Polar, Non-polar question(Gen.Chem)

[joonkimdds]

I want to ask a general chemistry question.
When I learned about General Chemistry, my teacher told me that symmetrical lewis structure is non-polar, and non-symmetrical is polar.
so...for example
.......F
.......|
....F-C-F
.......|
.......F

is symmetrical, thus non-polar.
If u guys have Kaplan Blue book, it's the actual practice test number 52.
it says "which of the following is the most polar molecular compound?"
The five choices are BF3, CF4, CBr4, CH2Cl2, Ch2Br2

I already showed u the first one up there, and the other ones are
.......F
.......|
.......B
...../...\
....F.....F

.......Br
.......|
...Br-C-Br
.......|
.......Br


.......H
.......|
...Cl-C-Cl
.......|
.......H


.......H
.......|
...Br-C-Br
.......|
.......H

my point is...they all look symmetrical, so I think all of them should be called non-polar. but the answer in the back of the book says D is the answer.

could someone explain to me why?

---

Members don't see this ad.

 

[curlykid]

think of all of these compounds in their 3D form (make a model, it will help you visualize them)

b,c, and d are all tetrahedral
b is perfectly symmetrical
c and d both have dipole moments towards Cl and Br (make a model of the tetralhedral shape, it'll make more sense)
however Br is more electronegative than Cl, that is why it has a greater dipole moment and is more polar

hope this helps!

I want to ask a general chemistry question.
When I learned about General Chemistry, my teacher told me that symmetrical lewis structure is non-polar, and non-symmetrical is polar.
so...for example
.......F
.......|
....F-C-F
.......|
.......F

is symmetrical, thus non-polar.
If u guys have Kaplan Blue book, it's the actual practice test number 52.
it says "which of the following is the most polar molecular compound?"
The five choices are BF3, CF4, CBr4, CH2Cl2, Ch2Br2

I already showed u the first one up there, and the other ones are
.......F
.......|
.......B
...../...\
....F.....F

.......Br
.......|
...Br-C-Br
.......|
.......Br


.......H
.......|
...Cl-C-Cl
.......|
.......H


.......H
.......|
...Br-C-Br
.......|
.......H

my point is...they all look symmetrical, so I think all of them should be called non-polar. but the answer in the back of the book says D is the answer.

could someone explain to me why?

 

[Divineimpetus]

Bromine is more electronegative than Chlorine?

I thought Chlorine was because it is closer to Flourine

 

[dat_student]

I want to ask a general chemistry question.
When I learned about General Chemistry, my teacher told me that symmetrical lewis structure is non-polar, and non-symmetrical is polar.
so...for example
.......F
.......|
....F-C-F
.......|
.......F

is symmetrical, thus non-polar.
If u guys have Kaplan Blue book, it's the actual practice test number 52.
it says "which of the following is the most polar molecular compound?"
The five choices are BF3, CF4, CBr4, CH2Cl2, Ch2Br2

I already showed u the first one up there, and the other ones are
.......F
.......|
.......B
...../...\
....F.....F

.......Br
.......|
...Br-C-Br
.......|
.......Br


.......H
.......|
...Cl-C-Cl
.......|
.......H


.......H
.......|
...Br-C-Br
.......|
.......H

my point is...they all look symmetrical, so I think all of them should be called non-polar. but the answer in the back of the book says D is the answer.

could someone explain to me why?

It's not about symmetry. If the sum of the vectors is equal to zero the molecule is non-polar. Vector has a magnitude and a direction.

When you have CCl4 the sum of two vectors cancels the sum of the other two vectors because the sums have the same magnitude but point in opposite directions. Remember that all angles are not 90 degrees. It's tetrahedral. So angles are 109.5 degrees.

Both CH2Cl2 and CH2Br2 are polar but Cl is more electronegative. Thus, CH2Cl2 is the most polar molecule.

 

[seoul]

I want to ask a general chemistry question.
When I learned about General Chemistry, my teacher told me that symmetrical lewis structure is non-polar, and non-symmetrical is polar.
so...for example
.......F
.......|
....F-C-F
.......|
.......F

is symmetrical, thus non-polar.
If u guys have Kaplan Blue book, it's the actual practice test number 52.
it says "which of the following is the most polar molecular compound?"
The five choices are BF3, CF4, CBr4, CH2Cl2, Ch2Br2

I already showed u the first one up there, and the other ones are
.......F
.......|
.......B
...../...\
....F.....F

.......Br
.......|
...Br-C-Br
.......|
.......Br


.......H
.......|
...Cl-C-Cl
.......|
.......H


.......H
.......|
...Br-C-Br
.......|
.......H

my point is...they all look symmetrical, so I think all of them should be called non-polar. but the answer in the back of the book says D is the answer.

could someone explain to me why?

You're right, all are indeed symmetrical, but:

only D and E are polar (strictly speaking) cuz in tetrahedral arrangement, the net polarity resulted by the halogen group will always be pointing in one specific direction (build a model to help visualize if you have the kit) no matter how you turn the molecule.

Since Cl is > electronegative than Br, the most polar molecule is no doubt D then.

 

[gatszu]

actually, Cl is more electronegative than Br. Thus, answer is CH2Cl2.

.........H
..........l
.....H--C--Cl
..........l
.........Cl

not really that symmetric anymore, now is it?

you should learn how to draw w/wedge & dash line to represent 3D.

 

[joonkimdds]

so...r u guys saying the way my teacher taught me about symmetrical = non polar and vice versa is the wrong way of learning this polarity stuffs?

 

[dat_student]

so...r u guys saying the way my teacher taught me about symmetrical = non polar and vice versa is the wrong way of learning this polarity stuffs?

Your teacher didn't know or you misunderstood him 😉

 

[I AM SARA]

so...r u guys saying the way my teacher taught me about symmetrical = non polar and vice versa is the wrong way of learning this polarity stuffs?

nope, your confused.

 

[Flipper405]

so...r u guys saying the way my teacher taught me about symmetrical = non polar and vice versa is the wrong way of learning this polarity stuffs?

I'm taking gen chem right now, and we just recently covered polarity. My professor also taught symmetry as a way to predict polarity. I think the thing you have to realize is that it's not symmetrical if all the outer elements are not the same... The first 3 all have the same elements around the central atom. The last two have Br/Cl and H though. Therefore, they are not symmetrical. Then comparing the last two, since Cl is more electronegative, that would be the most polar.

Good luck. 👍

 

[joonkimdds]

Do u guys actually think of these structures in 3D and compare their electronegetivities to find their polarity?

then how come my teacher and Flipper405's teacher teach us symmetric to decide polarity?

does this mean there are more than 1 way to find out polarity and the way i learned is not as good as the way u guys learned with using electronegetivity, 3D...and stuffs?

 

[curlykid]

think of all of these compounds in their 3D form (make a model, it will help you visualize them)

b,c, and d are all tetrahedral
b is perfectly symmetrical
c and d both have dipole moments towards Cl and Br (make a model of the tetralhedral shape, it'll make more sense)
however Br is more electronegative than Cl, that is why it has a greater dipole moment and is more polar

hope this helps!

oh goodness, shoot, you guys are right!!! I am sooooo dumb I wasn't paying attention, just looked that the answer and tried to explain it. Everyone else is right, Cl is definitely more electronegitive than Br and therefore more polar, I should have looked at a periodic table first. Sorry about that everyone! ::blushes:: 🙁

 

[curlykid]

Do u guys actually think of these structures in 3D and compare their electronegetivities to find their polarity?

then how come my teacher and Flipper405's teacher teach us symmetric to decide polarity?

does this mean there are more than 1 way to find out polarity and the way i learned is not as good as the way u guys learned with using electronegetivity, 3D...and stuffs?

yeah, it is best to look at these structures in 3D, and examine the symmtry of the 3D structure to decide polarity. A and B are symmtrical in their 3D structure as well as being equally electronegative in all directions so the dipole moments of all of the elements cancel each other out, while C and D, there isn't perfect symmetry of the distrubution electronegative elements so there is a dipole moment. Does that make sense? Is that clear? (sorry i'm really tired)