Intermolecular forces

Intermolecular forces Remember this phrase . . . Intermolecular forces

It’s the answer to (nearly) every question about physical properties. Why is H2O a liquid, but H2S is a gas? Intermolecular forces! Why is ethanol a liquid but ethane is a gas? Why wasn’t life on Earth wiped out during various ice ages? Intermolecular forces are forces between molecules which are not chemically bonded.

We split intermolecular forces into 3 types. The easiest to understand are permanent dipole – permanent dipole forces Sometimes we just call these dipole-dipole forces These forces are about 1% as strong as a covalent bond.

If a molecule is a dipole (make sure you know how to tell!) Then the +ve end of one molecule is attracted to the –ve end of another molecule. This builds up a whole network of forces holding molecules in position. Energy must be supplied to overcome these forces before a substance can melt or boil. Polar molecules have higher boiling points than non-polar molecules

Hydrogen bonds

A special case of dipole-dipole force is hydrogen bonding. A hydrogen bond is about 10% as strong as a covalent bond.

Hydrogen bonding happens when a very electronegative atom is bonded to a hydrogen atom. ONLY N, O or F The electronegative atom withdraws the bonding electrons Hydrogen has no other (shielding) electrons So the proton in the nucleus is exposed, Meaning that any electrostatic forces are relatively large.

Hydrogen bonding in water

Hydrogen bonding in ammonia

Hydrogen bonding in HF

Hydrogen bonding in organic chemicals

Because hydrogen bonding is a stronger force than dipole-dipole attraction, Molecules where hydrogen bonding is present have higher BPs

Why is ice less dense than water? The molecules of H2O in water are bouncing randomly around, which makes it a liquid. Usually molecules move closer together when a liquid becomes a solid, but because H2O molecules are strongly polarized, when they slow down and become a solid, they arrange themselves into a matrix that actually pushes them further apart--making ice less dense than water.

Also . . . What might happen to the bond angle as hydrogen bonds form? The lone pair electrons are pulled slightly further away from the oxygen atoms So they repel O-H bonds slightly less And the bond angle increases slightly So each water molecule takes up slightly more room.

Van der Waal’s Forces The third kind of intermolecular force is known as: Van der Waal’s forces London Forces Temporary dipole – induced dipole forces These are all the same thing!

Van der Waal’s forces exist between all molecules – even ones which aren’t polar! Molecules have lots of electrons which are in permanent random motion. At certain times (just by chance) more of the electrons will be on one side of a molecule And the molecule will have a temporary dipole

Temporary dipole

The temporary dipole attracts or repels the electrons in neighbouring molecules. This creates an induced dipole

Van der Waal’s forces are very weak – less than 1% the strength of a covalent bond They act over very short distances only The electrons don’t stay still, so the dipoles change.

The strength of Van der Waal’s forces depends on the number of electrons present. Bigger molecules (higher Mr) have more electrons, so they have greater VdW forces, and a higher BP. The strength of Van der Waal’s forces also depends on the amount of surface area in contact with another molecule. Long molecules have a higher BP than short or spherical molecules.

Explain why pentane has a BP of 309K, but dimethyl propane has a BP of 283K. Short answer – intermolecular forces! Long answer – not polar so only VdW’s forces. Same Mr, so no difference in number of electrons. Pentane is a straight chain molecule, so relatively large contact area. Dimethyl propane has side chains, so takes a spherical shape. Lower contact area, so weaker forces.