Aqa chem 4/ chem 5 june 2016 thread

I don't really understand the difference in buffer calculation when there is XS base or XS acid?
What I usually do:
1. Find moles of each acid/base given
2. Find the XS mol of whatever is in XS
Am I right in thinking you use Ka for the acid XS buffer?
I always get the final answer wrong :frown:

(

For a buffer calculation I break it into these steps:
1. Find the moles of the of the acid and base
2. Find out which one is in excess:

If Alkali is in excess you need to divide the excess moles by the volume to get you the concentration of OH- ions
Then apply the Kw equation to get you the pH

If the acid is in excess then you need to divide the excess moles by the total volume to get you the concentration of HA (I call it HA or HX)
Then you need to divide the (original moles you have calculated) moles of Alkali by the total volume to get you the concentration of A- and apply the Ka equation (once rearranged)

If none of this makes sense to you post a question you are having difficulty with I'll be happy to help! (ill break it down in steps)

Reply 101

DDan123

8

Is it okay to use pencil for the organic mechanisms?? I did that last year and nothing happened, but others are saying there's a chance it might not get scanned?

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Reply 102

ravichauhan11

4

Original post by mortal infinity

For the nucleophilic addition reaction of aldehydes and ketones with HCN, does it need a dilute HCl catalyst? Also it it better to use KCN/NaCN since HCN is more toxic?

Yeah although id say H2SO4

Reply 103

ravichauhan11

4

Original post by SubwayLover1

Does one ethyl group have the same positive inductive effect as two methyl groups ? I can't find an answer :frown:

No, two methyls have more inductive effect. Think of it as primary vs secondary

Reply 104

jammypancake

6

Original post by haj101

For a buffer calculation I break it into these steps:
1. Find the moles of the of the acid and base
2. Find out which one is in excess:

If Alkali is in excess you need to divide the excess moles by the volume to get you the concentration of OH- ions
Then apply the Kw equation to get you the pH

If the acid is in excess then you need to divide the excess moles by the total volume to get you the concentration of HA (I call it HA or HX)
Then you need to divide the (original moles you have calculated) moles of Alkali by the total volume to get you the concentration of A- and apply the Ka equation (once rearranged)

If none of this makes sense to you post a question you are having difficulty with I'll be happy to help! (ill break it down in steps)

Thanks so much!!

Reply 105

jammypancake

6

When writing an equation for the reduction of nitrobenzene to phenylamine, how do you know how many [H]'s to use? I can never seem to balance these equations correctly?

Reply 106

jammypancake

6

With amino acids, what happens when you add ethanoyl chloride to it? I saw it in a past paper and had no idea how you would work it out?

Reply 107

jammypancake

6

Original post by DDan123

Is it okay to use pencil for the organic mechanisms?? I did that last year and nothing happened, but others are saying there's a chance it might not get scanned?

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They usually say that's fine, if in doubt on the day though double check the front of the question paper and it will say there, or ask an invigilator.

Reply 108

username1133132

15

Original post by jammypancake

When writing an equation for the reduction of nitrobenzene to phenylamine, how do you know how many [H]'s to use? I can enever seem to balance these equations correctly?

Write the equation first (remember you form water) : balance out the oxygens first and then balance the hydrogens :smile:

Reply 109

Lilly1234567890

9

Original post by jammypancake

With amino acids, what happens when you add ethanoyl chloride to it? I saw it in a past paper and had no idea how you would work it out?

The amino acid is a primary amine right so what happens when you add a primary amine to an acyl chloride? Which past paper is this from ?

Reply 110

username1133132

15

Original post by jammypancake

With amino acids, what happens when you add ethanoyl chloride to it? I saw it in a past paper and had no idea how you would work it out?

Not 100% sure on this but amino acids do contain an Amine group...
So I'm guessing the lone pair of the nitrogen of the amine group attacks the carbon which is bonded to the oxygen and Chlorine...

Reply 111

jammypancake

6

Original post by Lilly1234567890

The amino acid is a primary amine right so what happens when you add a primary amine to an acyl chloride? Which past paper is this from ?

Jan 2011 Q4

Reply 112

jammypancake

6

Original post by haj101

Not 100% sure on this but amino acids do contain an Amine group...
So I'm guessing the lone pair of the nitrogen of the amine group attacks the carbon which is bonded to the oxygen and Chlorine...

ahh okay thanks just seemed quiet diffiut for 1 mark

Reply 113

jammypancake

6

Original post by haj101

Write the equation first (remember you form water) : balance out the oxygens first and then balance the hydrogens :smile:

Oh yeah! hahah I think ive been overthinking this a bit too much!

Reply 114

Gezza_O'Brien

5

In chapter 5, with Acyl Chorides, which is the most reactive nucleophile? The Nelson-Thornes book that school uses says

Ammonia>primary amine>alcohol>water

But that doesn't make sense to me since a primary amine has an alkyl group so has a greater positive inductive effect so the lone pair on the N is more available making it more reactive. Is the book wrong or have I missed something?

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Reply 115

SubwayLover1

3

Original post by haj101

Okay I think youve mixed up nucleophiles with bases...
The explanation you have given is correct and can be only applied to properties of bases not nucleophiles...

I disagree.

If you have a primary amine- the electron density is going to be pushed onto the N atom. This means that the lone pair of electrons on the N available are more ' available ' so surely this means it can act as a stronger nucleophile. why would ammonia be a stronger nucleophile ?

Reply 116

username1133132

15

Original post by SubwayLover1

I disagree.

If you have a primary amine- the electron density is going to be pushed onto the N atom. This means that the lone pair of electrons on the N available are more ' available ' so surely this means it can act as a stronger nucleophile. why would ammonia be a stronger nucleophile ?

Okay yes. As the electron density is increased on the nitrogen atom the lone pair of electrons are more available...
So I guess your right.. (Ammonia wouldn't be) Sorry for the confusion! :redface:

Reply 117

man i

3

need some help on this, the june 2011 paper

question 2ci)

At 25 °C, the acid dissociation constant Ka for ethanoic acid has the value 1.75 × 10–5 mol dm–3.2 (c) (i) Calculate the pH of the solution formed when 10.0 cm3 of 0.154 mol dm–3 potassiumhydroxide are added to 20.0 cm3 of 0.154 mol dm–3 ethanoic acid at 25 °C........................................................................................................................................

and 2cii)
(ii) Calculate the pH of the solution formed when 40.0 cm3 of 0.154 mol dm–3 potassiumhydroxide are added to 20.0 cm3 of 0.154 mol dm–3 ethanoic acid at 25 °C. At 25 °C, Kw has the value 1.00 × 10–14 mol2 dm–6.

the first one i got right you just rearrange the formula etc. the second one says that you need to divide 0.00308 (XS) by 60x1000 now im not really sure why thats the case and the CGP book doesent explain it well either. any help will be appreciated.

Reply 118

PleaseHelpMe:(

Hi.guys I need a really really big favour from everyone here. I am resitting all 9 of AS exams this year along with 9 A2 exams for bio chem and maths. I am really struggling with chem 4 and I keep getting <30 in all my past papers :,( . No matter how much I revise, I can't get anything into my head. So can any of you bright clever people post ur revision notes on here please. I will be so grateful. I really want to pass chemistry Thank you