Chemistry: Post your doubts here!

[Dexterwarrior12]

Hi everyone
I've started a site which will provide you with solved papers showing all the steps in the working and with useful tips on the side. The best part is that when you look at a solved paper, you can look at commonly made mistakes and learn from them.
Please do check it out!
https://sites.google.com/site/fromatoscom/home

Great step, but I'd advice that you do them with pen, ofcourse these have been done, but the later ones, if you ever add them, try doing them with pen because the contrast is quite low so its pretty difficult to read. Anyways, its really cool!

 

[AnujaK]

Thank you! Yeah I'll try and do that. Problem is that I scratch out a LOT while writing so that would be quite an eyesore. I'll make sure that I adjust the contrast and see if it's readable for the next papers. Thank you so much for the feedback, I appreciate it

Great step, but I'd advice that you do them with pen, ofcourse these have been done, but the later ones, if you ever add them, try doing them with pen because the contrast is quite low so its pretty difficult to read. Anyways, its really cool!

 

[Dexterwarrior12]

Is there some specific stuff I should really know of AS for the A2 paper? I know it can come from anywhere maybe, but can anyone tell me if there are some specific topics I should really revise. I personally think Halogens and Kc are asked quite often.

 

[itallion stallion]

Thank you! Yeah I'll try and do that. Problem is that I scratch out a LOT while writing so that would be quite an eyesore. I'll make sure that I adjust the contrast and see if it's readable for the next papers. Thank you so much for the feedback, I appreciate it

When are u gonna do the same for A2 papers!it would be awesome if u take the same initiative for A2 soon,cuz we have papers next week!As students gonna get a great help from your work!

 

[itallion stallion]

Can someone clear my confusion,when we have to calculate solubility of Ag2CO3 we divide the Ksp by 4'and take the cube root.now if we have to calculate solubility of Al2O3,by which no are,we gonna divide the Ksp with and which root are we gonna take!

 

[itallion stallion]

Can someone plz provide me link to any notes where I can understand how to complete the missing electrons questions in past papers,like showing electron sharing in N20 etc.thanks!

 

[Namehere]

Can anyone explain me if fluoromethane (CH3F) can hydrogen bond with water (H2O)? Here is my thinking... Water can form a hydrogen bond simply because it has a hydrogen attached to an oxygen, but this has to come into contact with a lone pair from a second electronegative atom, in this case F in fluromethane. So, in theory, shouldn´t there be a hydrogen bond? Though, CH3F is insoluble in H2O...

 

[Namehere]

Can anyone explain me if fluoromethane (CH3F) can hydrogen bond with water (H2O)? Here is my thinking... Water can form a hydrogen bond simply because it has a hydrogen attached to an oxygen, but this has to come into contact with a lone pair from a second electronegative atom, in this case F in fluromethane. So, in theory, shouldn´t there be a hydrogen bond? Though, CH3F is insoluble in H2O...

And if the cause that it doesn´t dissolve in water is because CH3F is a gas and you have to decrease the pressure blablabla... would CH3F dissolve due to hydrogen bonding with water if it were a liquid at rtp?

 

[AnujaK]

Hi I'm just in the AS so I havnt solved A2 papers yet. Sorry :/ I'll see if I can get some of my friends to send me their papers though

When are u gonna do the same for A2 papers!it would be awesome if u take the same initiative for A2 soon,cuz we have papers next week!As students gonna get a great help from your work!

 

[A star]

And if the cause that it doesn´t dissolve in water is because CH3F is a gas and you have to decrease the pressure blablabla... would CH3F dissolve due to hydrogen bonding with water if it were a liquid at rtp?

isnt hte hydrogen suppose to be directly attached to NOF ?

 

[Namehere]

isnt hte hydrogen suppose to be directly attached to NOF ?

Yep, it is in the water. You need to have a hydrogen attached to O,N or F which comes into close proximity to another electronegative atom with a lone pair, as is the case of CH3F and H2O. Am i right? Anyone knows if there is H bond there? And why is it insoluble in H2O?

 

[A star]

a complex ion defination is " a central TRANSITION metal ion surrounded by ions or molecules called ligands" do we have too add transition or only metal ion?

 

[Farez]

Yep, it is in the water. You need to have a hydrogen attached to O,N or F which comes into close proximity to another electronegative atom with a lone pair, as is the case of CH3F and H2O. Am i right? Anyone knows if there is H bond there? And why is it insoluble in H2O?

In order for a halogenoalkane to dissolve in water you have to break attractions between the halogenoalkane molecules (van der Waals dispersion and dipole-dipole interactions) and break the hydrogen bonds between water molecules. Both of these cost energy. Energy is released when new attractions are set up between the halogenoalkane and the water molecules. These will only be dispersion forces and dipole-dipole interactions. These aren't as strong as the original hydrogen bonds in the water, and so not as much energy is released as was used to separate the water molecules.

CH3F, or any other flourine containing hydrocarbons, don't form hydrogen bonds with water. The reason behind that is beyond our syllabus and they won't ask a question on it unless they give appropriate information.

 

[Namehere]

In order for a halogenoalkane to dissolve in water you have to break attractions between the halogenoalkane molecules (van der Waals dispersion and dipole-dipole interactions) and break the hydrogen bonds between water molecules. Both of these cost energy. Energy is released when new attractions are set up between the halogenoalkane and the water molecules. These will only be dispersion forces and dipole-dipole interactions. These aren't as strong as the original hydrogen bonds in the water, and so not as much energy is released as was used to separate the water molecules.

CH3F, or any other flourine containing hydrocarbons, don't form hydrogen bonds with water. The reason behind that is beyond our syllabus and they won't ask a question on it unless they give appropriate information.

Well, I´ve researched and apparently there are hydrogen bonds between CH3F and H2O molecules... So how do you get to know that "CH3F, or any other flourine containing hydrocarbons, don't form hydrogen bonds with water" ?

Here is one of the sites it does say it forms H-bonds: http://pubs.acs.org/doi/abs/10.1021/jp308533b

In case this is asked in an exam, should I say CH3F does not form H-bonds WITH H2O? (since obv they don´t form between CH3F molecules...)

 

[unique111]

Somebody help!
http://papers.xtremepapers.com/CIE/... AS Level/Chemistry (9701)/9701_s12_qp_43.pdf

Question 7c i

 

[Namehere]

Somebody help!
http://papers.xtremepapers.com/CIE/Cambridge International A and AS Level/Chemistry (9701)/9701_s12_qp_43.pdf

Question 7c i

You need to take into account the 79Br, 81Br, 35Cl and 37Cl isotopes, together with propene. If you add the Mr of propene with an isotope of Br and an isotope of Cl you get the answers.

 

[itallion stallion]

You need to take into account the 79Br, 81Br, 35Cl and 37Cl isotopes, together with propene. If you add the Mr of propene with an isotope of Br and an isotope of Cl you get the answers.

Can u plz clear my confusion stated on the previous page?thanks!

 

[unique111]

You need to take into account the 79Br, 81Br, 35Cl and 37Cl isotopes, together with propene. If you add the Mr of propene with an isotope of Br and an isotope of Cl you get the answers.

Thanks! Laina

 

[Namehere]

Can someone clear my confusion,when we have to calculate solubility of Ag2CO3 we divide the Ksp by 4'and take the cube root.now if we have to calculate solubility of Al2O3,by which no are,we gonna divide the Ksp with and which root are we gonna take!

For Ag2CO3 the solubility is just the cube root of the Ksp, nothing divided by 4, I think... For Al2O3 u take the fifth root, but they wouldn´t ask that, I think... WRONG

Ag2CO3 --> 2Ag + CO3 2-, Ksp = [Ag+] [CO3 2-], let conc (solubility) of Ag2CO3 = x

Therefore, (2x)^2 (x) = Ksp
4x^3 = Ksp
x = cube root (Ksp/4)

 

[Farez]

Well, I´ve researched and apparently there are hydrogen bonds between CH3F and H2O molecules... So how do you get to know that "CH3F, or any other flourine containing hydrocarbons, don't form hydrogen bonds with water" ?

Here is one of the sites it does say it forms H-bonds: http://pubs.acs.org/doi/abs/10.1021/jp308533b

In case this is asked in an exam, should I say CH3F does not form H-bonds WITH H2O? (since obv they don´t form between CH3F molecules...)

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).