HSC Chemistry Question Thread

[justwannawish]

Hey, for the Solvay modelling experimnt in Industrial, what would be some limitations for our method:

1. Place 2g of Sodium hydrogen carbonate precipitate into a large test tube and fit with a gas delivery tube and stopper. Clamp the test tube to a retort stand and half fill another test tube with limewater and place gas delivery tube in it
2. The carbon dioxide produced was bubbled through limewater to ensure the reaction was progressing
3. Light a Bunsen burner below the test tube such the tip of the flame just touches the base of the test tube. Heat the NaHCO3 strongly to convert it to Na2CO3. Record any observations

[SanaBanana]

hi i have a question about answering chem questions. i generally answer my chem questions much like how i answer my bio questions. so, say you get a question that says for example, explain why cellulose is classified as a condensation polymer
what i would do is define condensation polymerisation and then explain why it fits that crtieria, is that just a waste of words? is chem a subject where more info is preferred or is it the more direct the better?

[Dragomistress]

Hi,
Why doesn't the net ionic equation of water get separated into ions? For example, 2NaCl+2H2O->Cl2+h2+2NaOH is 2Cl-+2H2O->Cl2+H2+2OH-

[jazcstuart]

hi i have a question about answering chem questions. i generally answer my chem questions much like how i answer my bio questions. so, say you get a question that says for example, explain why cellulose is classified as a condensation polymer
what i would do is define condensation polymerisation and then explain why it fits that crtieria, is that just a waste of words? is chem a subject where more info is preferred or is it the more direct the better?

I'm not totally sure, however I do know for chem your answers need to be succinct and logically structured. For the example question you gave, I agree you would define condensation polymerisation then talk about why cellulose is a condensation polymer. It would probably even be good to include the chemical equation.
For other questions, it is great if you can use different methods, for example use a table (particularly for compare questions) or use diagrams.

Hi,
Why doesn't the net ionic equation of water get separated into ions? For example, 2NaCl+2H2O->Cl2+h2+2NaOH is 2Cl-+2H2O->Cl2+H2+2OH-

In net ionic equations you are determining which ions undergo a reaction. I you just look at water, in reacts to form new products:
2H2O ---> H2 + 2OH-
you can't split H2O into ions because it doesn't exist as ions, instead H and O are covalently bonded, which is why it remains as H2O on the left side of the net ionic equation.
Hope this answers your question

[Dragomistress]

Hello again!
Why is the answer C?

[jazcstuart]

Hello again!
Why is the answer C?

Total dissolved solids is a measure of the ions in the solution. A higher soncentration of ions increases conductivity and vice versa, so C is the correct answer. A and B are incorrect because not all dissolved solid affect pH, for example NaCl is neutral. D is also incorrect because AAS only measures the exact concentration of one specific ion (and one with small concentrations), but we need to find the total concentration of all dissolved solids.

[aadharmg]

Solvay process question. Is the use of trona as a source of sodium carbonate better than the Solvay process due to the purity of the product in any way? Like does it have any benefits in terms of this or not?

[Mate2425]

I doubt you'd lose a mark for it as the marking feedback didn't state that better responses did not extrapolate, but a good rule of thumb is unless you know for CERTAIN that the relationship is linear all the way to infinity (generally by an equation), then don't extrapolate.

Here's an example of why extrapolating when unsure could give you very wrong results.

(Image removed from quote.)

(This is a physics example and not a chemistry example, I know, sorry, but it conveys my point)

As you can see, when we plot the electric magnetic field magnitude with distance of a conducting sphere, it starts out very linear, but once the distance is greater than the radius of the conductor (R), it decreases exponentially. So, if you were given 5 points that all lied between 0 and R, you'd believe that it was linear, however, since we aren't CERTAIN of the equation, we shouldn't extrapolate, because in this case, we would be very wrong. This is similar to why we don't do it in the chemistry question.

As for Q20 in 2016, AAS works by comparing the same wavelength before,and after to see how much of the intensity was absorbed, you wouldn't get any proper results comparing the intensities of 2 different wavelengths, because well, they are different, and different wavelengths absorb differently.

Lastly, HFC's are the offical way for improving, and yeah you are correct, however the equation would be CH4  +  Cl2 -> CH3Cl  +  HCl. This is producing hydrochloric acid! As the question wants the one that reduces the environmental impact the most, I'd go with B.

Hope this helps!

Many thanks, your help has made an impact! 

[horse9996]

(Q27b from the 2003 exam)

I'm stuck on the second part. I know its about not repeating it so its not reliable, but how do I write that for 3 marks?

[clovvy]

(Q27b from the 2003 exam)

I'm stuck on the second part. I know its about not repeating it so its not reliable, but how do I write that for 3 marks?

Hey there, I am by no means expert at HSC chem but I'll try my best... To get 3 marks, I believe you need to explain the relationship between repetition and reliability and make judgements based on your explanation of the matter. You might want to talk about how reliability depends on repetition of the procedure.

So I'll just start you off with a few dot-points:
- fertiliser may not have been completely soluble in water, therefore mixtures must be filtered
- Acid should have been used to help dissolve and remove carbonate ions and prevent phosphate ions from precipitating
- Use Ba_2Cl to precipitate sulfates (explain why)
- Discuss the unreliability of the experiments:- technique used to collect the precipitate that can cause precipitates of formed by other impurities, lack of proof that 50mL of Barium Chloride was sufficient to precipitate all sulfate ions..
- Then discuss how to make the experiment more valid and increase the reliability by averaging out results from repeated experiment (also note that if the procedure is not done properly, the results are not valid)..

hopefully that helps

[horse9996]

Hey there, I am by no means expert at HSC chem but I'll try my best... To get 3 marks, I believe you need to explain the relationship between repetition and reliability and make judgements based on your explanation of the matter. You might want to talk about how reliability depends on repetition of the procedure.

So I'll just start you off with a few dot-points:
- fertiliser may not have been completely soluble in water, therefore mixtures must be filtered
- Acid should have been used to help dissolve and remove carbonate ions and prevent phosphate ions from precipitating
- Use Ba_2Cl to precipitate sulfates (explain why)
- Discuss the unreliability of the experiments:- technique used to collect the precipitate that can cause precipitates of formed by other impurities, lack of proof that 50mL of Barium Chloride was sufficient to precipitate all sulfate ions..
- Then discuss how to make the experiment more valid and increase the reliability by averaging out results from repeated experiment (also note that if the procedure is not done properly, the results are not valid)..

hopefully that helps

I read a sample answer that had some of this stuff like adding acid, but I thought that was validity when this question is just reliability?

[clovvy]

I read a sample answer that had some of this stuff like adding acid, but I thought that was validity when this question is just reliability?

If the experiment is invalid, how could the results be reliable than? This question requires you to make judgement...  Your results will be more reliable through repetition of a valid experimental procedure if they consistently produce similar results... 

[horse9996]

If the experiment is invalid, how could the results be reliable than? This question requires you to make judgement...  Your results will be more reliable through repetition of a valid experimental procedure if they consistently produce similar results...

So basically an experiment can't be reliable unless its valid, therefore discussion on validity is relevant here?

[clovvy]

So basically an experiment can't be reliable unless its valid, therefore discussion on validity is relevant here?

I believe yes..  Because you can have a lot of other crap in that fertiliser that also precipitates with sulfate ions...  So if your procedure is invalid, your results is more likely to be inconsistent and it will be unreliable..  While validity and reliability is different, they do relate to each other...  This is based on using  judgement

[horse9996]

I believe yes..  Because you can have a lot of other crap in that fertiliser that also precipitates with sulfate ions...  So if your procedure is invalid, your results is more likely to be inconsistent and it will be unreliable..  While validity and reliability is different, they do relate to each other...  This is based on using  judgement

Yea that makes sense, thanks