VCE Chemistry Question Thread

[VanillaRice]

Given that aldehydes have no hydrogen directly bonded to a oxygen fluorine or neon I thought that aldehydes are highly polar but don't experience hydrogen bonding. Could someone explain why they think hydrogen bonding is present?

The hydrogen in question is actually on the water molecule (H-O). I believe the only other requirement is that the delta positive region around the hydrogen interacts with the lone pair electrons of another atom. In this case, it is interacting with the lone pairs on the oxygen atom of C=O, so it doesn't really matter that the oxygen is bonded to a carbon, since it's the lone pairs that are interacting.

[Lavar Big BBB Balls]

Hi, I need some help with 2 problems

http://imgur.com/a/9s3Lc

Q3) the answer mentions that a volumetric flask needs to be washed with water, while this is true the question makes no mention of a volumetric flask being needed, or am I missing something?

Q4b) is the answer making the assumption that the standard solution is acidic?

If it were basic, then the titre would be larger wouldn't it?

Thanks

[Sine]

Hi, I need some help with 2 problems

http://imgur.com/a/9s3Lc

Q3) the answer mentions that a volumetric flask needs to be washed with water, while this is true the question makes no mention of a volumetric flask being needed, or am I missing something?

Q4b) is the answer making the assumption that the standard solution is acidic?

If it were basic, then the titre would be larger wouldn't it?

Thanks

3. The standard solution is in the volumetric flask.

4. So if it is washed with deionised water it really just dilutes the aliquot in that particular pipette. Thus the determiend concentation will be lower than the actual value. (titre will be smaller)

[Lavar Big BBB Balls]

3. The standard solution is in the volumetric flask.

4. So if it is washed with deionised water it really just dilutes the aliquot in that particular pipette. Thus the determiend concentation will be lower than the actual value. (titre will be smaller)

Thanks.

But for Q4, im still confused. Wouldn't there only be a smaller titre if the standard solution was acidic and the unknown solution was basic?

If the standard solution was basic, the deionised water dilute it so it would require a greater titre if the unknown solution was acidic?

[Syndicate]

Thanks.

But for Q4, im still confused. Wouldn't there only be a smaller titre if the standard solution was acidic and the unknown solution was basic?

If the standard solution was basic, the deionised water dilute it so it would require a greater titre if the unknown solution was acidic?

The pipette is rinsed deionised water, hence the aliquot will be diluted (so the solution with the known volume), hence it will have a lower concentration. The titre will be lower in value no matter it being an acid or a base.

c = n/v. So the lower the titre value, the higher the determined concentration of the unknown solution.

I think you are misinterpreting between what is a standard solution and titrant. The standard solution is the one being pipetted into an aliquot (it has a known concentration), whereas titrant is the one which has the unknown volume. You would be correct only if the titrant was diluted (this is the one placed in the burette), because then there would be a greater titre value.

[Sindhupriya]

Hey Guys,
can someone please suggest me a topic for my Unit 3 & 4 extended practical investigation?

Thank You in advance !

[Syndicate]

Hey Guys,
can someone please suggest me a topic for my Unit 3 & 4 extended practical investigation?

Thank You in advance !

If its under food chemistry, I suggest:
- How different pH levels, temperatures, acids and bases might decrease the effectiveness of an enzyme.
- Oxidation of vitamins (under different temperatures, lights, pH)
- Iron, sodium content of different foods

[Bri MT]

Hey Guys,
can someone please suggest me a topic for my Unit 3 & 4 extended practical investigation?

Thank You in advance !

-electrolysis
-heat of combustion of ethanol/propanol
-calorimetry
-is odor of esters related to the alcohol or acid or both?
-finding K for a equilibrium reaction

[tinagranger]

Hey guys, how come aldehydes can't hydrogen bond if there is O and H in every molecule? Couldn't the H hydrogen bond with the O of another aldehyde molecule?

[VanillaRice]

Hey guys, how come aldehydes can't hydrogen bond if there is O and H in every molecule? Couldn't the H hydrogen bond with the O of another aldehyde molecule?

The requirements for hydrogen bonding go beyond the presence of oxygen and hydrogen atoms. In the aldehyde group, the H is bonded to a C. The low(er) electronegativity difference between hydrogen and carbon means that the delta positive region that is present around the hydrogen in O-H bonds is not as strong. Therefore, hydrogen bonding cannot really occur with this particular hydrogen.
Hydrogen bonding requires this significant delta positive region around the hydrogen, which is caused by an electronegative atom which is bonded to the hydrogen.

Hope this helps 

[Lavar Big BBB Balls]

Hi,

http://imgur.com/a/myYM1

For part c) I just need to confirm that the splitting pattern for peak B doesn't correspond to anything within the given molecule? The maximum split you could get is 5.

Thanks

[keltingmeith]

Hi,

http://imgur.com/a/myYM1

For part c) I just need to confirm that the splitting pattern for peak B doesn't correspond to anything within the given molecule? The maximum split you could get is 5.

Thanks

I'm assuming you've been given the molecule? If so, reckon you could show it to us. Because I think what's happening is you're misunderstanding the question, but I need to see the actual molecule to know for sure.

[Lavar Big BBB Balls]

I'm assuming you've been given the molecule? If so, reckon you could show it to us. Because I think what's happening is you're misunderstanding the question, but I need to see the actual molecule to know for sure.

Yup, it's the 2nd image. You need to click the "show more images" blue button after clicking the link. The molecule is below the NMR graph.

[pmmenotes]

Anyone had a sac on organic compunds/reactions yet? (chap 10/11) if so what did they test you on. Feel like my teacher isnt giving us much of a guide on what to expect :/

[Syndicate]

Anyone had a sac on organic compunds/reactions yet? (chap 10/11) if so what did they test you on. Feel like my teacher isnt giving us much of a guide on what to expect :/

- How certain functions groups form (ie. amide)
- oxidation of alcohols and qualititative observations (ie. colour change of a MnO4^- -> clear to purple)
- bonding types
- types of functional groups
- reaction pathways given two compounds and/ or observations (ie. can react with a base, meaning it is a carboxylic acid)
- naming compounds

I think I have pretty much covered it all. Good luck with your SAC