VCE Chemistry Question Thread

[jyce]

Hey larissaaa! So good that we're seeing new people here on AN!
Okay so back titration is basically a titration in which one reactant is added in excess and that excess if then titrated with another reagent. Back titrations are required if the end point in a titration is not sharp and the titre value obtained may have a degree of error associated with it, thus affecting the accuracy of the calculated analyte concentration.

This is the case when a weak acid is titrated against a strong base or a weak base is titrated against a strong acid. Also if the substance is insoluble in water (e.g calcium carbonate) and cannot be made into solution to titrate, excess acid can be added to react with the base and the residue acid titrated against a strong base to calculate the excess acid.

Simply put, back titrations are where you add a reactant (e.g HCl) in excess to the sample (e.g CaCO3), allowing to react, then titrating the excess substance (that's left over from the initial reaction with the sample) with another substance (NaOH). We can then use the titre obtained from the HCl and NaOH reaction to calculate the amount of HCl that was left over after the initial reaction. Since we know how much HCl we added initially, we can now calculate the amount of HCl that reacted with the sample by:



Also this vid is pretty neat in explaining back titrations, go watch it : https://www.youtube.com/watch?v=J91n8RUkhKc

Your explanation is pretty top-notch, Elizawei. But just one thing: having ONE OF a weak acid OR a weak base does not mean that a back titration must be used; for example, the reaction between ethanoic acid and sodium hydroxide has a sharp end point above a pH of 7 and therefore no back titration is needed. Instead, a back titration is required when you have both a weak acid AND a weak base. Other situations in which a back titration would be used include when you have an insoluble substance, as you already mentioned, and also when you have a volatile (highly reactive) substance, or even just when the rate of reaction would be very slow using a standard titration procedure.

[Elizawei]

the reaction between ethanoic acid and sodium hydroxide has a sharp end point above a pH of 7 and therefore no back titration is needed.

Oh wow I didn't know that, thought all weak/strong combos don't have sharp end point 
Thanks jyce!

[jyce]

This is more for a confirmation, but has crackling and a specific knowledge of the hemoglobin reaction been taken off the study design? Thanks

Yes, you don't need to not any of that - not in specific detail anyway. They could always use those concepts to assess your more general knowledge of proteins, balancing chemical equations, etc.

Is the purpose of the flame in AAS to atomise the sample not ionise it?

Correct - the flame reduces metal ions to metal atoms.

Moderator notice: Please try to avoid double posting - you can edit/include multiple quotes in the same post.

[kimmie]

hey can someone help me with this

[jyce]

hey can someone help me with this

n(NaOH) = 0.55 M x 0.0295 L = 0.016225 mol

A triprotic acid donates three protons per molecule, while each NaOH accepts one proton only

Therefore, one mole of citric acid reacts with three moles of NaOH

n(citric acid) in 25 mL = 1/3n(NaOH) = 0.016225 mol x 1/3 = 0.005408333 mol

m(citric acid) in 25 mL = 0.005408333 mol x 192.12 g mol^-1 = 1.039049 g

c(critic acid) = 1.039049 g / 25 mL = 1.039049 g x 1000 mL/25 mL = 41.6 g L^-1

The answer is B.

[larissaaa_]

Hey larissaaa! So good that we're seeing new people here on AN!
Okay so back titration is basically a titration in which one reactant is added in excess and that excess if then titrated with another reagent. Back titrations are required if the end point in a titration is not sharp and the titre value obtained may have a degree of error associated with it, thus affecting the accuracy of the calculated analyte concentration.

This is the case when a weak acid is titrated against a strong base or a weak base is titrated against a strong acid. Also if the substance is insoluble in water (e.g calcium carbonate) and cannot be made into solution to titrate, excess acid can be added to react with the base and the residue acid titrated against a strong base to calculate the excess acid.

Simply put, back titrations are where you add a reactant (e.g HCl) in excess to the sample (e.g CaCO3), allowing to react, then titrating the excess substance (that's left over from the initial reaction with the sample) with another substance (NaOH). We can then use the titre obtained from the HCl and NaOH reaction to calculate the amount of HCl that was left over after the initial reaction. Since we know how much HCl we added initially, we can now calculate the amount of HCl that reacted with the sample by:



Also this vid is pretty neat in explaining back titrations, go watch it : https://www.youtube.com/watch?v=J91n8RUkhKc

Thank you so much

[HopefulLawStudent]

Hey guys. I have a bunch of dumb questions[size=0.5pt]please don't think poorly of me for asking them.[/size]

When the study design mentions "condensation reactions that produce lipids (limited to triglycerides)", what reaction are they talking about?

Also, for the attached portion of the study design, do we *only* need to know the reactions? Or do we need to know the theory and stuff attached to it. The textbook (and my teacher) went into extreme detail but the study design sorta makes it sound like we only really need to know the reactions and maybe some of the basics behind it?

Last question: When the study design says "function of organic molecules in the design and synthesis of medicines including the production of aspirin from salicylic acid", do they mean we only need to know about aspirin and no other knowledge about any other medicines are required (asking cos my class also briefly looked at Penicillin and a bunch of other medicines)? Also, in terms of equations we need to remember for the production of aspirin, is it just:

Salicylic Acid + Ethanoic Anhydride --> Acetysalicylic Acid + Ethanoic Acid?

Btw: Do we need to know the states for any of those? Do they even have states? When we write the equation up in class we draw out the structures soooo idk.

Do I need to also know either:

Ethanoic Acid + Ethanoic Acid --> Ethanoic Anhydride + Water?

or

That equation to do with the production of salicylic acid (attached)?

[blacksanta62]

Hey guys. I have a bunch of dumb questions[size=0.5pt]please don't think poorly of me for asking them.[/size]

When the study design mentions "condensation reactions that produce lipids (limited to triglycerides)", what reaction are they talking about?

Also, for the attached portion of the study design, do we *only* need to know the reactions? Or do we need to know the theory and stuff attached to it. The textbook (and my teacher) went into extreme detail but the study design sorta makes it sound like we only really need to know the reactions and maybe some of the basics behind it?

Last question: When the study design says "function of organic molecules in the design and synthesis of medicines including the production of aspirin from salicylic acid", do they mean we only need to know about aspirin and no other knowledge about any other medicines are required (asking cos my class also briefly looked at Penicillin and a bunch of other medicines)? Also, in terms of equations we need to remember for the production of aspirin, is it just:

Salicylic Acid + Ethanoic Anhydride --> Acetysalicylic Acid + Ethanoic Acid?

Btw: Do we need to know the states for any of those? Do they even have states? When we write the equation up in class we draw out the structures soooo idk.

Do I need to also know either:

Ethanoic Acid + Ethanoic Acid --> Ethanoic Anhydride + Water?

or

That equation to do with the production of salicylic acid (attached)?

1) The synthesis of lipids is an esterification reaction
2) Go with what the study design says. Imo, if you do enough questions, the pathways/reactions shouldn't be a problem. You don't need to go so deep as to know why we use a nickel catalyst or a iron catalyst but you should know each reaction (esp. fermentation of glucose since it seems to come up quite a bit, know the others too since they're just as, if not more important)
3) My teacher said that if they ever bring up any other drug besides aspirin, it would be application of our knowledge of drug making/esters etc. Like, they would make you find bonds in the drug and functional groups. Things like that. With that said, he did tell us to memorise the structure of ethanoic anyhydride since a company paper gave a 2 mark question for simply drawing it. VCAA could ask a legitimate question on it. However, you don't need to memorise the structure of salicylic acid or know how it's synthesised
4) I think the states are either (aq) or all (l). Hope someone else can clarify this

Edit: Just attached a visual aid for any future viewers might have to zoom in

[Elizawei]

Hey guys. I have a bunch of dumb questions[size=0.5pt]please don't think poorly of me for asking them.[/size]

When the study design mentions "condensation reactions that produce lipids (limited to triglycerides)", what reaction are they talking about?

Also, for the attached portion of the study design, do we *only* need to know the reactions? Or do we need to know the theory and stuff attached to it. The textbook (and my teacher) went into extreme detail but the study design sorta makes it sound like we only really need to know the reactions and maybe some of the basics behind it?

Last question: When the study design says "function of organic molecules in the design and synthesis of medicines including the production of aspirin from salicylic acid", do they mean we only need to know about aspirin and no other knowledge about any other medicines are required (asking cos my class also briefly looked at Penicillin and a bunch of other medicines)? Also, in terms of equations we need to remember for the production of aspirin, is it just:

Salicylic Acid + Ethanoic Anhydride --> Acetysalicylic Acid + Ethanoic Acid?

Btw: Do we need to know the states for any of those? Do they even have states? When we write the equation up in class we draw out the structures soooo idk.

Do I need to also know either:

Ethanoic Acid + Ethanoic Acid --> Ethanoic Anhydride + Water?

or

That equation to do with the production of salicylic acid (attached)?

Hahaha I'm gonna say the cliche phrase........... NO QUESTIONS ARE DUMB :}

When the study design mentions condensation reactions that produce triglycerides, it's referring to the various fatty acid chains reacting with glycerol to form a triglyceride. Now if you have a look at the past papers, in the past they've asked questions regarding the formation of these triglycerides, bromine/iodine numbers etc. The main point is that you understand how to draw them, understand the saturated/unsaturated stuff (especially for those bromine/iodine/hydrogenation questions). All the info is given in the data book regarding fatty acids, so you just gotta refer to the DB when needed.

Another thing that might ask in regards to fatty acids could be to do with biodiesel? (someone correct me if i'm wrong). So basically animal fats/used oil etc are hydrolysed into their fatty acids, then methanol is added and methy esters are formed.

Hmm as for the attached equations, I don't think you need to know the production of salicylic acid from phenol in that much detail (our teacher didn't even mention it), however it's good to understand the basic theory behind it. However, you do need to know about the production of aspirin (there's two main ways), and be able to draw the structures of aspirin, salicylic acid, and ethanoic anhydride. You also need to know about why a salt of aspirin is more soluble than normal aspirin, as well as the functional groups of aspirin and salicylic acid.

Since it's the last year of the current study design that contains aspirin, I bet that there's gonna be a question on aspirin (since it's the last year they can test it). As for the function of organic molecules in the design and synthesis of medicines , the main focus would be on aspirin, but a few years ago they did ask stuff about another substance, but you should be fine dealing with it with your knowledge of how functional groups react and behave.  Yeah, penicillin and some other stuff are on the textbooks but as long as you know the theory in regards to functional groups and properties you should be good. (I don't think we need to worry about the production of penicillin and other stuff, but I'll ask my teacher to double check )

As for the production of aspirin, you may need to know the other way of producing it (the slower way), because they might pull a question asking for the differences between the two reactions? Not too sure, just our teacher taught us this reaction too:

Salicylic acid + ethanoic acid --> aspirin + water

As for states, I think it's all (aq)? Not too sure, someone please clarify My teacher wasn't too fussy on states for this.
And yeah , knowing the production of ethanoic anhydride is good, definitely within the SD and something VCAA might ask.

I think we also need to know how aspirin works (in terms of how it provides pain relief), and the hydrolysis of aspirin

[HasibA]

are we supposed to know each type of primary and secondary cells in regards to electrolysis etc. thank you

[zsteve]

are we supposed to know each type of primary and secondary cells in regards to electrolysis etc. thank you

Hey, primary and secondary cells aren't related to electrolysis (instead, primary cells are cells which are discharge-only, secondary cells can be recharged).
But no, you don't need to know specific examples of primary, secondary, or electrolytic cells. The details will be given to you in exam questions.

[HopefulLawStudent]

Thanks blacksanta62 and Elizawei!

[HasibA]

Hey, primary and secondary cells aren't related to electrolysis (instead, primary cells are cells which are discharge-only, secondary cells can be recharged).
But no, you don't need to know specific examples of primary, secondary, or electrolytic cells. The details will be given to you in exam questions.

whooopss hahaha i meant when talking about unit 4 aos 2 - didnt rmbr the name - thanks for the correction thought! xD my bad!!!

[blacksanta62]

Thanks blacksanta62 and Elizawei!

All g! And like Elizawei said, there are no dumb questions

P.s.That's my 300th post I'm so happy and it's on the chem thread, that makes it even better 

Edit: 0.952 posts per day. Momma, we made it!

[Apink!]

I have a question.
Why is that acids with different Ka values need the same amount of let's say 0.1 M of NaOH to neutralize it?