understand how and why a compound can react with itself when the anode and cathod reactinos are th same. E.g. Fe turns to Fe2+ ions at te anode,and Fe2+ ions turn into Fe at th cathode.(Specifically VCAA 2014 q9d)

For the question you referred to, it appears the cathode reaction is the reduction of Mg²⁺ ions (with an iron electrode), and the preferential anode reaction is the oxidation of iron metal (Fe).

Regardless, consider that the anode and cathode reactions are not occurring in the same location. Reduction at the anode, oxidation at the cathode. Fe²⁺ can be reduced at the anode, and Fe metal can be oxidised at the cathode. They are not reacting at the same electrode.

Hope this helps

[-elcee]

Has anyone done the 2011 vcaa exam 1 ? Is Q4 short answer about gravimetric analysis ? That's not on this year's study design right ?

[anotherworld2b]

I was wondering if someone could check whether I wrote hydrolysis reactions correctly

[Gogo14]

Carbon monoxide poisoning and its competing equilibrium appears in the study design under AoS 2 Outcome 2:
Le Chatelier’s principle: identification of factors that favour the yield of a chemical reaction, representation of
equilibrium system changes using concentration-time graphs and applications, including competing equilibria
involved in the occurrence and treatment of carbon monoxide poisoning resulting from incomplete combustion
of fuels.

So it is fair game for VCAA to ask you questions about this.
Not sure on whether they will supply you with the equations, but they aren't too complex, so they shouldn't be too hard to memorise.
For the question you referred to, it appears the cathode reaction is the reduction of Mg²⁺ ions (with an iron electrode), and the preferential anode reaction is the oxidation of iron metal (Fe).

Regardless, consider that the anode and cathode reactions are not occurring in the same location. Reduction at the anode, oxidation at the cathode. Fe²⁺ can be reduced at the anode, and Fe metal can be oxidised at the cathode. They are not reacting at the same electrode.

Hope this helps

But they are teh same redox equation.there is no voltage difference between them. By this principle, why isnt Mg and Mg2+ reacting at both the anode and cathode? Why does Mg2+ and Cl- react instead?
Bit lost sorry

[VanillaRice]

I was wondering if someone could check whether I wrote hydrolysis reactions correctly

Looks good, but note for the first reaction in each question, the water on the left hand side isn't balanced. Generally, you can omit the water in such cases (of ionisation), since when we write (aq), we are assuming the reaction is happening in an aqeuous environment i.e. water.

But they are teh same redox equation.there is no voltage difference between them. By this principle, why isnt Mg and Mg2+ reacting at both the anode and cathode? Why does Mg2+ and Cl- react instead?
Bit lost sorry

Apologies, it seems I've misunderstood your question. Are you asking why, under the conditions of Q9d, the reduction and oxidation of iron is happening at both the anode and cathode?
By replacing the anode with an iron electrode, we now have two 'competing' reactions: the oxidation of either Fe metal or Cl^-. We can see from the electrochemical series that Fe metal would be preferentially oxidised. So now chloride ions no longer become oxidised at the anode.
Due to the production of Fe²⁺ ions (at the anode, they will migrate to the cathode), there are now two 'competing' reactions at the cathode: either the reduction of Mg²⁺ or Fe²⁺. From the electrochemical series, we can see that Fe²⁺ would be preferentially reduced, so this reaction occurs instead.

The question asks about the potential implications of replacing the graphite anode with an iron anode, and we can see that it will no longer result in the reduction of magnesium ions, or the oxidation of chloride ions, due to the competing reactions.

Hope this answers your question

[Rieko Ioane]

Hi, could someone help me out (quite a lot of Qs, but they're small): https://imgur.com/a/RsyhD

Q1a iii) I didn't know the reason, but the answer (attached) seems dodgy. It talks about renewability, does this explain why it can be considered bioethanol? Is my answer correct?

Q1b ii) Similiar problem to aiii, is my answer correct?

ii) Is 10.0kg 2sf or 3sf? I think it is 2 sf but not entirely confident since the answer thinks otherwise.

Q8a) For the bonds, for VCAA exams do I need to draw all the unbonded pairs (seen in the red)? If I drew a ether link as C-O-C exactly like this, would be okay?

Also, the answer says the product of the hydrolysis of a carb is glucose, this isn't strictly true though? Is monosaccharides a better answer?

8bi) Do I need to explicitly mention the lack of hydroxyl groups? or is my answer passable?

Q10a) Would my answer be fine? It wasn't really what the answer had.

Thanks

[ardria]

Q1a iii) I didn't know the reason, but the answer (attached) seems dodgy. It talks about renewability, does this explain why it can be considered bioethanol? Is my answer correct?
Q1b ii) Similiar problem to aiii, is my answer correct?

My teacher has said all year that "ethanol" and "bioethanol" mean the exact same thing. I don't see why your answers would be incorrect, but the answers aren't wrong for indicating that it's reasonable to explain that the source is easily replenishable.

ii) Is 10.0kg 2sf or 3sf? I think it is 2 sf but not entirely confident since the answer thinks otherwise.

It is definitely 3sf! This is how sf works:

0.0001 = 1sf
0.00010 = 2sf
10.000 = 5sf

Q8a) For the bonds, for VCAA exams do I need to draw all the unbonded pairs (seen in the red)? If I drew a ether link as C-O-C exactly like this, would be okay?

Don't know, sorry!

Also, the answer says the product of the hydrolysis of a carb is glucose, this isn't strictly true though? Is monosaccharides a better answer?

Yes I would agree monosaccharides is a better answer. Glucose is the "main" monosaccharide but not the only one that is produced.

8bi) Do I need to explicitly mention the lack of hydroxyl groups? or is my answer passable?

Personally I would definitely have said that the non-polar hydrocarbon part of the molecule dominates over the polar hydroxyl group, to be safe.

Q10a) Would my answer be fine? It wasn't really what the answer had.

What did the answers say? Your Error 1 seems right but your Error 2 is much more questionable and likely not passable.


What company is this? Seems a touch dodgy.

[Phenomenol]

Hi, could someone help me out (quite a lot of Qs, but they're small): https://imgur.com/a/RsyhD

Q1a iii) I didn't know the reason, but the answer (attached) seems dodgy. It talks about renewability, does this explain why it can be considered bioethanol? Is my answer correct?

Q1b ii) Similiar problem to aiii, is my answer correct?

ii) Is 10.0kg 2sf or 3sf? I think it is 2 sf but not entirely confident since the answer thinks otherwise.

Q8a) For the bonds, for VCAA exams do I need to draw all the unbonded pairs (seen in the red)? If I drew a ether link as C-O-C exactly like this, would be okay?

Also, the answer says the product of the hydrolysis of a carb is glucose, this isn't strictly true though? Is monosaccharides a better answer?

8bi) Do I need to explicitly mention the lack of hydroxyl groups? or is my answer passable?

Q10a) Would my answer be fine? It wasn't really what the answer had.

Thanks

For Q1 a. iii. I think your answer is fine. Renewability is probably not 100% relevant for the given question.

For Q1 b. ii. Same as above, your answer is also fine.

ii. I would say 3 sig figs here.

Q8 a. I thiiiink both ways would be fine.

Monosaccharides would indeed be a better answer.

Q8 b. i. Mentioning lack of hydroxyl groups does help here, but I think for a 1 mark answer yours suffices.

Q10 a. Error 1 is good. In Error 2, I don't think increasing the electrode surface area would affect the cell voltage, so I would say it is not a valid argument. I believe in general the overall cell voltage doesn't depend on the shape/size of the electrodes.

[angela666]

Can I use pencils in the chemistry and physics exams? I have been using pencils all year but have just realised that the exam says to write in pens. I dont think I can change it now. Would it be alright if I just use a really dark pencil?

[bentennason]

Q10a) Would my answer be fine? It wasn't really what the answer had.

Thanks

Increasing electrode size will not increase cell potential difference:
cell potential difference (voltage) = energy per unit of charge (joules/coulomb), coulomb = unit of electrical charge that can be carried by an electron or proton

Increasing the size of one electrode whilst keeping all other conditions constant will enable the electrode to produce more charge (electrons). However, the ENERGY per unit of charge will be identical when using a smaller, larger, tiny, or minuscule electrode. Essentially, you're just increasing the capacity of the electrode and not its standard reduction/oxidation potential.

In regards to rate of reactions for galvanic cells, I'd advise to read the relevant section on the study design as there are a multitude of factors that will do this.

[bentennason]

Can I use pencils in the chemistry and physics exams? I have been using pencils all year but have just realised that the exam says to write in pens. I dont think I can change it now. Would it be alright if I just use a really dark pencil?

I used pencil. Just ensure you completely rub out the mistakes or cross them out. Personally I feel more sure when writing with a pencil because i know I can rub any bs sentences out lol

[Butterflygirl]

Hey guys, how would you distinguish between fossil fuels and biofuels (I'm not sure what they're definitions are)?

Thanks!!

[Syndicate]

Hey guys, how would you distinguish between fossil fuels and biofuels (I'm not sure what they're definitions are)?

Thanks!!

Basically, fossil fuels are non-renewable* fuels (they aren't that sustainable** either), whilst biofuels are renewable fuels produced from biomass (are quite sustainable).

Some fossil fuels you should know about:
- coal, sourced from coal mines, and is often considered to contain carbon only (can contain other compounds such sulphur and nitrogen as well). It is a non-renewable fuel, as it basically takes millions of years to form.
- crude oil, contains several hydrocarbons, which are separated using fractional distillation (LPG, petrodiesel, petrol etc... are obtained from it)
- Natural gas (CH4), obtained through a process of fracking.

Biofuels:
- Biogas (CH4), obtained through large digesters (tanks) containing anaerobic bacteria.
- Bioethanol, produced from the fermentation of glucose (sugar canes is a good example). glucose -> 2 x ethanol + 2 x carbon dioxide
- Biodiesel, produced through a transesterification reaction between a triglyceride and methanol. The fatty acid, methyl ester is the biodiesel (Remember: you would require a KOH catalyst for the reaction to proceed)

Note: You'll have to know how they are sourced, and their affects on the environment and economy etc...
* can be replenished in a short amount of time through natural processes (ie biomass).
** Is enough for our lifetime use