VCE Chemistry Question Thread

[Callum@1373]

If it's a smaller volume of the unknown species, wouldn't you get a lower than actual concentration?

Experimental errors are going to be the death of me.

So if a similar question were to pop up in a VCAA exam, how would I know whether to circle A or B? Or would they only have one or the other in the exam?


Also:

What's the difference between an amide and a peptide linkage?

**Sorry for spamming you all with questions.

If a lower volume is needed for the same number of mol of solute, then the concentration must be higher (hope that makes sense!)

Also I'm pretty sure amide linkages are the same as peptide linkages, except peptide linkages are specifically referring to the amide linkages between amino acids in a polypeptide.

[zsteve]

If a lower volume is needed for the same number of mol of solute, then the concentration must be higher (hope that makes sense!)

Also I'm pretty sure amide linkages are the same as peptide linkages, except peptide linkages are specifically referring to the amide linkages between amino acids in a polypeptide.

Correct. An amide functional group is very general, and refers to -CONH2, -CONHR, and -CONR2 groups (you only cover secondary amides, -CONH- in VCE) where R is some other group (e.g. alkyl group). The term peptide bond/linkage applies specifically to those between amino acids.

[HopefulLawStudent]

Correct. An amide functional group is very general, and refers to -CONH2, -CONHR, and -CONR2 groups (you only cover secondary amides, -CONH- in VCE) where R is some other group (e.g. alkyl group). The term peptide bond/linkage applies specifically to those between amino acids.

If a lower volume is needed for the same number of mol of solute, then the concentration must be higher (hope that makes sense!)

Also I'm pretty sure amide linkages are the same as peptide linkages, except peptide linkages are specifically referring to the amide linkages between amino acids in a polypeptide.

Ohmygosh. Thank you. That makes so much sense now.

[Hopelesshopefull]

quick question- how do we determine what units to leave equilibrium questions in? i'd give examples but can't find any right not- but for example, sometimes they would leave the unit in M^-2, M^-1, M etc. is there a way to determine what to leave it as?

also why is this question's final unit M^-2 ? thanks (question 21)

If you mean how to determine units for K , then :
-u just write out your K expression
-substitute M for all the elements, still keeping them to their powers( that come from their stoichiometric ratios)
-simplify (u need to apply Index laws and stuff) till u get M to the power of whatever.
-this becomes ur units for K
Hope that helps

[HopefulLawStudent]

Hey guys. Please help.

How do  I know that the following is a back titration question? Like what's the hint in the Q that tells me it's a back titration?

Also, in the answers, for that question, they say

Direct titrations involving weak acids or bases do not usually result in a sharp end point. If a sharp end point cannot be achieved, an accurate concentration for the unknown solution cannot be determined. In cases such as these, a back titration is used.

But can't you do a direct titration with a weak acid with a strong base? I know it doesn't give as steep a curve as a strong acid strong base but I thought if it was a weak acid strong base, than usually you could probably do a direct titration cuz the curve was steep enough? Asking cuz in my notes, I wrote that back titrations are for weak acid weak base; need to know if I need to fix up my notes...

I ask too many questions...

[HasibA]

-snip-

thanks

[zsteve]

Hey guys. Please help.

How do  I know that the following is a back titration question? Like what's the hint in the Q that tells me it's a back titration?

Also, in the answers, for that question, they say

But can't you do a direct titration with a weak acid with a strong base? I know it doesn't give as steep a curve as a strong acid strong base but I thought if it was a weak acid strong base, than usually you could probably do a direct titration cuz the curve was steep enough? Asking cuz in my notes, I wrote that back titrations are for weak acid weak base; need to know if I need to fix up my notes...

I ask too many questions...

We know it's a back titration by looking at the procedure - we add aspirin to excess NaOH before titrating with HCl. That gives it away.

The terms 'strong' and 'weak' acids are extremely vague, Ka or pKa (Kb/pKb) values are much more useful in describing strength of acids and bases.

Weak/weak titrations usually don't work, so back titrations must be used. Strong/weak titrations may/may not work specifically depending on the identities of the acids/bases involved. For instance, NaOH/CH3COOH is a strong/weak titration that is fine to be done directly. For some other weak acids, I have a feeling this may not be the case, depending on the pKa of the acid.

In any case, you're trying to analyse aspirin, which isn't particularly soluble in water. All the more reason to do a back titration.

[HopefulLawStudent]

We know it's a back titration by looking at the procedure - we add aspirin to excess NaOH before titrating with HCl. That gives it away.

The terms 'strong' and 'weak' acids are extremely vague, Ka or pKa (Kb/pKb) values are much more useful in describing strength of acids and bases.

Weak/weak titrations usually don't work, so back titrations must be used. Strong/weak titrations may/may not work specifically depending on the identities of the acids/bases involved. For instance, NaOH/CH3COOH is a strong/weak titration that is fine to be done directly. For some other weak acids, I have a feeling this may not be the case, depending on the pKa of the acid.

In any case, you're trying to analyse aspirin, which isn't particularly soluble in water. All the more reason to do a back titration.

So strong/weak combinations don't always yield a steep enough curve to do a direct titration? Huh. Did not know that. Thank you.

[zsteve]

So strong/weak combinations don't always yield a steep enough curve to do a direct titration? Huh. Did not know that. Thank you.

I have a hunch that this is the case, don't know of any examples though. You'll always need to look at titration curves.

[HopefulLawStudent]

I have a hunch that this is the case, don't know of any examples though. You'll always need to look at titration curves.

That's alright. Thank you.


Question:

Give the chemical formulae for any two of the three waste products that are formed when proteins are completely digested or eliminated from the body. (attached the image of the protein they were dealing with in the question if it helps).

We never covered this in class and I can't find anything on it in either my notes nor my textbook. Is this even within the parameters of the study design?

[zsteve]

That's alright. Thank you.


Question:

Give the chemical formulae for any two of the three waste products that are formed when proteins are completely digested or eliminated from the body. (attached the image of the protein they were dealing with in the question if it helps).

We never covered this in class and I can't find anything on it in either my notes nor my textbook. Is this even within the parameters of the study design?

Don't think so... from my sketchy knowledge of digesion and metabolism (thanks to BIOL10004), I know that digestion of proteins can form ammonia (toxic) or urea (inert). I might be wrong, but you can google it.

definitely not on the study design

[HopefulLawStudent]

The answers go on a whole spiel about carbon dioxide and water and something about urea (C(NH2)2O ) and I honestly understood none of what they were saying... Thank God it's not on the study design.

Thank you!

[HasibA]

quick q:
can anyone explain why a decrease in temp results in a net forward reaction- can you also give your reasoning for it as in-depth as possible? ty
edit: more q's
quote from textbook 'loss of chemical on the left side or a reaction causes a net backwards reaction according to Le Chaleteliers princple' is this true in  opposite? i.e loss of chemical on the right side causes a net forward reaction- can anyone clarify this point a bit more (struggling a bit) thanks )
also: K will incrase as Temp decreases, is this always true and is it always true in reverse? i.e K decreases as temp increases !? ty

[Sine]

quick q:
can anyone explain why a decrease in temp results in a net forward reaction- can you also give your reasoning for it as in-depth as possible? ty

In terms of LCP
The reaction is exothermic therefore it is producing thermal energy. Therefore decreasing temperature will result in the system attempting to partially appose this change therefore net forward reaction results to produce more thermal energy to increase temperature.

Q and K
For exothermic reactions a decrease of temperature will K will increase therefore Q will try to increase aswell. Therefore net forward reaction required to increase Q.

[HasibA]

sorry for heaps of q's guys xD
but part c- not sure what concentrations to use/ how to do it?