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

• Why are some substances gases at room temperature, but others are liquid or solid?

• What gives metals the ability to conduct electricity?

• What makes non-metals brittle? • The answers have to do with …

Intermolecular forces

Questions

Introduction• There are 2 types of attraction in molecules:

intramolecular bonds & intermolecular forces• We have already looked at intramolecular

bonds (ionic and covalent bonding)• INTRAmolecular forces are the forces of

attraction within a molecule or ionic compound

• These are the forces that hold molecules and compounds together

INTERMOLECULAR FORCES

INTERMOLECULAR FORCES

• Intermolecular forces (IMF) have to do with the attraction between molecules (vs. the attraction between atoms in a molecule)

• These forces are weaker than intramolecular bonding forces

• There are 3 types of IMF’s (from weakest to strongest) 1) van der Waal’s

2) dipole-dipole3) hydrogen-bonding

1. Van der Waal’s forces

• Non-polar molecules do not have dipoles like polar molecules.

• How, then, can non-polar compounds form solids or liquids?

• What holds the molecules close to one another when in the liquid or solid state?

Van der Waal’s forces• Van der Waal’s forces are due to small

temporary dipoles that exist in non-polar molecules

• Because electrons are moving around in atoms there will be instants when the charge around an atom is not symmetrical ie not evenly distributed

• The resulting tiny dipoles cause attractions between neighbouring atoms/molecules

vanderWaal’s forces

Instantaneous dipole: Induced dipole:

Eventually electrons are situated so that tiny dipoles form

A dipole forms in one atom or molecule, inducing a

dipole in the other

• What trends do you notice?

:// . . / / /http wps prenhall com wps media objects

The boiling points (and melting points) of the halogens DECREASES

as their MOLAR MASS increases (ie DOWN the group)

2. Dipole - Dipole attractions•We have seen that bonds within molecules can have a separation of charge that creates dipole moments

• If these dipoles combine in a non-symmetrical manner, then a polar molecule results.

H Cl

+ –

Polar and non-polar molecules

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2. Dipole - Dipole attractions• Neighbouring molecules are attracted to one another through dipole-dipole forces of attraction

• The greater the ∆EN of the dipole moment, the stronger the dipole-dipole forces of attraction between molecules, and the higher the boiling point, because it takes more energy to move molecules apart.

+–

+ –

+ –

+ –

The greater the ∆EN of the dipole moment, the stronger the dipole-dipole forces of attraction

between molecules

• H-Cl EN = 0.8 Strongest

dipole

Highest Boiling Point

H-Br EN = 0.7

H-I EN = 0.4 Weakest dipole

Lowest Boiling Point

Recall: the 3 states of matter

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3. Hydrogen-bonding

• H-bonding is a special type of dipole - dipole attraction that is very strong

• NOTE: it is not a “Bond” but actually an intermolecular force of attraction between neighbouring molecules

• It only occurs between molecules that have an O-H, N-H or F-H bond which are all highly polar

bonds with large EN values

3. Hydrogen-bonding• Compare the EN for H-Cl and O-H bond

in H2OH-Cl = 2.9-2.1 = 0.8, O-H = 3.5-2.1 = 1.4

• The high EN of N-H, O-H, and H-F bonds cause hydrogen bonding forces to be the strongest IMF (about 5x stronger than normal dipole-dipole forces)

The hydrogen-bonding attraction between molecules of water account for its high surface tension.

NOTE: the hydrogen “bond” is between molecules of water – it is not a TRUE

bond!!

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Water Water has a very high melting and boiling point due to extensive hydrogen bonding. Having two hydrogens it can form two H- bonds per molecule allowing a lattice diamond-like structure to be built up, as shown below in ice. Note that each O atom is “bonded” to 4 H’s either through H-bonding or a covalent bond. This causes the EXPANSION of water when it freezes.

• http://ibchem.com/IB/ibnotes/brief/bon-sl.htm

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H – bonding and boiling pointPredicted and actual boiling points

-200

-150

-100

-50

0

50

100

Period

Bo

ilin

g p

oin

t Group 4

Group 5

Group 6

Group 7

2 3 4 5

Trends in Boiling Points

Boiling points increase down a group (as period increases) for two reasons: 1) EN tends to increase and 2) size increases. A larger size means greater vanderWaal’s forces.

Boiling points are higher than expected for H2O, HF, and NH3 because these are capable of hydrogen-bonding with neighbouring molecules (high EN), creating very strong intermolecular forces. This makes it more difficult to move molecules apart in the evaporation process, resulting in a high boiling point.

Ionic-Dipole Attractive Forces

• Positive and negative ions that are present in ionic compounds will be attracted to dipoles such as those of a polar molecule like water

• This “ionic-dipole” attraction between positively and negatively charged centers accounts for the solubility of most ionic compounds in polar solvents

Ionic – dipole attractive forces allow ionic compounds to

dissolve in a polar solvent like water

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Ionic-Dipole Attraction