How do you determine polarity of substance? (1 Viewer)

[redhat]

How do you determine whether a substance is polar or not?

I have no idea how the hell symmetry can be used to determine it because it is confusing.

Can someone please give me an Idiots guide in how to determine it?

thanks in advance..

Conq. Chem + macmillan + contexts isn't very helpful!

 

[howareya]

it's in there read it

 

[PoLaRbEaR]

A substance is polar if it has poles, that is one side is more negative than the other...a substance such as ethane is non-polar because all the electron are equally shared between the atoms..
An substance like ethanol is polar because, due to the high electronegativity of oxygen, most electrons are attracted towards it...
if you look at water, the more negative side would be at the oxygen and positive at the hydrogen...

 

[+:: $i[Q]u3 ::+]

well, O is much more electronegative than C or H, right..
so if you have an O atom stuck next to a C/H atom, then you'll know that part of the molecule is more electronegative.

 

[mitochondria]

*Recall from the preliminary course* when you look at the periodic table, the further to the lright and the closer to the top, the more electronegative the element will be. The most electronegative element is Flourine and it has a EN of 4. This means when other elements are combined with F, electrons are more likely to be found as it get closer to the F atom.

However, when it comes to determining the polarity, it does not solely depend on the negativity of a substance but also the angles, thus, the structure of the substance. For example, when considering a methane - CH4 molecule, even though C is more electronegative than H, this molecule is NONPOLAR due to its structure. In other words, there are only ends of +'s on the outside, and for a polar molecule you are required to have opposite ends of +'s and -'s.

Here are more examples... Let's consider a NH3 molecule in a 3D point of view, it's like some sort of alien with 3 legs separated by angles of 120 degrees that are directly connected to it's head. (Do you get what I mean? If you can't ask your teacher for those cool little toys in chemistry - molecular modeling - hehee.. I love MolyMod and make an NH3) Since N is more electronagtive than H, electrons are "drawn" to the N molecule, together with its structure they allow the molecule to be polar.
Note: in this case N is NOT IN THE MIDDLE.

For further consideration with respect to the NH3 molecule (hehee... here's where my teacher's little jokes come in). Imagine the N molecule is sharing its lunch (electrons) with 3 H's. In all 3 pairs they each contribute half of the lunch (one election). Now one H+ molecule (pratically a proton) comes and join with the sharing. BUT, but, it's not sharing any lunches and just pinch two electrons from the NH3 molecules and now you have a NH4+ molecule. So, back to the alian description of the NH3, N is now OFFICIALLY in the middle and it is not polar anymore.

The COH bond as mentioned above is also a really good example for a polar structure. Technically, when you are illustrating the COH bond at the ENDS of alkanols or similar substances they should be drawn at an angle. If it was linear, there will be no polarity.


You are not an idiot

 

[PoLaRbEaR]

*Recall from the preliminary course* when you look at the periodic table, the further to the left.The most electronegative element is Flourine and it has a EN of 4...

you mean the further to the right =)
and why wouldnt chlorine be more electronegative than fluorine?

Here are more examples... Let's consider a NH3 molecule in a 3D point of view, it's like some sort of alien with 3 legs separated by angles of 120 degrees that are directly connected to it's head..

i guess you can say tetrahedral? or pyramidal? like the substructure of diamond...

 

[Exeter]

yes he means further to right, and its more electronegative than chlorine because its closer to the top

 

[PoLaRbEaR]

Originally posted by Exeter
yes he means further to right, and its more electronegative than chlorine because its closer to the top

oh yeh it is too...i always thought Cl was above F...=|

 

[Exeter]

hehe, i had to check my periodic table, good ol RACI

 

[mitochondria]

ahhhh, what a mistake that I have made there... lol, maybe I better go back to kindy and do something about let and right so that I wont' get things wrong in the HSC... Well, yes yes, the more to the RIGHT and the more the to TOP the more electronegarive the element is. Therefore Flourine is the most electronegative element oin the periodic table

Originally posted by Exeter
hehe, i had to check my periodic table, good ol RACI

I was just interested... what''s the RACI periodic table like? I've always been trying to get a good one with all the information i.e. b.p. m.p. density, radii blah blah blah.. has the RACI one got hose on it?? And if so, where do you get it? Hehehe... thank you...

Originally posted by PoLaRbEaR
i guess you can say tetrahedral? or pyramidal? like the substructure of diamond...

lol... I have never learnt those shapes properly maybe I should go and do some now.. ooh, I mean after the trial so here you go, I HAVE LEARNT SOMETHING TODAY The NH3 molecule is... whoops.... what is it? ahhhhhh..... a triangular prism (a more mathematical approach )

 

[Exeter]

hey mitochondria, the RACI periodic table is just the ones they give out at chemistry quiz's

it just have the mass of elements to 4 sig figs, and on the back they have alot of constants and stuff

 

[mitochondria]

ahhhhhh...... thank you exeter heheee... I have four of them in my wallet already *blink*

Just a note on this.. I don't know if you have previously read a message about a syllabus note team before, but what we are aiming to do is to produce syllabus notes for a variety of subjects (currently: Biology, Chemistry, English - it's just general techniques, and probably no notes related to the syllabus will be published, Maths - except for general, Physcis... Prospective subjects include: Business Studies, Legal Studies, Ancient/Mordern History, General Maths...) So if you are interested in joining us, please visit the link in my signature.. quote from Frigid: most people are "selfish selfish beings"... let's not be one of those will ya? Thank you!

 

[inasero]

look for symettry in the molecule....thinks such as methane do in fact have dipoles but these add up to give a net zero dipole....whereas say for water, consider the shape of the moecule and the atoms involved.....the H atoms are positive with respect to the oxygen (R-E-S-T-E-C-P!). Yeah....anyways so you can vectorially add up the component dipoles to see that overall, the H end is positive and the oxygen end is negative....

 

[iambored]

if the molecule is symmetrical then generally it'll be non-polar. this is because there is no charge over to one particular side, if u know what i mean

 

[spice girl]

for organic molecules, a quick rule:

the part of the molecule containing only carbon and hydrogen is the non-polar part. all weird and wonderful add-on substituents make the molecule polar.