Polarisation and Intermolecular Forces

Question 1

Electronegativity

Answer 1

An atom’s ability to attract the electron pair in a covalent bond

Question 2

Strongest electronegative element

Three other strong electronegative elements

Answer 2

Fluorine

Oxygen, nitrogen and chlorine

Question 3

Polar Bonds

Answer 3

In a covalent bond between two atoms of different electronegativities, the bonding electrons will be pulled towards the more electronegative atom

Question 4

Non-polar bonds between the same atoms

Answer 4

A covalent bond between two atoms of the same element (eg H2) is non polar:

because the atoms have equal electronegativities, so the electrons are equally attracted to both nuclei

Question 5

Dipole

Answer 5

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

Question 6

Permanent Dipole

Answer 6

The difference in electronegativity between the two atoms in a polar bond causes a permanent dipole

Question 7

Describe the forces in substances made up of molecules with permanent dipoles

Answer 7

There will be weak electrostatic forces of attraction between the delta positive and delta negative charges on neighbouring molecules

Question 8

Three Types of Intermolecular Forces

Answer 8

Induced dipole-dipole or van der Waals forces

Permanent dipole-dipole forces (caused by polar molecules)

Hydrogen bonding

(these are all forces BETWEEN the molecules)

Question 9

Van der Waals forces

Answer 9

Uneven distribution of electrons in one molecule induces a dipole on neighbouring molecules.

Van der Waals forces are found between all atoms and molecules

Question 10

Van der Waals forces and molecules

Answer 10

Van der Waals forces can hold molecules in a lattice

Question 11

Stronger van der Waals forces

Answer 11

Higher boiling points

due to larger electron clouds

Question 12

Shape of molecules and Van der Waals forces

Answer 12

Long-straight molecules can lie closer together than branched ones.

The closer together two molecules get, the stronger the forces between them are

Question 13

Boiling a liquid and Van der Waals forces

Answer 13

When you boil a liquid, you need to overcome the intermolecular forces, so that the particles can escape from the liquid surface.

This is why you need more energy to overcome stronger intermolecular forces, so liquids tend with stronger van der Waals forces will have higher boiling points

Question 14

Hydrogen Bonding

Answer 14

Only occurs when hydrogen is covalently bonded to fluorine, nitrogen or oxygen (NOF)

Question 15

Why does hydrogen bonding happen

Answer 15

Fluorine, nitrogen and oxygen are very electronegative, so they draw bonding electrons away from the hydrogen atom.

The bond is polarised, and hydrogen has such a high charge density (as it’s so small) that the hydrogen atoms form weak bonds with lone pairs of electrons on the NOF atoms of other molecules

Question 16

Substances with hydrogen bonds

Why is this

Answer 16

Have higher boiling and melting points than other similar molecules.

because of the extra energy needed to break the hydrogen bonds between the molecules

Question 17

Describe ice and hydrogen bonds

Answer 17

As liquid water cools to form ice, the molecules make more hydrogen bonds and arrange themselves into a regular lattice structure