HSC Chemistry Question Thread

[Happy Physics Land]

Hi can u check my 6 marks response? question is attached below

Le Chatalier’s principle applies in step 1 and 2 since reactions are in equilibrium, but it is not applied in step 3 since reaction goes to completion.
 At step 1, low pressure and low temperature will cause reaction to shift towards the formation of NO(g). When heat is removed, reaction, being exothermic, will shift to the right to make up some of the reduced heat. When pressure is reduced, the reaction shifts right as 9 moles of gas react to form 10 moles of gas which results in increase in pressure.

The answer include “these conditions reduce the rate of reaction”  which I don’t really understand
 
 At step 2, the production of NO2(g) is favoured at low temperature and high pressure. This is because the removal of heat will cause the exothermic reaction to shift right to counteract some of the lost heat, and increase in pressure cause reaction to shift right since 3 moles of gas reacts to form 2 moles of gas which reduce some of the increased pressure.
At step 3, high pressure and high temperature will favour the formation of HNO3. The increase in pressure also increases the collision between gas molecules and water, and increase in temperature will increase the average kinetic energy of particles which leads to more successful collisions, hence reaction will shift right.
I don’t get this bit included in the answer  “as this step is not an equilibrium, the highest yield and rate of formation of nitric acid will occur when reactants undergo more collisions per second. “

-Doesn’t change in pressure, temperature, concentration or volume have no effect on reactions that go to completion?
-Pressure only applies for gas molecules right?

Hey Katherine!

A very good answer, pretty much encompasses everything. You have provided the correct answers and correct reasoning. If I am to recommend anything, I would suggest putting in "According to Le Chatelier's principle" every time you refer to an equilibrium shift and at the end of your reasoning just put "... to minimise the disturbance". If I am to be picky, I would be careful with your final statement in step 3 where you stated "hence the reaction will shift right". Step three goes to completion, hence the reaction would always be going to the right, it would be considered incorrect to state "shift right" because this would indicate that you are considering this equation as an equilibrium reaction.

1. To answer your first concern, when you decrease your temperature (i.e. if you put the equilibrium mixture in a cold condition), the particles would be deprived of their kinetic energy and hence there will be less effective collisions. As we know from year 11 chemistry, rate of reaction is dependent upon the amount of effective collisions that take place, hence decrease in temperature would reduce rate of reaction. Less pressure (i.e. increase in volume) would provide a larger amount of space for gas molecules to flow around, hence decrease the chance of effective collision taking place (imagine yourself in a crowded party that takes place in a small space, there is a high chance you would walk into someone. Now imagine yourself in the same party with same amount of people in a much larger space, it is less likely that you would walk into someone). Hence decreased pressure would also cause reduced rate of reaction.

2. You have essentially said it in your answer without realising ! You have correctly state that rise in temperature and rise in pressure would both promote effective collisions and with more effective collisions occurring between reactants, more products will be formed and the rate of reaction will increase! Temperature rise and pressure certainly changes the position of equilibrium, but they apply universally to all reactions! You may like to have a look at your year 11 content on the factors that influence the rates of reactions. Think about when we havent learnt equilibrium reactions back in year 11, and how we applied those factors such as concentration, pressure and temperature to complete reactions.

3. In our HSC chemistry, we can assume that pressure only applies for gases, because it has the greatest influence on gases than any other states of matter (However in reality, pressure do have an impact upon liquids. This is what we call hydraulic pressure and is a dominant concept in mechanical and automotive industries. But in this case you dont have to worry about this too much.) Because we increased pressure, gas particles would be more concentrated in a smaller amount of space and hence it's not the water molecules that are colliding into gas particles, but it is the gas particles that are colliding into water molecules to generate the products. If you wish, you may also mention that there is some impact of pressure upon water molecules, but for the sake of HSC, you dont have to include it.

Hope my explanation was clear! If you have further questions please dont hesitate to ask!

Best Regards
Happy Physics Land

[amandali]

Hi i dont really understand how to determine suitable indicator to use for solutions.

[Happy Physics Land]

Hi i dont really understand how to determine suitable indicator to use for solutions.

Hey Amanda!

If we have a look at the indicator. It is able to distinguish between strong acids and weak acids (0.1M HCl reflects a pink colour, 0.001M HCl reflects a pale yellow colour), however it is unable to differentiate between neutral substances and bases (colour observed for distilled water is the same as the colour observed for bases). This means, that any reactions that would have an equivalence point of 7 or more than 7 would be unsuitable because there wont be any colour change to indicate whether neutrality has been achieved or basicity has been achieved. Hence we can first eliminated B) which is a strong acid + strong base reaction. This is going be neutral and hence the indicator wont be suitable. We can eliminate C) because this is a weak acid + weak base reaction, which will also yield a pH around 7 (be mindful however, it is difficult to conduct titration with weak acid + weak base due to a lack of distinct colour change, so in fact no indicator would be very suitable). We can also eliminate D) because this is a weak acid + strong base reaction which would yield an equivalence point above pH 7. As mentioned before, because there is no colour change to distinguish between neutral equivalence point and basic equivalence point, therefore D would also be unsuitable.

So this leaves us with A as our answer. However, before we put that down as our answer, we must also check whether we can determine the equivalence point of A using our indicator. Clearly, there is a colour change from strong acid to weak acid. Hence we are able to determine the equivalence point around pH 3 - 6 (i.e. weak acid pH range). Since A) is a strong acid + weak base reaction, its equivalence point would be around pH 3 - 6 (weak acid pH range) and hence A is our final answer.

Great question! Hope you understood my explanation! If you are still a little confused about some of my explanations please dont hesitate to ask!

Best Regards
Happy Physics Land

[amandali]

Whats the difference between PHB and PHB-V? Do i need to know how to draw the structure of these polymers ?

[katherine123]

Hi can u check my response
Is it too long and how do you allocate marks?

Question: 7 marks With reference to the underlying chemistry and with relevant equations, assess the impacts on society of two uses of ethanol

Ethanol is a renewable resource produced by fermentation of sugary crops such as sugar can and corn
C6H12O6(aq) --> 2C2H6O(aq) + 2CO2(g)
Carbon dioxide and water is released when ethanol is combusted hence it is a useful car fuel.
C2H6O(l) + O2(g)--> 2CO2(g) +3H2O(l)

Theoretically, the carbon dioxide released from combustion of ethanol balances with the CO2 absorbed during photosynthesis. Hence, ethanol is considered carbon neutral as it does not lead to increase in CO2 in the atmosphere, thus prevents global warming and has a significant positive impact on society.

However, fossil fuels are used during the farming process, distillation and distribution of ethanol. Hence, it does lead to a net increase in CO2 into the atmosphere. Moreover, production of ethanol requires the clearing of vast areas of arable land which can impact on food production, causes environmental problems like soil erosion and salinity and destructs animal habitats.

Weighing up the pros and cons, using ethanol as fuel has a negative impact on society as it does lead to increase in CO2 when the energy output of its production is taken into consideration which can elevate global warming and in addition it requires clearing of arable land which also leads to severe environmental problems.

Ethanol is also used as solvent for polar substances like water, and non-polar substances like oil and grease. It has non-polar region (-C2H3) which interacts with the non-polar substances via dispersion forces and also has a polar-region (OH-) which forms hydrogen bonds or dipole-dipole interact with polar substances.
(including a diagram  of ethanol bonding with polar/non-polar sub)

Therefore, ethanol is used as a solvent in cosmetic, perfumes and paints which create a positive impact on society by making lives easier with these products being available for use.

However, ethanol is derived from petroleum involving the process of cracking and dehydration. The mining of petroleum is environmentally damaging as it can cause problems like contamination of soil, erosion and oil leaks. Moreover, ethanol is volatile and flammable thus explosion may occur when it comes into contact with heat or flames if it is not stored properly which pose a severe risk to society.

Weighing up the pros and cons, ethanol is not good to be used as solvents as it poses a great risk to society due to its flammability which outweighs its positive impact of making lives easier.

[katherine123]

Hey Katherine!

A very good answer, pretty much encompasses everything. You have provided the correct answers and correct reasoning. If I am to recommend anything, I would suggest putting in "According to Le Chatelier's principle" every time you refer to an equilibrium shift and at the end of your reasoning just put "... to minimise the disturbance". If I am to be picky, I would be careful with your final statement in step 3 where you stated "hence the reaction will shift right". Step three goes to completion, hence the reaction would always be going to the right, it would be considered incorrect to state "shift right" because this would indicate that you are considering this equation as an equilibrium reaction.

1. To answer your first concern, when you decrease your temperature (i.e. if you put the equilibrium mixture in a cold condition), the particles would be deprived of their kinetic energy and hence there will be less effective collisions. As we know from year 11 chemistry, rate of reaction is dependent upon the amount of effective collisions that take place, hence decrease in temperature would reduce rate of reaction. Less pressure (i.e. increase in volume) would provide a larger amount of space for gas molecules to flow around, hence decrease the chance of effective collision taking place (imagine yourself in a crowded party that takes place in a small space, there is a high chance you would walk into someone. Now imagine yourself in the same party with same amount of people in a much larger space, it is less likely that you would walk into someone). Hence decreased pressure would also cause reduced rate of reaction.

2. You have essentially said it in your answer without realising ! You have correctly state that rise in temperature and rise in pressure would both promote effective collisions and with more effective collisions occurring between reactants, more products will be formed and the rate of reaction will increase! Temperature rise and pressure certainly changes the position of equilibrium, but they apply universally to all reactions! You may like to have a look at your year 11 content on the factors that influence the rates of reactions. Think about when we havent learnt equilibrium reactions back in year 11, and how we applied those factors such as concentration, pressure and temperature to complete reactions.

3. In our HSC chemistry, we can assume that pressure only applies for gases, because it has the greatest influence on gases than any other states of matter (However in reality, pressure do have an impact upon liquids. This is what we call hydraulic pressure and is a dominant concept in mechanical and automotive industries. But in this case you dont have to worry about this too much.) Because we increased pressure, gas particles would be more concentrated in a smaller amount of space and hence it's not the water molecules that are colliding into gas particles, but it is the gas particles that are colliding into water molecules to generate the products. If you wish, you may also mention that there is some impact of pressure upon water molecules, but for the sake of HSC, you dont have to include it.

Hope my explanation was clear! If you have further questions please dont hesitate to ask!

Best Regards
Happy Physics Land

Does yield apply for reactions in equilibrium   If not, why?

[Happy Physics Land]

Whats the difference between PHB and PHB-V? Do i need to know how to draw the structure of these polymers ?

Hey Amanda!

Sorry for the late reply! PHB and PHBV are both common biopolymers that possess contrasting physical properties! PHB is otherwise known as 3-hydroxybutyrate, which is stiff and brittle. Because of these properties, PHB have limited usage in commodities and hence we dont usually discuss about PHB. In contrast, PHBV, or biopol, is stronger and more flexible. Because of these desirable physical properties, PHBV have widespread use in medical implants (prosthetic devices) , specialty packaging and tissue repairing. The structure of these polymers are beyond the scope of chemistry syllabus and looking at past questions they have never asked for the structure of these, so no you wouldnt need to, but I guess you can look up the structure if you are interested and just in case they do ask you in an exam. What you definitely do need to know is the specific name of the bacteria that is involved in the production of PHB and PHBV, in this case the bacteria is called Cupriavidus Necator.

Good luck studying!

Best Regards
Happy Physics Land

[Happy Physics Land]

Does yield apply for reactions in equilibrium   If not, why?

Hey Katherine!

Yes yield definitely do apply for reactions in equilibrium (as stated by le chatelier's principle, shifting of equilibrium to a certain side will favour the yield of that side), and it also applies to complete reactions as well! With the changes in temperature, changes in concentration and change in temperature, both equilibrium reactions and complete reactions will have different yields of products. You can refer to my previous answer for explanations on this!

Best Regards
Happy Physics Land

[Happy Physics Land]

Hi can u check my response
Is it too long and how do you allocate marks?

Question: 7 marks With reference to the underlying chemistry and with relevant equations, assess the impacts on society of two uses of ethanol

Ethanol is a renewable resource produced by fermentation of sugary crops such as sugar can and corn
C6H12O6(aq) --> 2C2H6O(aq) + 2CO2(g)
Carbon dioxide and water is released when ethanol is combusted hence it is a useful car fuel.
C2H6O(l) + O2(g)--> 2CO2(g) +3H2O(l)

Theoretically, the carbon dioxide released from combustion of ethanol balances with the CO2 absorbed during photosynthesis. Hence, ethanol is considered carbon neutral as it does not lead to increase in CO2 in the atmosphere, thus prevents global warming and has a significant positive impact on society.

However, fossil fuels are used during the farming process, distillation and distribution of ethanol. Hence, it does lead to a net increase in CO2 into the atmosphere. Moreover, production of ethanol requires the clearing of vast areas of arable land which can impact on food production, causes environmental problems like soil erosion and salinity and destructs animal habitats.

Weighing up the pros and cons, using ethanol as fuel has a negative impact on society as it does lead to increase in CO2 when the energy output of its production is taken into consideration which can elevate global warming and in addition it requires clearing of arable land which also leads to severe environmental problems.

Ethanol is also used as solvent for polar substances like water, and non-polar substances like oil and grease. It has non-polar region (-C2H3) which interacts with the non-polar substances via dispersion forces and also has a polar-region (OH-) which forms hydrogen bonds or dipole-dipole interact with polar substances.
(including a diagram  of ethanol bonding with polar/non-polar sub)

Therefore, ethanol is used as a solvent in cosmetic, perfumes and paints which create a positive impact on society by making lives easier with these products being available for use.

However, ethanol is derived from petroleum involving the process of cracking and dehydration. The mining of petroleum is environmentally damaging as it can cause problems like contamination of soil, erosion and oil leaks. Moreover, ethanol is volatile and flammable thus explosion may occur when it comes into contact with heat or flames if it is not stored properly which pose a severe risk to society.

Weighing up the pros and cons, ethanol is not good to be used as solvents as it poses a great risk to society due to its flammability which outweighs its positive impact of making lives easier.

Hey Katherine!

Ok so let's start off breaking down the question and analysing the mark distribution:

Straight away, 2 marks for providing two equations for the two uses. (However, if your content isnt satisfactory, teacher may only award one mark for the equations because the equations are meant to be there to support your explanations. If your explanation isnt too desirable, it deprives the meaning of having an equation included).
1 mark for assessing (i.e. making a judgment)
2 marks for identifying the use of ethanol and explaining its impact on society (you may not be awarded any mark for just simply identifying the use, nor would you be awarded any mark for explaining impacts upon environment, but something you can relate to is that CO2 continuously contribute to greenhouse effect, and this will making the Earth environment inhospitable for human society.)
2 marks for identifying a second use of ethanol and explaining its impact on society.

What I do like about your response, is your frequent use of cause and effect language and your ability to make several judgments. These are evidences of you clearly understanding how to answer an explain/assess question! Well done! You have also included several social impacts and have also explained why these impacts are positive/negative. Another positive aspect of your response is the fact that you have provided the chemistry principles behind these uses, such as your successful identification of the -OH hydroxyl group and the hydrocarbon group. This answers the part "using underlying chemistry".

A fatal error (please dont be scared off by this harsh tone its still an articulate response!) in your writing is that you are overly focusing on environmental impacts. This is a question asking about social impacts, as soon as you mention the word environment, you are off-topic. If this question is on "society and environmental impacts", I would give 7/7 for your response, beyond all doubts. Even if you want to include environmental impacts, you will still need to ultimately relate it back to effect upon society, just like what you have done with the global warming concept. When you talked about arable land, I thought you would follow up with the idea "the use of land and agricultural products for the production of ethanol would create competition with human consumption" (this is a good idea for social impact if you wanna put it in). But instead you made a great point on environmental impact, but again it's not relevant to the question.

A small technicality error is your chemical formula for ethanol. You wrote C₂H₆O, but the more correct way of expressing it is C₂H₅OH. Sure, these two formulae have exactly the same number of hydrogen atoms, but we must not ignore the significance of the -OH hydroxyl group in ethanol, because that is what allows ethanol to dissolve in water! By writing C₂H₆O, you are ignoring the significance of -OH and it leaves a chance for the teacher to deduct you a mark there!

Overall, I would personally give a 5/7. Your teacher may give a 4-5/7. So essentially your response is a brand 5 range response, which is definitely good at this stage. If you apply my feedbacks, this would definitely be a band 6 response! Well done overall!

Best Regards
Happy Physics Land

[atarz]

I'm very confused... I have done two practice half-yearly tests from James Ruse. One said to use a stopper in the production of an ester while another said not to. Also I have seen esterification using thermometers. What should be used in esterification and why?

[RuiAce]

I'm very confused... I have done two practice half-yearly tests from James Ruse. One said to use a stopper in the production of an ester while another said not to. Also I have seen esterification using thermometers. What should be used in esterification and why?

The whole point of the condenser is to not only allow for high temperatures, but ALSO to avoid a buildup of pressure within the system. Otherwise, the system will explode. You will have glass everywhere.

The stopper, is a beautiful way to PROMOTE this buildup of pressure. Therefore it is very very very VERY bad for the process of reflux.

Now, perhaps a bit more importantly, if you look carefully at your diagram that is not a stopper. That is just the top of the condenser. I think you just got the diagram confused.

Note: The thermometer is optional. I've never refluxed with one but it's probably advantageous if you want your system to be heated to a specific temperature for reflux. It may promote safety, but I don't believe it's necessary.

[atarz]

Thank you!

The whole point of the condenser is to not only allow for high temperatures, but ALSO to avoid a buildup of pressure within the system. Otherwise, the system will explode. You will have glass everywhere.

The stopper, is a beautiful way to PROMOTE this buildup of pressure. Therefore it is very very very VERY bad for the process of reflux.

Now, perhaps a bit more importantly, if you look carefully at your diagram that is not a stopper. That is just the top of the condenser. I think you just got the diagram confused.

Note: The thermometer is optional. I've never refluxed with one but it's probably advantageous if you want your system to be heated to a specific temperature for reflux. It may promote safety, but I don't believe it's necessary.

[amandali]

a) citric acid = weak, triprotic
b) acetic acid = weak, monoprotic
c)sulfuric acid=diprotic , strong
d) HCl = monoprotic strong

ive managed to rule out HCl and acetic acid  but im not sure why its citric
isnt citric really weak so its going to partially ionise producing much more less H+ than H2SO4

[Happy Physics Land]

a) citric acid = weak, triprotic
b) acetic acid = weak, monoprotic
c)sulfuric acid=diprotic , strong
d) HCl = monoprotic strong

ive managed to rule out HCl and acetic acid  but im not sure why its citric
isnt citric really weak so its going to partially ionise producing much more less H+ than H2SO4

Hey Amanda!

Well citric acid isnt REALLY weak, like l mean its still weak but its definitely not the weakest acid you will ever see. But then thats not the point in this question! Here, the amount of base required will depend on the stoichiometry of the reaction (i.e. the number of ionisable protons in the acid). Since citric acid has the most protons per mole (because its triprotic), it would require the most amount of base to change the colour of indicator.

But yes I do understand where your reasoning came from. Keep in mind however this question is about a neutralisation reaction, not so much about whether an acid is weak or strong. If we compare the balanced chemical equation for citric acid + NaOH (C₆H₈O_7(aq) + NaOH_(aq) --> NaC₆H₅O_7(aq) + 3H₂O_(l)) and the balanced chemical equation for sulfuric acid + NaOH (H₂SO_4(aq) + 2NaOH_(aq) --> Na₂SO₄(aq) + 2H₂O_(l)), we can see that 3 moles of NaOH is required to neutralise one mole of citric acid, and 2 moles of NaOH are required to neutralise one mole of citric acid. So I think this explains why the answer is citric acid not sulfuric acid more clearly for you.

If you still find it a bit hard to understand, please let me know!

Best Regards
Happy Physics Land

[RuiAce]

a) citric acid = weak, triprotic
b) acetic acid = weak, monoprotic
c)sulfuric acid=diprotic , strong
d) HCl = monoprotic strong

ive managed to rule out HCl and acetic acid  but im not sure why its citric
isnt citric really weak so its going to partially ionise producing much more less H+ than H2SO4

Precisely what HPL was trying to say.

Because we're titrating with a strong base, the reaction goes to completion REGARDLESS of the strength of the acid.

So since the acids are both equal in volume AND equal in concentration, the triprotic one will take more NaOH to effectively be neutralised.