Structure & propertiesINTERMOLECULAR FORCES

INTRAMOLECULAR FORCES

Ionic bonds, covalent bonds, metallic bonding are all examples of bonds within a molecule

intermolecular forces

Forces BETWEEN molecules

Weaker than inTRAmolecular forces, but still very important

QuickTime™ and a decompressor

are needed to see this picture.

intermolecular forces!• Featuring the exciting world of ...

• Dipole dipole, hydrogen bonding, ion dipole, Van der Waal’s/London dispersion forces!

dipole-dipole

One of the strongest of the intermolecular forces

Result from uneven distribution of electrons, creating a dipole

Molecules will arrange themselves in solution according to the dipoles present

ie) ICl

impact on mp/bp

Dipoles will INCREASE melting and boiling points

The molecules are now stuck together more tightly (opposites attract!) and so more energy is required to melt/boil the substance

hydrogen bondingA special type of dipole that only occurs between hydrogen and F, O, N atoms

Usually represented by a dashed line

One of the reasons water has such a high boiling point is this interaction

Impact on mp/bpConsider the effect hydrogen bonding has on melting and boiling points

 state at room

Tboiling point

(°C)H2O liquid 100H2S gas -61CH3–O–CH3 gas -24CH3–CH2OH liquid 78NH3 gas -28PH3 gas -90HF gas 21HCl gas -79CH3COOH liquid 118C2H6 gas -89

ion dipole forces

- force of attraction of ion and polar molecule- ionic compounds dissociate in polar solvents since ion (cation/anion) dipole (polar molecule) force overcomes force binding ions together

- ex) NaCl dissolved in water

Think about it! • You are making some pasta. The

instructions indicate you should boil the pasta in salted water.

• You are in a rush and want to eat as quickly as possible!

• Should you add salt BEFORE you boil the water or AFTER the water has started to boil?

• Explain why!

van der waal’s forcesThese occur in every molecule!

At any given time, there may be more electrons in a particular area than another, creating an ever-so-slight dipole

This creates a chain reaction which can have a large effect in molecules

impact

The impact is larger with larger molecules, since there more electrons and a higher chance they will be unbalanced

Who has the highest bp? H2O vs H2S H2O due to hydrogen

bonding,

NH3 vs PH3 NH3 due to hydrogen bonding

CHCl3 vs CH4 CHCl3 due to dipole-dipole

CH4 vs C10H20 C10H20 due to larger VDW forces

try it!

try it!

Nelson Chemistry 12

Read p.257-265

Try p.260 #1-5; p.264 #9,11