unit 4 questions (im confused!)

hi! i have the following questions regarding unit 4 of A2 chemistry (9080); I know there are a lot of questions, but I find different things in different textbooks and sometimes even the markschemes contradict each other:

1. i know that dry ether is used in the production of a grignard reagent; but is it used in the reactions involving grignard reagents as well (e.g. with carbon dioxide or aldehydes)?

2. do the conditions of preparation of a grignard reagent involve heating under reflux and iodine catalyst?

3. for hofmann degredation, do you use concentrated NaOH (aq) or just dilute, and do you just warm the mixture or do you heat it under reflux. Also I have seen in mark schemes that bromine water is not right, does bromine water mean Br2 (aq) and if you put down Br2 (l) is this right?

4. in the reaction of acid chlorides with ammonia, is the ammonia concentrated?

5. does the reaction of acid chlorides with amines (which produces N-sustituted amides) only happen with primary amines?

6. if asked for just reagents, do you specify if he reagent is aqueous or dilute/concentrated (e.g. do you say HCl (aq) or just HCl)?

7. what is the reagent used in the acidic hydrolysis of an ester (the equation just has water in it, and since the acid is used as a catalyst, wouldn’t this be a condition rather than a reagent)?

8. What type of reaction is taking place when methyl ketones form an iodoform (e.g. is it nucleophilic substitution or something else)?

9. if you are asked to write an expression for the Kc of an acid (not Ka), do you include water?

10. do you include gases in your expression for Kc?

11. does pressure affect a heterogeneous reaction involving gases and liquids; if so, how? If you are still looking at which side contains the higher number of moles, then do you include the number of moles of the liquid?

12. there are so many equations for the reaction of aluminium chloride with water, does any one know which ones are accepted by edexcel (for the reaction with a limited amount of water and excess water)?

Thnx a lot for your help, its really appreciated :confused:

Reply 1

Rags241

1. No
2. It involves both reflux and the traces of iodine as a catalyst.
3. Make sure you put down liquid bromine, bromine water is bromine in water – so its aqueous. As for NaOH it has to be aqueous, I was taught concentrated but generally its aqueous in the mark schemes.
4. I don’t think you need to know that, I just have it down as ammonia.
5. I think so, and to be honest, you only need to know the reaction with a primary amine, so don’t worry about with any other amines.
6. If you’ve been taught whether or not its aqueous then yes.
7. the reagent is water. Dilute acid is the catalyst.
8. Im not actually sure about that one.
9. Yes
10. Err crap, I think so yes. Its for Kp that you don’t include solids.
11. Not sure about that one.
12. The one I remember generally is:

AlCl3 (s) + 6H2O (l) + aq à [Al(H2O)6]3+ (aq) + 3Cl – (aq)

Hope it helped a bit.

Reply 2

thelostchild

11) yes it does as you are essentially increasing the conc of the gasses, therefore the equilibrium will shift to the side with fewest moles of gas (you ignore liquids)

10) yea you do its just solids you dont put in :smile:

Reply 3

aash18

im still not sure about a few of these, so does any1 else have any views???

Reply 4

thelostchild

aash18

im still not sure about a few of these, so does any1 else have any views???

which ones?

Reply 5

aash18

1 to 9 (excluding 6) and 12

Reply 6

rohitkhannak

1. i know that dry ether is used in the production of a grignard reagent; but is it used in the reactions involving grignard reagents as well (e.g. with carbon dioxide or aldehydes)?

AFAIK, I don't think so. Once the Grignard Reagent has been formed, the dry ether requirement has been fulfilled.

2. do the conditions of preparation of a grignard reagent involve heating under reflux and iodine catalyst?

Yes, they do. Under reflux in Dry ether with a trace of iodine catalyst I think.

3. for hofmann degredation, do you use concentrated NaOH (aq) or just dilute, and do you just warm the mixture or do you heat it under reflux. Also I have seen in mark schemes that bromine water is not right, does bromine water mean Br2 (aq) and if you put down Br2 (l) is this right?

Well, Make The Grade says concentrated NaOH, but the Mark Scheme for 2006 Jun says '(aqueous) sodium hydroxide' :s-smilie:

. I've also seen that some MSs say 'bromine water' is not right, probably because its bromine dissolved in water which makes it aqueous bromine. Safe side is just if you say bromine or Br2 is guess. MTG says 'liquid bromine'.

4. in the reaction of acid chlorides with ammonia, is the ammonia concentrated?

I dunno.

5. does the reaction of acid chlorides with amines (which produces N-sustituted amides) only happen with primary amines?

Well, I don't really know. Primary amines are the only amines in the syllabus, maybe that's why ...

6. if asked for just reagents, do you specify if he reagent is aqueous or dilute/concentrated (e.g. do you say HCl (aq) or just HCl)?

Well I usually just say the reagent when I don't know whether its concentrated or dilute, but if I know whether it is concetrated or not or whether a reagent being concentrated is important or not (e.g. in formation of ester, Conc H2SO4 is a catalyst, conc. being important here). However, like in the case with acid chloride + ammonia, I dont know whether Ammonia is conc/dilute so I just write ammonia

7. what is the reagent used in the acidic hydrolysis of an ester (the equation just has water in it, and since the acid is used as a catalyst, wouldn’t this be a condition rather than a reagent)?

Acid is a catalyst as the reaction would be too slow without action of an acid. So I think we write H20 + ester <---> (on top of arrow, H+ [acid] and heat under reflux) alcohol + acid.

8. What type of reaction is taking place when methyl ketones form an iodoform (e.g. is it nucleophilic substitution or something else)?
I dont know ...

9. if you are asked to write an expression for the Kc of an acid (not Ka), do you include water?
Yes, you include all components of a reaction except solids for Kc

10. do you include gases in your expression for Kc?
Yes,I think so.

11. does pressure affect a heterogeneous reaction involving gases and liquids; if so, how? If you are still looking at which side contains the higher number of moles, then do you include the number of moles of the liquid?
Not sure

12. there are so many equations for the reaction of aluminium chloride with water, does any one know which ones are accepted by edexcel (for the reaction with a limited amount of water and excess water)?
Not sure

Reply 7

Rags241

for hofmann degredation, do you use concentrated NaOH (aq) or just dilute, and do you just warm the mixture or do you heat it under reflux. Also I have seen in mark schemes that bromine water is not right, does bromine water mean Br2 (aq) and if you put down Br2 (l) is this right?

you have to say liquid bromine, and the sodium hydroxide is in fact aqueous. On the syllabus it says we need to know amides with bromine in aqueous alkali

Reply 8

ConservativeNucleophile

aash18

3. for hofmann degredation, do you use concentrated NaOH (aq) or just dilute, and do you just warm the mixture or do you heat it under reflux. Also I have seen in mark schemes that bromine water is not right, does bromine water mean Br2 (aq) and if you put down Br2 (l) is this right?

For Hofmann rearrangement to occur, water has to be reacted with the intermediate isocyanate compound formed. I'm not sure if this water is provided separately or as the diluent in the NaOH solution. As far as I can see, either could be possible.

aash18

4. in the reaction of acid chlorides with ammonia, is the ammonia concentrated?

It would make sense. Water reacts with acid chlorides, so you would get a mixture of products if the ammonia were dilute. Concentrated ammonia negates this problem.

aash18

5. does the reaction of acid chlorides with amines (which produces N-sustituted amides) only happen with primary amines?

It is the lone pair of electrons on the nitrogen atom which initiates the reaction with acid chlorides, and this lone pair is still present in secondary and tertiary amines. However, the electron density around the nitrogen can be withdrawn by species with high electronegativity, and increased by electron-inducing species such as a methyl group.

Reply 9

NotoriousRB

i thought that for both Kc and Kp if it is heterogeneous then u just put whatever the odd one out is...so for example if all reactants n products were gas except for 1 which was solid u would have the solid as jus '1'

2SO2(s) + O2(g) <-> 2SO3 (g) ps i knw u cnt have so2 as a solid js cnt fink of a hetergeneous eqn

kp = p(so3)^2
-----------
p(so2)^2 X 1

correct me if i am wrong and am totally missin teh point here thanks