Intermolecular Forces 3

7/30/2019 Intermolecular Forces 3

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Why do some solids dissolve in water but others do not?

Why are some substances gases atroom temperature, but others are liquidor solid?

What gives metals the ability to conductelectricity, what makes non-metals

brittle? The answers have to do with

Intermolecular forces

Questions


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Overview There are 2 types of attraction in molecules:intramolecular bonds & intermolecular forces

We have already looked at intramolecular bonds (ionic, polar, non-polar) Intermolecular forces (IMF) have to do with

the attraction between molecules (vs. theattraction between atoms in a molecule)

IMFs come in six flavours: 1) ionic, 2) dipole- dipole, 3) H-bonding, 4) London forces, 5)

covalent (network solids), 6) metallic



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Ionic, Dipole - Dipole attractions We have seen that moleculescan have a separation of charge

This happens in both ionic andpolar bonds (the greater the EN,

the greater the dipoles) H Cl

+

Molecules are attracted to each other in acompound by these +ve and -ve forces

+ +


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H - bonding H-bonding is a special type of dipole - dipole

attraction that is very strong It occurs when N, O, or F are bonded to HQ- Calculate the EN for HCl and H 2 O

A- HCl = 2.9-2.1 = 0.8, H 2 O = 3.5-2.1 = 1.4 The high EN of NH, OH, and HF bonds

cause these to be strong forces (about 5x

stronger than normal dipole-dipole forces) They are given a special name (H-bonding)because compounds containing these bondsare important in biological systems


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London forces Non-polar molecules do not have dipoles

like polar molecules. How, then, can non-polar compounds form solids or liquids?

London forces are named after Fritz London(also called van der Waal forces)

London forces are due to small dipoles thatexist in non-polar molecules

Because electrons are moving around inatoms there will be instants when the chargearound an atom is not symmetrical

The resulting tiny dipoles cause attractionsbetween atoms/molecules

Read 10.3 (pg. 351 - 355) and answer


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London forces

Instantaneous dipole: Induced dipole:

Eventually electronsare situated so that

tiny dipoles form

A dipole forms in one atomor molecule, inducing a

dipole in the other


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Testing concepts1. Which attractions are stronger: intermolecular or

intramolecular?2. How many times stronger is a covalent bond

compared to a dipole-dipole attraction?3. What evidence is there that nonpolar molecules

attract each other?4. Which chemical in table 10.1 has the weakest

intermolecular forces? Which has the strongest?How can you tell?

5. Suggest some ways that the dipoles in Londonforces are different from the dipoles in dipole-dipoleattractions.

6. A) Which would have a lower boiling point: O 2 or F 2?Explain. B) Which would have a lower boiling point:

NO or O 2? Explain.


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7. Which would you expect to have the higher meltingpoint (or boiling point): C 8H18 or C 4H10 ? Explain.

8. What two factors causes hydrogen bonds to be so

much stronger than typical dipole-dipole bonds?9. So far we have discussed 4 kinds of intermolecular forces: ionic, dipole-dipole, hydrogen bonding, andLondon forces. What kind(s) of intermolecular

forces are present in the following substances:a) NH 3, b) SF 6, c) PCl 3, d) LiCl, e) HBr, f) CO 2(hint: consider EN and molecular shape/polarity)

Challenge: Ethanol (CH 3CH 2OH) and dimethyl ether

(CH 3OCH 3) have the same formula (C 2H6O).Ethanol boils at 78 C, whereas dimethyl ether boilsat -24 C. Explain why the boiling point of the ether is so much lower than the boiling point of ethanol.

Challenge: try answering the question on the next slide.


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Testing concepts1. Intramolecular are stronger.2. A covalent bond is 100x stronger.3. The molecules gather together as liquids or

solids at low temperatures.4. Based on boiling points, F 2 (-188) has the

weakest forces, H 2S has the strongest (-61).5. London forces

Are present in all compounds

Can occur between atoms or molecules Are due to electron movement not to EN Are transient in nature (dipole-dipole are

more permanent ). London forces are weaker


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6. A) F 2 would be lower because it is smaller.

Larger atoms/molecules can have their electron clouds more easily deformed andthus have stronger London attractions and

higher melting/boiling points.B) O 2 because it has only London forces. NOhas a small EN, giving it small dipoles.

7. C 8H18 would have the higher melting/boilingpoint. This is a result of the many more sitesavailable for London forces to form.

8. 1) a large EN, 2) the small sizes of atoms.

Testing concepts


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9. a) NH 3: Hydrogen bonding (H + N), London.

b) SF 6: London only (it is symmetrical).c) PCl 3: EN=2.9-2.1. Dipole-dipole, London.d) LiCl: EN=2.9-1.0. Ionic, (London).

e) HBr: EN=2.8-2.1. Dipole-dipole, London.f) CO 2: London only (it is symmetrical)

Challenge: In ethanol, H and O are bonded (thelarge EN results in H-bonding). In dimethylether the O is bonded to C (a smaller ENresults in a dipole-dipole attraction rather than hydrogen bonding).

Testing concepts


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H bonding and boiling point

See pg. 369 Q why does BP as period ,why are some BP high at period 2?

Predicted and actual boiling points

-200-150

-100

-50

0

50100

Period

B o

i l i n g p o

i n t Group 4

Group 5

Group 6

Group 7

2 3 4 5


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Testing conceptsBoiling points increase down a group (asperiod increases) for two reasons: 1) ENtends to increase and 2) size increases. Alarger size means greater London forces.

Boiling points are very high for H 2O, HF, and

NH3 because these are hydrogen bonds(high EN), creating large intermolecular forces

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Documents

Intermolecular Forces 3