The Student Room Group

Dissociation chemistry

Hi, please could someone just clarify something? So my teacher was talking about electron clouds and how it affects how HCL dissociates. So she said in HCL chlorine has a higher electronegativity or electron density. Please could somoen just clarify which one it is and what’s the difference? She then said chlorine pills the electrons of hydrogen towards it so the electron cloud is sort of a weird shape where it bulges near the chlorine and then it splits? I didn’t write it down because she was explaining it but I just wanted to check whether this is correct? Also could someone please clarify if it is electonegstivity that increases from bottom to top right of periodic table? Please could someone also explain what happens in a carboxylic acid when it tries to dissociate? So I know that the hydrogen leaves the hydroxide so the oxygen has a negative charge so does that mean the chloride pulls the negative charge because it is not electronegative thus making the bond stronger? Sorry for all the questions but thank you so much for your help, I’m really stuck so this would be highly appreciated!!
(edited 8 months ago)
Original post by Ashirs
Hi, please could someone just clarify something? So my teacher was talking about electron clouds and how it affects how HCL dissociates. So she said in HCL chlorine has a higher electronegativity or electron density. Please could somoen just clarify which one it is and what’s the difference? She then said chlorine pills the electrons of hydrogen towards it so the electron cloud is sort of a weird shape where it bulges near the chlorine and then it splits? I didn’t write it down because she was explaining it but I just wanted to check whether this is correct? Also could someone please clarify if it is electonegstivity that increases from bottom to top right of periodic table? Please could someone also explain what happens in a carboxylic acid when it tries to dissociate? So I know that the hydrogen leaves the hydroxide so the oxygen has a negative charge so does that mean the chloride pulls the negative charge because it is not electronegative thus making the bond stronger? Sorry for all the questions but thank you so much for your help, I’m really stuck so this would be highly appreciated!!

1) she said it has a higher electronegativity. Electronegativity - affinity of an atom to attract a bonded pair of electrons in a covalent bond. Electron density - you will see it in organic chemistry, in the bonding of benzene and alkenes (describes how close the electrons are). For example, pi bonds in alkene have an electron density above and below the sigma bond between the 2 carbons.
2) Yes that is correct. Chlorine is more electronegative than hydrogen, so it pulls the ‘electron cloud’ (you can imagine it like that to remember it)
3) Electronegativity - is dependent on atomic radius and nuclear charge. The smaller the atomic radius, the stronger the attraction (less electron shielding). The stronger the nuclear charge, the greater the attraction.
So it goes up and across to the right of the periodic table (fluorine being the most polar). We dont count the noble gases as they dont normally form bonds.
4) COOH is a weak acid so partially dissociates in aqueous solution… COOH -> COO- + H+.
I dont understand what you mean by COOH dissociating and ‘chloride pulls the negative charge because it is not electronegative thus making the bond stronger?’

Read https://www.chemguide.co.uk/atoms/bonding/electroneg.html
When I was studying A Level chemistry, our teacher recommended chemguide. It has a nice 2000’s feel of a website
Although it had knowledge beyond A Level as well which was nice.

If you learnt about electron orbitals than consider this:
Carbon has electron configuration of 1s^2 2s^2 2p^2
In the second shell, carbon has 2 unpaired electrons (in the p orbitals). But you learnt to form a bond - 2 orbitals must overlap, each containing 1 electron. So how does carbon form 4 bonds, when it has 2 unpaired electrons? It is knowledge beyond A Level but it is nice to know (your teacher may touch on promotion in organic chemistry, with alkane and alkene. Our teacher did it with ethene, when we learnt about pi bonds)
https://www.chemguide.co.uk/basicorg/bonding/methane.html
(edited 8 months ago)
Reply 2
Original post by BankaiGintoki
1) she said it has a higher electronegativity. Electronegativity - affinity of an atom to attract a bonded pair of electrons in a covalent bond. Electron density - you will see it in organic chemistry, in the bonding of benzene and alkenes (describes how close the electrons are). For example, pi bonds in alkene have an electron density above and below the sigma bond between the 2 carbons.
2) Yes that is correct. Chlorine is more electronegative than hydrogen, so it pulls the ‘electron cloud’ (you can imagine it like that to remember it)
3) Electronegativity - is dependent on atomic radius and nuclear charge. The smaller the atomic radius, the stronger the attraction (less electron shielding). The stronger the nuclear charge, the greater the attraction.
So it goes up and across to the right of the periodic table (fluorine being the most polar). We dont count the noble gases as they dont normally form bonds.
4) COOH is a weak acid so partially dissociates in aqueous solution… COOH -> COO- + H+.
I dont understand what you mean by COOH dissociating and ‘chloride pulls the negative charge because it is not electronegative thus making the bond stronger?’

Read https://www.chemguide.co.uk/atoms/bonding/electroneg.html
When I was studying A Level chemistry, our teacher recommended chemguide. It has a nice 2000’s feel of a website
Although it had knowledge beyond A Level as well which was nice.

If you learnt about electron orbitals than consider this:
Carbon has electron configuration of 1s^2 2s^2 2p^2
In the second shell, carbon has 2 unpaired electrons (in the p orbitals). But you learnt to form a bond - 2 orbitals must overlap, each containing 1 electron. So how does carbon form 4 bonds, when it has 2 unpaired electrons? It is knowledge beyond A Level but it is nice to know (your teacher may touch on promotion in organic chemistry, with alkane and alkene. Our teacher did it with ethene, when we learnt about pi bonds)
https://www.chemguide.co.uk/basicorg/bonding/methane.html


THIS IS SO HELPFUL thank you so much it’s helped me way more than Google has!
I just have a couple of follow on questions if you don’t mind me asking sorry.
if the electronegativity depends on nuclear charge and atomic radius, if the atomic radius is smaller and therefore the attraction is greater, does the mean electronegativity is greater? How come fluorine has a higher electronegativity than say francium in group 1 because although francium has a higher atomic radius so the attraction is lower, the nuclear charge is greater so that increases the attraction?

also answering the question of you don’t u don’t and what I meant, I don’t really get it but my teacher said CH2ClCOOH cannot dissociate. Since as you said COOH particularly dissociates and loses a hydrogen forming O- she then said something about the chlorine and the electrons moving and the bond getting stronger so the molecule cannot dissociate but I don’t know how to piece these words together of if the explanation is right??

also thank you for the extra bit, I’ll sure have a look at that seems very interesting.

thanks again for your help ☺️
Original post by Ashirs
THIS IS SO HELPFUL thank you so much it’s helped me way more than Google has!
I just have a couple of follow on questions if you don’t mind me asking sorry.
if the electronegativity depends on nuclear charge and atomic radius, if the atomic radius is smaller and therefore the attraction is greater, does the mean electronegativity is greater? How come fluorine has a higher electronegativity than say francium in group 1 because although francium has a higher atomic radius so the attraction is lower, the nuclear charge is greater so that increases the attraction?

also answering the question of you don’t u don’t and what I meant, I don’t really get it but my teacher said CH2ClCOOH cannot dissociate. Since as you said COOH particularly dissociates and loses a hydrogen forming O- she then said something about the chlorine and the electrons moving and the bond getting stronger so the molecule cannot dissociate but I don’t know how to piece these words together of if the explanation is right??

also thank you for the extra bit, I’ll sure have a look at that seems very interesting.

thanks again for your help ☺️

If the distance between two point charges (in this case the shared pair of electrons in the outermost quantum shell and the atomic nucleus) is small, the attraction will be stronger. This statement arises from something called coulomb’s law (which you are not expected to know unless you do A level physics, though some A level chemistry textbooks may show the equation anyway to help give a mathematical understanding of why it has such an effect).

Francium is on period 7 of the periodic table, so it has 7 occupied quantum shells - as such, it is a huge atom and has many electrons (so a lot of shielding) and these factors outweigh the greater nuclear charge. As such, the attraction between the outer electron in francium that would form the covalent bond in the definition and the atomic nucleus would be weak and so the electronegativity of francium is lower.

EDIT: that particular example is quite complex. If you are asking why only the O-H bond in CH2ClCOOH can dissociate and why the C-H bonds can’t dissociate, there are several factors in play. The very simplified answer is that C-H bonds aren’t polar so they aren’t terribly susceptible to dissociation, whereas the oxygen in the O-H bond attracts the pair of electrons closer to itself, so the dipole formed means that O-H bond is more easily strained by surrounding molecules of water (due to hydrogen bonding) and it breaks more easily.
(edited 8 months ago)

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