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Topic: polarity  (Read 361 times)

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Offline Timson28

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polarity
« on: October 15, 2020, 07:47:36 AM »

The question is: Arrange the sequences below according to increasing polarity by giving them a number from 1 to 3. 1 = least polar 3 = most polar

explanation: CH4 is non-polar because C pulls equally hard on each C and it is also symmetrical. And (C) -En (H) = 0.3 this so what the tipping point in our book we consider this just as polar.

CH3Br(2): En(C)-En(H)= 0.3 en En(Br)-En(c)=0.3 dus het is sowiso een polaire molecule want C trekt even hard aan H maar Br trekt aan C dus het is assymetrisch dus polair.

CH3Cl is the most polar.

Sorry for the bad vocabulary but I am Belgian.

Is this correct?

Offline chenbeier

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Re: polarity
« Reply #1 on: October 15, 2020, 08:27:19 AM »
Methane is a symmetric molecule this is non polar.
Methylchloride has one electronegativ atom (chlorine) What gives the molecule least some polarity.
Methylene bromide has two polar atoms( bromine) and make it almost polar.

Offline Timson28

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Re: polarity
« Reply #2 on: October 15, 2020, 08:38:30 AM »
So CH4(1) because it's nonpolar  CH3Cl(3) and CH3Br(2) ?

Offline chenbeier

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Re: polarity
« Reply #3 on: October 15, 2020, 08:46:56 AM »
I would exchange 2 and 3.
Dibrommethane 3 > Methylchloride 2 > Methane 1

Offline mjc123

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Re: polarity
« Reply #4 on: October 15, 2020, 08:55:09 AM »
That's bromomethane, not dibromomethane. OP keeps writing (2) after it because they think it's the second most polar molecule.

Offline chenbeier

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Re: polarity
« Reply #5 on: October 15, 2020, 09:07:23 AM »
Oh sorry , I saw some dirt on the scanned photo as a 2 behind bromine.

You are right.

Methylchloride Dipolmoment 1,89
Methyl bromide Dipolmoment 1,82
Methane Dipolmoment 0



Offline Timson28

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Re: polarity
« Reply #6 on: October 15, 2020, 09:19:28 AM »
so my order is correct?

How can I best formulate this because we have not yet learned how to calculate the dipole moment.

Offline Corribus

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Re: polarity
« Reply #7 on: October 15, 2020, 09:31:45 AM »
Use electronegativity as a general guide.
What men are poets who can speak of Jupiter if he were like a man, but if he is an immense spinning sphere of methane and ammonia must be silent?  - Richard P. Feynman

Offline Enthalpy

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Re: polarity
« Reply #8 on: October 18, 2020, 07:30:18 AM »
[...] how to calculate the dipole moment.

The dipole moment is much experimental.

Software can make a prediction based on first principles, or worse, on additive rules. It does have some success, but I don't trust it to distinguish 1.82 from 1.89.

Hand estimates would be less accurate. Electronegativity, or the physically meaningful electron affinity and ionisation energy, can give an indication, but they don't suffice, because the distance over which the charge is displaced counts too, and this favours the bigger bromine.

Offline Corribus

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Re: polarity
« Reply #9 on: October 19, 2020, 10:20:09 AM »
Absolutely true, enthalpy. Dipole moment is basically related to charge differential divided by distance. So, the bond length can't be ignored. However, electronegativity of atoms changes to much greater extent than bond length does, so all things being equal the charge differences usually play a bigger role in determining polarity. Usually you will get the right answer in general chemistry if you focus on electronegativity... unless the two EN values are very close together.

Considering that the halogen-carbon bond lengths in bromomethane and chloromethane are about 1.94 and 1.79 Angstrom, respectively, that's only a bond length difference of ~7-8%. By the same token the respective ENs of bromine and chlorine are 3.16 and 3.98. With the EN of carbon being 2.55, the ΔEN for C-Br and C-Cl are 0.61 and 1.43, a considerable difference. For these two molecules, the electronegativity differences of Br and Cl are far more important than the longer bond length imposed by the bigger bromine atom, which is why chloromethane is the more polar molecule despite having the shorter bonds.

These arguments are only qualitative of course and the dipole moment is measured for the entire molecule, not just the carbon-halogen atom. But this is general chemistry so we need not be so focused on fine details.
What men are poets who can speak of Jupiter if he were like a man, but if he is an immense spinning sphere of methane and ammonia must be silent?  - Richard P. Feynman

Offline mjc123

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Re: polarity
« Reply #10 on: October 19, 2020, 02:52:54 PM »
Quote
Dipole moment is basically related to charge differential divided by distance

MULTIPLIED by distance!

Offline Corribus

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Re: polarity
« Reply #11 on: October 19, 2020, 03:07:05 PM »
Sorry, right, bad error :D
What men are poets who can speak of Jupiter if he were like a man, but if he is an immense spinning sphere of methane and ammonia must be silent?  - Richard P. Feynman

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