Chemistry unit 4 AQA

Whats are the rough grade boundaries for this unit, i know they change year to year but would you expect an A to be 72 ~ 77

Reply 381

swert

does anybody have the june 2011 paper??

Reply 382

NightBear

12

Original post by tigger18

Whats are the rough grade boundaries for this unit, i know they change year to year but would you expect an A to be 72 ~ 77

http://web.aqa.org.uk/over/stat_grades_archive.php
You can search for the grade boundaries here. And, I think an A is around about what you said. A* is usually 80-82+

Reply 383

JesusIsMyHomeboy

9

Do not want to do this exam!!

Reply 384

itsbea

Original post by Erotas

I think you just need to say that amino acids have ionic bonding whereas alcohols have hydrogen bonding, and ionic bonds are stronger than hydrogen bonds.

Does anyone know if there are grade boundaries for the specimen paper?

Why isn't there hydrogen bonding in Amino acids? They contain NH CO which are also electronegative enough to show hydrogen bonding. I thought we only mention VDW, Permanent Dipole and Hydrogen when it comes to boiling points. Lol I'm so confused, can you explain this please!

Reply 385

Besakt

3

Really ****ting myself about this exam. :frown:

Reply 386

NewCrack

6

Original post by itsbea

Why isn't there hydrogen bonding in Amino acids? They contain NH CO which are also electronegative enough to show hydrogen bonding. I thought we only mention VDW, Permanent Dipole and Hydrogen when it comes to boiling points. Lol I'm so confused, can you explain this please!

I think it's because the hydrogen bonding in Amino acids and alcohol have the same effect so you're not really explaining why the Boilining temperature is higher. I think Because amino acids can exist as zwitterions then Ionic bonding gives them that 'extra edge', so they have a higher boiling point.

The other person will probably explain it better lol :tongue:

Reply 387

rommy123

Does anyone have the mark scheme for June 2011 paper??

Reply 388

itsbea

Original post by NewCrack

I think it's because the hydrogen bonding in Amino acids and alcohol have the same effect so you're not really explaining why the Boilining temperature is higher. I think Because amino acids can exist as zwitterions then Ionic bonding gives them that 'extra edge', so they have a higher boiling point.

The other person will probably explain it better lol :tongue:

Thanks! That makes more sense. I feel like I know nothing about the "obvious" stuff.

Reply 389

eesha_25

Hiya, hope somebody can help me: I did the june 2010 paper today morning and am now very confused about q 2bii: why did they not include the volume of 5dm^3 for the Kc question?? Help would be greatly appreciated!!!

(edited 12 years ago)

Reply 390

JesusIsMyHomeboy

9

Does anyone know a simple way to tackle the calculations? I never know which one to use and when things like pH=pKa are relevant..

Reply 391

isitinyet?

13

for the the reactions of alcohols to aldehydes and ketones, is sulfuric acid the reagent or the catalyst (or both, awful at chemistry!)?

Reply 392

eesha_25

Original post by JesusIsMyHomeboy

Does anyone know a simple way to tackle the calculations? I never know which one to use and when things like pH=pKa are relevant..

pH= pKa
Ka= H+
A- = HA

All 3 of the above are only relevant for the half neutralisation stuff.

The normal ones are

pH = -log[H+]
Ka = [H+][A-]
----------
[HA]

Kw= [H+][OH-]

and for buffer

Ka= [H+][Salt]
--------------
[acid]

and for dilution

[new conc]= (old vol/new vol) x old conc

thats all you need to know pH i think?

Reply 393

eesha_25

Original post by isitinyet?

for the the reactions of alcohols to aldehydes and ketones, is sulfuric acid the reagent or the catalyst (or both, awful at chemistry!)?

Primary alcohol to Aldehyde

Reagents: K₂Cr₂O₇ and acidified with dil H₂SO₄
Conditions: Heat at beginning then distil out at formed
Type of reaction: Partial oxidation

Aldehyde to Carboxylic Acid

Reagents: K₂Cr₂O₇ and acidified with dil H₂SO₄
Conditions: Heat under reflux
Type of reaction: Full Oxidation

Primary alcohol to carboxylic acid

Reagents: K₂Cr₂O₇ and acidified with dil H₂SO₄
Conditions: Heat under reflux
Type of reaction: Full oxidation

Secondary alcohol to Ketone

Reagents: K₂Cr₂O₇ and acidified with dil H₂SO₄
Conditions: Heat under reflux
Type of reaction: Full oxidation

Sorry, just read your question properly...there isn't a catalyst for this reaction I think. So it's a reagent

(edited 12 years ago)

Reply 394

student1717

Do we need to know specific indicators and between what pH's they change colour?

Reply 395

MuffinMonster

Whenever I see [HA], in my head, I just imagine the textbook laughing at me -_-

Reply 396

eesha_25

Original post by student1717

Do we need to know specific indicators and between what pH's they change colour?

I think we need to know that phenolphthalein is for strong bases and weak acids and that methyl orange is for strong acids and weak bases...but the others should be given. That's my understanding anyway..

Reply 397

wibletg

3

Original post by eesha_25

Primary alcohol to Aldehyde

Reagents: K₂Cr₂O₇ and acidified with dil H₂SO₄
Conditions: Heat at beginning then distil out at formed
Type of reaction: Partial oxidation

Aldehyde to Carboxylic Acid

Reagents: K₂Cr₂O₇ and acidified with dil H₂SO₄
Conditions: Heat under reflux
Type of reaction: Full Oxidation

Primary alcohol to carboxylic acid

Reagents: K₂Cr₂O₇ and acidified with dil H₂SO₄
Conditions: Heat under reflux
Type of reaction: Full oxidation

Secondary alcohol to Ketone

Reagents: K₂Cr₂O₇ and acidified with dil H₂SO₄
Conditions: Heat under reflux
Type of reaction: Full oxidation

Sorry, just read your question properly...there isn't a catalyst for this reaction I think. So it's a reagent

You can just say acidified potassium dichromate if that makes things easier :tongue:

Reply 398

Nickini

2

When it comes to test tube reagents what do you guys put when it comes to "nothing happens"? The mark schemes seem to differ on this, sometimes they prefer "no observation" and sometimes it's "no reaction". :/

Reply 399

rommy123

Original post by Nickini

When it comes to test tube reagents what do you guys put when it comes to "nothing happens"? The mark schemes seem to differ on this, sometimes they prefer "no observation" and sometimes it's "no reaction". :/

I would put no reaction

Chemistry unit 4 AQA

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