Sapling #8

(Polar molecules, Non-polar molecules, etc.)

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Sapling #8

Postby EmilyGillen_1A » Mon Nov 23, 2020 11:51 am

I am having trouble getting Sapling #8 right (for week 7/8 homework). The problem gives 2 molecules, BeCl2 and H2O. It asks you to specify if the bonds are non-polar or polar, and if the molecule is polar or non-polar. I thought BeCl2 has polar bonds but is non polar, and that H2O has polar bonds and is polar, but when I put that as my answer it was incorrect. Can anyone explain how to determine polar vs non-polar bonds / explain how to answer this question correctly?

Thank you!

Melody Wu 2L
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Re: Sapling #8

Postby Melody Wu 2L » Mon Nov 23, 2020 11:58 am

This problem also took me a bunch of tries.

For both molecules, the BONDS are POLAR because the two atoms on either side of the bond have different electronegativities. The only time a bond would be nonpolar is if the two atoms on either side are the same (or if the difference is very small at least).

Now for looking at whether the molecule is polar or nonpolar, you have to consider the geometry. BeCl2 is a linear molecule so the dipoles cancel out due to the symmetry. In H2O, it has a bent shape due to the lone pairs so the dipoles do not cancel out; therefore it is a polar molecule.

I just realized that this is the answer you came up with, so maybe try the combo again?

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Re: Sapling #8

Postby anikamenon2H » Mon Nov 23, 2020 12:29 pm

Yeah I would try changing your answer slightly, maybe there's a typo or something? BeCl2 has two polar bonds with dipoles towards the Cl but they cancel because they're going in the opposite direction so the molecule as a whole is nonpolar. H2O also has two polar bonds and with the dipoles towards the O and the lone pair on the O makes it bent so the dipoles don't cancel --> H2O is polar.

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Re: Sapling #8

Postby OwenSumter_2F » Mon Nov 23, 2020 1:02 pm

Also check that you didn't accidentally switch the answers for the two molecules, because what you got is correct. Also you could try refreshing the page or something.]

Madisen Brown -1C
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Re: Sapling #8

Postby Madisen Brown -1C » Mon Nov 23, 2020 1:33 pm

The bonds of both molecules are polar because their electronegativities are different. For BeCl2, the shape of the molecule is linear because there are no lone pairs on the central atom that would cause the molecule to bend or be angular. In a linear shape, the dipoles cancel each other out therefore the molecule is nonpolar (in nonpolar molecules, the dipoles cancel out). For H2O however, there are lone pairs on the central atom which causes the shape to be bent/angular. In the angular shape, you can visually see that the bonds are asymmetrical and thus the dipoles do not cancel out. In a molecule where the dipoles do not cancel out, the molecule is polar.
Hope this helps!

Emily Jacobo 1C
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Re: Sapling #8

Postby Emily Jacobo 1C » Mon Nov 23, 2020 5:08 pm

I had the exact same issue! It took me a lot of tries for some reason even though I had the right answer! I finally had to exit the tab and come back.

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