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Chemistry: Post your doubts here!

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hela

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Jinosupreme said:
X2_zpsde01ddaf.png


Guys! Need help! For the 3rd one, why is it no reaction? Isn't it carboxylic acid + alcohol reaction?
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Phenol
 
H

hela

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what chapter do yoou think we have to focus on for A level chemistry do you think
for varaint 1
if you have any idea please post it
thank you
 
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hela

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paper 4 winter 2013 is unusual with a lot of AS level Material
As if you are doing AS level and not A level ?
 
Farez

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hope4thebest said:
Why amides give pH=7 while amines are basic in nature? Logic?
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We need to look at the bonding in the -CONH2 group.

Like any other double bond, a carbon-oxygen double bond is made up of two different parts. One electron pair is found on the line between the two nuclei - this is known as a sigma bond. The other electron pair is found above and below the plane of the molecule in a pi bond.

A pi bond is made by sideways overlap between p orbitals on the carbon and the oxygen.

In an amide, the lone pair on the nitrogen atom ends up almost parallel to these p orbitals, and overlaps with them as they form the pi bond.

image: http://www.chemguide.co.uk/organicprops/amides/amidedeloc.gif

The result of this is that the nitrogen lone pair becomes delocalised - in other words it is no longer found located on the nitrogen atom, but the electrons from it are spread out over the whole of that part of the molecule.

This has two effects which prevent the lone pair accepting hydrogen ions and acting as a base:
  • Because the lone pair is no longer located on a single atom as an intensely negative region of space, it isn't anything like as attractive for a nearby hydrogen ion.
  • Delocalisation makes molecules more stable. For the nitrogen to reclaim its lone pair and join to a hydrogen ion, the delocalisation would have to be broken, and that will cost energy.
hope4thebest said:
http://papers.xtremepapers.com/CIE/Cambridge International A and AS Level/Chemistry (9701)/9701_s11_qp_42.pdf Question 3 (c) part?
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Compare E values of the metal getting reduced at the cathode with that of H2 (which is zero). So if it's positive, the metal gets discharged. If negative, hydrogen gets discharged.

Ag+ + e– ⇌ Ag E = +0.80 so Ag discharged
Fe2+ + 2e– ⇌ Fe E = –0.44 so H2 discharged
Mg2+ + 2e– ⇌ Mg E = –2.38 so H2 discharged

Compare E values of the anion with that of
O2 + 4H+ + 4e– ⇌ 2H2O E = +1.23

F2 + 2e– ⇌ 2F– E = +2.87 more positive so O2 discharged
S2O82– + 2e– ⇌ 2SO42– E = +2.01 more positive so O2 discharged
Br2 + 2e– ⇌ 2Br– E = +1.07 less positive so Br2 discharged
 
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unique111 said:
How do you get that? I don't understand the concept? And the diagram?
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the percentage abundance of X=(height of X/height of Y +height of X )*100 and vice versa . you can also tell it by the distance covered by the peaks i:e time but its the same in this case. I was taught this in applications but honestly i didnt apply the formulae just took the heights and a random guess :p
 
goodluckayesha

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Plz someone solve my doubts:

1. papers.xtremepapers.com/CIE/Cambridge%20International%20A%20and%20AS%20Level/Chemistry%20(9701)/9701_w03_qp_4.pdf for this one explain Q4ci and 4cii and also in question one i had a doubt regarding if we can use tollens, K2Cr2O7 smthn like tht as well?
2. http://papers.xtremepapers.com/CIE/...d AS Level/Chemistry (9701)/9701_w05_qp_4.pdf
Q5ci plz explain
3. and i wanted to ask is there a difference between H3O+ and H+ because for hydrolysis of amide diff ms have diff so if they r different when do we use each of them?
 
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goodluckayesha said:
Plz someone solve my doubts:

1. papers.xtremepapers.com/CIE/Cambridge%20International%20A%20and%20AS%20Level/Chemistry%20(9701)/9701_w03_qp_4.pdf for this one explain Q4ci and 4cii and also in question one i had a doubt regarding if we can use tollens, K2Cr2O7 smthn like tht as well?
2. http://papers.xtremepapers.com/CIE/Cambridge International A and AS Level/Chemistry (9701)/9701_w05_qp_4.pdf
Q5ci plz explain
3. and i wanted to ask is there a difference between H3O+ and H+ because for hydrolysis of amide diff ms have diff so if they r different when do we use each of them?
Click to expand...
2) its the same thing H3O or H+(aq) if you use the latter then the state symbol becomes essential otherwhise it will be taken as a free radical or nascent hdrogen
 
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itallion stallion said:
Can u help me how to do q1 b II.i just don't know where would a coordinate bond be formed,other question like it where we have to form double bond! Are there any specific examples which I should learn or just try my luck in the paper!
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its usually common sense you have too apply in this one its common knowledge the NH4+ has a dative covalent bond (H atom donates to Cl-. in others for example CO you have no choice but to draw a simple double bond it would be accepted (mind you the structure is abit different in reality but not needed by cie) and try t see if the valency is being satisfied or not
 
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