2021 BoS Chemistry Trial (1 Viewer)

idkkdi

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If by "merit" you mean marks in an exam, marking can only be done based on what is written in responses, not on what an examiner might think the student understands, etc. So, a response that answers the question will always gain credit. Verbatim memorisation becomes much less necessary, however, with understanding. By understanding what a prac is about and how it works, much of the sequence of steps in a method becomes necessary consequences and so can be deduced. For example, washing techniques for a titration... once someone understands that the concentration of the solution in the pipette and burette must not be changed for accurate results, washing each with the solution they are to contain becomes clear - using anything else will cause a change in concentration by dilution or reaction or both. In the conical flask, by contrast, all of the reacting solutions must come from the pipette and the burette, so any residue from washing the CF must be water. This does dilute the substance in the CF but that doesn't matter as the chemical amount / moles is unchanged and its volume when it was in the pipette is known.

If you find yourself needing to memorise a procedure, I suggest you reflect on which steps you don't understand. If the reason for a step is clear, its position in the sequence should be clear. A good example is in purifying an ester product, where you should be able to answer:
  • why is simple distillation inappropriate (2 reasons)?
  • why neutralise with Na2CO3 or NaHCO3 (at least 2 reasons)?
  • why can you use a separating funnel? What does it achieve?
  • why anhydrous calcium sulfate (or equivalent)?
  • why is distillation now appropriate (2 reasons, including describing what is different at this point than at the start)?
Being able to answer these questions should also explain why a different sequence would be problematic, meaning the order need not be memorised.
what are the two reasons for neutralising with na2co3?

im only aware that u use it to neutralise the acids and then rinse the new salt off with water.
 

Mowhuss

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Screen Shot 2021-11-26 at 10.37.43 am.png

For this question I'm confused as to why it is A (I have read a previous post explaining this to be correct but still don't understand it) and not B. My reasoning being, that we could react ethanoic acid and sodium hydroxide to produce sodium ethanoate (basic salt). Then we could use an ethanoate ions equilibrium reaction, and add ethanoic acid to make this an effective buffer solution (ethanoate ions and ethanoic acid are equimolar).
 

CM_Tutor

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what are the two reasons for neutralising with na2co3?

im only aware that u use it to neutralise the acids and then rinse the new salt off with water.
I meant in the context of purifying an ester. The mixture after reflux includes unreacted carboxylic acid and sulfuric acid. Neutralisation removes the sulfuric acid catalyst, meaning that any subsequent movement in the position of equilibrium will be slower. Neutralising the unreacted carboxylic acid will prevent any movement in the forward reaction as the reactant is missing. It means the system will start to shift left, but that can be stopped by removing the water product (hence drying the organic layer) so that you have an organic layer of only ester and unreacted alcohol. This can be distilled without risk of losing ester as it is no longer an equilirbium system as you have only one reactant and one product. If you distill the entire product mixture, the heating (in the presence of a catalyst) will result in removal of the alcohol (typically lowest BP) and a consequent shift left, with loss of ester product.

Talking about spills, neutralising with something like Na2CO3 is best done with a solid (to prevent spreading) and to produce a non-harmful product (sodium salt) that you can sweep up and discard, so there are at least two reasons there too.
 

CM_Tutor

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Could someone help me out with the last two parts of this question?
Also would the name of Compound Y be 2,2-dimethyl butanoic acid?
Yes, Y was 2,2-dimethylbutanoic acid, though the question was meant to ask for the name of Z.

The oxidant was the diamminesilver(I) cation, [Ag(NH3)2]+.

It was formed, as stated in the question, by reaction of silver(I) nitrate with sodium hydroxide to form silver oxide:

2 AgNO3 (aq) + 2 NaOH (aq) -----> Ag2O (s) + 2 NaNO3 (aq) + H2O (l)

or

2 Ag+ (aq) + 2 OH- (aq) -----> Ag2O (s) + H2O (l)​

which is then re-dissolved in ammonia:

Ag2O (s) + 4 NH3 (aq) + H2O (l) -----> [Ag(NH3)2]+ (aq) + 2 OH- (aq)​

The question did not ask for the rest of the redox reaction, but if it had, the reactions would be:

[Ag(NH3)2]+ (aq) + e- -----> Ag (s) + 2 NH3 (aq)​

and

CH3-CH2-C(CH3)2-C(=O)H (aq) + H2O (l) -----> CH3-CH2-C(CH3)2-CO2- (aq) + 3 H+ (aq) + 2e-

for an overall reaction of

CH3-CH2-C(CH3)2-C(=O)H (aq) + 2 [Ag(NH3)2]+ (aq) + H2O (l) -----> CH3-CH2-C(CH3)2-CO2- (aq) + 2 Ag (s) + 3 NH4+ (aq) + NH3 (aq)​
 

CM_Tutor

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View attachment 34205

For this question I'm confused as to why it is A (I have read a previous post explaining this to be correct but still don't understand it) and not B. My reasoning being, that we could react ethanoic acid and sodium hydroxide to produce sodium ethanoate (basic salt). Then we could use an ethanoate ions equilibrium reaction, and add ethanoic acid to make this an effective buffer solution (ethanoate ions and ethanoic acid are equimolar).
You can indeed prepare a buffer solution with ethanoic acid and sodium hydroxide. As you say, it would be an effective buffer when ethanoate ions and ethanoic acid are present in equimolar quantities.

The problem is that the pH of the buffer would be 4.76, and thus it would be an acidic buffer, when the question seeks a buffer which is effective in a pH range that is basic.
 

Mowhuss

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You can indeed prepare a buffer solution with ethanoic acid and sodium hydroxide. As you say, it would be an effective buffer when ethanoate ions and ethanoic acid are present in equimolar quantities.

The problem is that the pH of the buffer would be 4.76, and thus it would be an acidic buffer, when the question seeks a buffer which is effective in a pH range that is basic.
Would the basic salt not increase the pH of the buffer?
 

CM_Tutor

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Would the basic salt not increase the pH of the buffer?
Yes. Without the basic salt, the pH of the acetic acid alone would be somewhere near 3 (concentration depending, of course).
 

CM_Tutor

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when are the solutions going to be up?
The marked papers for those who did the exam, which include individual comments, will be posted to the forum where the exam was done as soon as I know that they can be accessed.

I'm also going to make some general comments today, and will answer specific questions.
 

CM_Tutor

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I have uploaded for each student who did the BoS 2021 Chemistry trial your marked paper into the forum where you submitted the papers.

It is my understanding that notifications that they are uploaded aren't coming through, so if anyone can't find there paper, send me a message.

The comments that I made on individual papers will hopefully be expanded on in the general comments that I am posting question by question in the other thread in the Chemistry forum.
 

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