Polarisation and Intermolecular Forces

Question 1

Define electronegativity

Answer 1

Electronegativity is an atom’s ability to attract the electron pair in a covalent bond

Question 2

What is the most electronegative element?

Answer 2

Flourine

Question 3

Electronegativity is measured on what?

Answer 3

The Pauling scale

Question 4

Give examples of the most electronegative elements and their score on the Pauling scale

Answer 4

Flourine - 4.0
Oxygen - 3.4
Chlorine - 3.0
Nitrogen - 3.0
Carbon -2.5
Hydrogen -2.2

Question 5

Can covalent bonds be polarised?

Answer 5

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

Question 6

Are covalent bonds with atoms of the same element polar?

Answer 6

A covalent bond between two atoms of the same element is non-polar because the atoms have equal electronegativities, so the electrons are equally attracted to both nuclei

Question 7

Are covalent bonds with atoms of similar electronegativities polar?

Answer 7

Some elements, such as carbon and hydrogen, have similar electronegativities are essentially non-polar

Question 8

What happens in a polar bond?

Answer 8

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

Question 9

What is a dipole?

Answer 9

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

Question 10

What does a greater difference in electronegativity mean in terms of a polar bond?

Answer 10

The greater the difference in electronegativity between the atoms, the more polar the bond

Question 11

Can a whole molecule be polar?

Answer 11

If you have a molecule that contains polar bonds, you can end up with an uneven distribution of charge across the whole molecule. When this happens the molecule is polar

Question 12

Are all molecules that contain polar bonds, polar?

Answer 12

Not all molecules that contain polar bonds are polar. If the polar bonds are arranged symmetrically in the molecule, then the charges cancel out and the is no permanent dipole

Question 13

Polar molecules have permanent dipole-dipole forces, explain

Answer 13

In a substance made up of molecules that have permanent dipoles, there will be weak electrostatic forces of attraction between the S+ and S- charges on neighbouring molecules

Question 14

Are intermolecular forces strong or weak?

Answer 14

Very weak

Question 15

What are intermolecular forces?

Answer 15

Intermolecular forces are forces between molecules

Question 16

Name three types of intermolecular forces

Answer 16

1. van der Waals (induced dipole-dipole)
2. Permanent dipole-dipole
3. Hydrogen bonding

Question 17

Intermolecular forces affect what properties of a compound?

Answer 17

The physical properties

Question 18

Van der Waals forces cause all atoms to be attracted to each other. Explain

Answer 18

Electrons in charge clouds are always moving really quickly. At any moment, the electrons in an atom are likely to be more at one side than the other. At this moment, the atom has a temporary dipole. This dipole causes another temporary dipole and the opposite direction of the neighbouring atom. The two dipoles are attracted to each other. The second dipole causes another temporary dipole, and so on. Because the electrons are constantly moving, the dipoles are being created and destroyed all the time. Even though the dipoles keep changing, the overall effect is for the atoms to be attracted to each other.

Question 19

What can van der Waals forces hold molecules in?

Answer 19

A lattice

Question 20

Explain why Iodine is solid at rooms temperature

Answer 20

Iodine atoms are held together in pairs by strong covalent bonds to form I2 molecules. But the molecules are held together in a molecular lattice arrangement by weak van der Waals forces

Question 21

What increases the strength of van der Waals forces?

Answer 21

1. Larger molecules have larger electron clouds, meaning stronger van der Waals forces
2. Long, straight molecules can lie closer together than branched ones - the closer two molecules can get, the stronger the van der Waals forces

Question 22

How do van der Waals forces affect boiling points?

Answer 22

Stronger van der Waals forces mean higher boiling points. This is because you need more energy to overcome the intermolecular forces so that particles can escape from the liquid surface

Question 23

What is the strongest intermolecular force?

Answer 23

Hydrogen bonding

Question 24

When does hydrogen bonding happen?

Answer 24

Hydrogen bonding only happens when hydrogen bonding is covalently bonded to flourine, nitrogen or oxygen

Question 25

Hydrogen bonding has a huge effect on what?

Answer 25

The properties of substances

Question 26

How does hydrogen bonding affect the melting and boiling points of substances?

Answer 26

Substances with hydrogen bonding have higher melting and boiling points because of the extra energy to break the hydrogen bonds

Question 27

What happens when water forms ice?

Answer 27

When water cools to form ice, the molecules make more hydrogen bonds and arrange themselves into regular lattice structure. In this structure the water molecules are further apart on average than molecules in liquid water - this makes ice less dense than water