Chemistry: Post your doubts here!

[LeoMessi]

Have you read the chapter on partition coefficients? This page does a great job in explaining the concept and how to do the calculations. http://www.chemguide.co.uk/CIE/section112/learningf.html

Read that page and try to figure out how to solve the question yourself. It's mostly math anyway.



No need to memorize colours of complexes, other than Cu2+ complexes, and iron (ii)/(iii) ones. Make sure you know the reason behind the colours as that is a very common question.

As for your second question, it's rather subjective. The colour of the Fe2+ solution really depends on the concentration of the ions. If it's dilute, the colour would be harder to notice than the very prominent purple of the KMnO4 solution. This is what I think to make the most sense. I have no idea what the examiner had in mind lol.

Thanks alot!!!

 

[Namehere]

Flourocarbons are hydrophobic.

http://en.wikipedia.org/wiki/Fluorine

"Also, adding fluorine to organics increases their lipophilicity (ability to dissolve in fats) because the carbon–fluorine bond is even more hydrophobic than the carbon–hydrogen bond."

http://en.wikipedia.org/wiki/Fluorocarbon

"They have very low solubility in water, and water has a very low solubility in them (on the order of 10 ppm)"

"As a result, fluorocarbons have low intermolecular attractive forces and are lipophobic in addition to being hydrophobic/non-polar."

http://en.wikipedia.org/wiki/Organofluorine_chemistry

"Fluorine has the highest electronegativity of all elements: 3.98.[1] This causes the high dipole moment of C-F bond (1.41 D[1]).

Fluorine has the lowest polarizability of all atoms: 0.56 10−24 cm3.[1] This causes very weak dispersion forces between polyfluorinated molecules and is the reason for the often-observed boiling point reduction on fluorination as well as for the simultaneous hydrophobicity and lipophobicity of polyfluorinated compounds whereas other perhalogenated compounds are more lipophilic."

This obviously means they are insoluble in water, and that they don't form hydrogen bonds.

This is why they are used in sorbents to absorb oil from oil spills, and not the water (page 137 of the application booklet).

Ok, I need explanations... "the carbon–fluorine bond is even more hydrophobic than the carbon–hydrogen bond." "Fluorine has the highest electronegativity of all elements: 3.98.[1] This causes the high dipole moment of C-F bond" - Isn´t this contradictory? If the bond is polar, shouldn´t it be hydrophilic?

Plus, I still don´t get why it doesn´t form H-bonds with water. Can you explain me clearly why it doesn´t, please? I also don´t get why its hydrophobic, if you could explain that too..

Thank you sooo much!

 

[Farez]

Ok, I need explanations... "the carbon–fluorine bond is even more hydrophobic than the carbon–hydrogen bond." "Fluorine has the highest electronegativity of all elements: 3.98.[1] This causes the high dipole moment of C-F bond" - Isn´t this contradictory? If the bond is polar, shouldn´t it be hydrophilic?

Plus, I still don´t get why it doesn´t form H-bonds with water. Can you explain me clearly why it doesn´t, please? I also don´t get why its hydrophobic, if you could explain that too..

Thank you sooo much!

It feels contradictory to me as well, but this is what makes sense to me:

Fluorine has the lowest polarizability of all atoms. From this statement, we can say that the C-F bond isn't polarized enough to form the strong hydrogen bonds i.e. the δ- of the F isn't negative enough to form a strong hydrogen bond with the H's δ+.

It's hydrophobic because of the organic part - the CH3 group for example. And if the molecule has a lot of carbons, the hydrophobic property increases. So increasing carbon number will make it more insoluble.

 

[A star]

Ok, I need explanations... "the carbon–fluorine bond is even more hydrophobic than the carbon–hydrogen bond." "Fluorine has the highest electronegativity of all elements: 3.98.[1] This causes the high dipole moment of C-F bond" - Isn´t this contradictory? If the bond is polar, shouldn´t it be hydrophilic?

Plus, I still don´t get why it doesn´t form H-bonds with water. Can you explain me clearly why it doesn´t, please? I also don´t get why its hydrophobic, if you could explain that too..

Thank you sooo much!

look here is the deal flourine has a different chemistry i have a feeling that its explaination lies in concepts we avent studied yet so its bettter 2 days before the exam to leave it then ask your respective sir teacher about it

 

[hope4thebest]

Why amides give pH=7 while amines are basic in nature? Logic?

 

[A star]

Why amides give pH=7 while amines are basic in nature? Logic?

they are neutaral compounds, the nitrogen cannot use ts lone pair

 

[hope4thebest]

they are neutaral compounds, the nitrogen cannot use ts lone pair

Why cant they donate lone pair in case of amides?

 

[hela]

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

Phenol

 

[hela]

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

 

[hela]

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

 

[hela]

I WANT TO OPEN A NEW THREAD FOR A LEVEL CHEMISTRY PAPER 4
IF ANY ONE HAVE A SUGGESTION

 

[hela]

i do not know how to open a new thread
can someone help me

 

[hope4thebest]

http://papers.xtremepapers.com/CIE/... AS Level/Chemistry (9701)/9701_s11_qp_42.pdf Question 3 (c) part?

 

[Farez]

Why amides give pH=7 while amines are basic in nature? Logic?

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.

http://papers.xtremepapers.com/CIE/Cambridge International A and AS Level/Chemistry (9701)/9701_s11_qp_42.pdf Question 3 (c) part?

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

 

[unique111]

http://papers.xtremepapers.com/CIE/... AS Level/Chemistry (9701)/9701_w10_qp_43.pdf
Ques 10a iii

 

[A star]

http://papers.xtremepapers.com/CIE/Cambridge International A and AS Level/Chemistry (9701)/9701_w10_qp_43.pdf
Ques 10a iii

thes distance of the peaks is the same so take the ratio of the heights . mine came around 58 and 42(x)

 

[A star]

58% Y peak and 42 % X peak*

 

[unique111]

58% Y peak and 42 % X peak*

How do you get that? I don't understand the concept? And the diagram?

 

[A star]

How do you get that? I don't understand the concept? And the diagram?

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

 

[goodluckayesha]

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?