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Polar Molecules
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Polar Molecules. We already know that it is the difference in electronegativities between elements that determines the type of bonding. If there is a.

Dec 24, 2015

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Pierce Curtis
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Page 1: Polar Molecules. We already know that it is the difference in electronegativities between elements that determines the type of bonding. If there is a.

Polar Molecules

Page 2: Polar Molecules. We already know that it is the difference in electronegativities between elements that determines the type of bonding. If there is a.

• We already know that it is the difference in electronegativities between elements that determines the type of bonding.

• If there is a large difference in electronegativities . . .

• Ionic bonding• Complete transfer of electrons• The more electronegative element takes the

electrons

Page 3: Polar Molecules. We already know that it is the difference in electronegativities between elements that determines the type of bonding. If there is a.

• No difference in electronegativities• Pure covalent bonding• Equal share of electrons

• In real life though, most molecules are somewhere in between these 2 extremes.

• They are polar covalent• They share electrons, but not equally• The more electronegative element has a

greater share of the electrons.

Page 4: Polar Molecules. We already know that it is the difference in electronegativities between elements that determines the type of bonding. If there is a.

X Y X Y Y+X- Y+X-

Electronegativity Difference0 4

Pure covalent Polar covalentElectrons not equally shared

Polar ionicDistorted ions

Pure ionic

Polarisation of covalent bonds

Polarisation of ions

Favoured by small, highly charged +ve ions, e.g. Li+, Be2+

- +

Page 5: Polar Molecules. We already know that it is the difference in electronegativities between elements that determines the type of bonding. If there is a.

Non-polar bond

Polar bond

Page 6: Polar Molecules. We already know that it is the difference in electronegativities between elements that determines the type of bonding. If there is a.

• We can show that a bond is polar in a number of ways:

Page 7: Polar Molecules. We already know that it is the difference in electronegativities between elements that determines the type of bonding. If there is a.

This is the commonest way.The funny symbol is pronounced “delta”

Page 8: Polar Molecules. We already know that it is the difference in electronegativities between elements that determines the type of bonding. If there is a.

This symbol actually shows that there is an OVERALL imbalance in the distribution of charge within a molecule.

The molecule is a dipole

Page 9: Polar Molecules. We already know that it is the difference in electronegativities between elements that determines the type of bonding. If there is a.

• It is important to know • (because the IB keep asking about it!)• That even though it may contain individual bonds

that a polar, a molecule may not be a dipole.

• Consider CO2

• Each C=O bond is polar (how do we know?)• But the symmetrical shape of the molecule means

electrons are pulled equally in both directions• And the molecule is not a dipole

Page 10: Polar Molecules. We already know that it is the difference in electronegativities between elements that determines the type of bonding. If there is a.

CO+-

CO2

Bonds: polarMolecule: non-polar

O-

Page 11: Polar Molecules. We already know that it is the difference in electronegativities between elements that determines the type of bonding. If there is a.

• Compare this with H2O• This also contains three atoms , but this time

the arrangement is not symmetrical,• So the molecule IS a dipole

Page 12: Polar Molecules. We already know that it is the difference in electronegativities between elements that determines the type of bonding. If there is a.

OHH

+ +

-

H2O

Bonds: polarMolecule: polar

Page 13: Polar Molecules. We already know that it is the difference in electronegativities between elements that determines the type of bonding. If there is a.

• Comment on whether tetrachloromethane, CCl4, is a dipole

• How about trichloromethane, CHCl3 ?

Page 14: Polar Molecules. We already know that it is the difference in electronegativities between elements that determines the type of bonding. If there is a.

CCl4

Bonds: polarMolecule: non-polar

C

- -

+

Cl-

ClCl

Cl-

tetrachloromethane

Symmetrical

Page 15: Polar Molecules. We already know that it is the difference in electronegativities between elements that determines the type of bonding. If there is a.

• Not symmetricalTrichloromethane

CHCl3

Yes – it’s a dipole!

Page 16: Polar Molecules. We already know that it is the difference in electronegativities between elements that determines the type of bonding. If there is a.

How can we tell if a liquid is polar or non-polar?

• Could you think of an experiment to tell if an unknown liquid is polar or not?

• The charge we have talked about isn’t just theory – it’s a real charge.

• If we hold an electrostatically charged rod near a thin stream of the liquid . . .

Page 17: Polar Molecules. We already know that it is the difference in electronegativities between elements that determines the type of bonding. If there is a.

• It will be either attracted or repelled if it is a polar liquid.

• We can do this with water!

Page 18: Polar Molecules. We already know that it is the difference in electronegativities between elements that determines the type of bonding. If there is a.