12. Intermolecular Forces

What is the VSEPR theory based on?

The principle that electron pairs in the outer shell of an atom repel each other, so they are arranged as far away from each other as possible.

What are the steps for predicting the shape of a molecule?

Draw the Lewis structure, count the amount of ‘things’ on the central atom, determine shape and bond angles.

What is the bond angle for linear molecules?

180˚

What is the bond angle for trigonal planar molecules?

120˚

What is the bond angle for tetrahedral molecules?

109.5˚

What is the bond angle for bent molecules with 1 lone pair?

<120˚

What is the bond angle for bent molecules with 2 lone pairs?

104.5˚

What is the bond angle for trigonal pyramidal molecules?

107˚

What do the shapes of molecules play a role in determining?

The intermolecular forces between molecules and physical properties such as polarity, solubility, density, vapour pressure, MP and BP.

How does the strength of intermolecular forces compare to intramolecular forces?

They are 10-100 times weaker than bonds like ionic, covalent and metallic bonds.

What is vapour pressure?

The pressure exerted by particles which have escaped from the surface liquid in a closed container colliding with the container walls.

How do intermolecular forces affect vapour pressure?

Liquids with strong IMFs have molecules held together tightly, so it is harder for them to escape the liquid’s surface, meaning vapour pressure is lower.

What is electron density?

The measure of the probability of an electron being present at a particular location within an atom.

How does electrostatic attraction arise in intermolecular forces?

When electrons are unevenly distributed due to the shape of a molecule and the electronegativity of its atoms, partial positive and negative charges occur, which attract opposite charges in other molecules.

What are non-polar molecules?

Molecules in which there is an even distribution of valence electrons among atoms, meaning there is no charge on either end.

Why are bonds between different atoms typically polar?

One atom has a higher electronegativity than the other, meaning it has a stronger pull on the electron, causing it to have a partial negative charge while the other atom has a partial positive charge.

Why are intermolecular bonds weaker than ionic bonds?

The partial charge on a polar molecule are smaller than the charges on ions.

Steps for comparing polarity of molecules

Find electronegativites of each atom, subtract the lower value from the higher value, determine which has a greater difference.

What do properties of polyatomic molecules depend on?

The shape of the molecule and the polarity of the bonds.

What are symmetrical molecules?

Molecules with evenly-distributed polar bonds, causing the bond dipoles to cancel out.

How to determine solubility from polarity?

Polar substances tend to dissolve in polar solvents, non-polar substances tend to dissolve in non-polar solvents - like dissolves like.

What is an electric dipole?

The occurrence of a positive charge and an equal negative charge separated by a fixed distance.

What is a dipole?

A difference in charge between the two atoms, caused by a shift in the electron density in the bond.

At what ∆EN is a bond polar/non-polar?

∆EN ≤ 0.4 = non-polar

1.7 ≥ ∆EN ≥ 0.5 = polar

What are the 3 main types of intermolecular forces?

Dipole-dipole, hydrogen bonding, dispersion.

What do dipole-dipole forces result from?

The attraction between positive and negative ends of polar molecules.

How strong are dipole-dipole forces and why?

Relatively weak because of partial charges

How does polarity of molecules affect MP/BP?

More polar molecules have stronger dipole-dipole forces, meaning more heat energy is required to break them.

What is hydrogen bonding?

A special form of dipole-dipole force which only occurs in molecules in which a H atom is covalently bonded to a N, F or O atom.

How does hydrogen bonding occur?

Because O, F and N atoms are small and highly electronegative, they strongly attract electrons, leaving H with a significant positive charge. This charge is electrostatically attracted to lone pairs of electrons on N, O and F atoms in neighbouring molecules, resulting in a relatively strong intermolecular bond.

What are the 2 key requirements for H bonding to occur?

A H atom covalently bonded to a F, O or N atom, and a lone pair of electrons on the F, O or N atom.

What are dispersion forces?

The forces of attraction between non-polar molecules, resulting from temporary dipoles caused by the random movement of electrons surrounding molecules.

How does molecule size affect dispersion forces?

As molecule size increases, strength of dispersion force increases because larger molecules have more electrons so it is easier to produce temporary dipoles.

How does molecule shape affect dispersion forces?

Molecules that form long chains have more surface area to interact with neighbouring molecules, meaning they have stronger dispersion forces.

In what type/size of molecules can dispersion forces dominate other types of forces?

Large polar molecules.