4.4: Intermolecular forces

Question 1

Intermolecular forces meaning

Answer 1

Forces that exist between molecules

Question 2

3 types of intermolecular forces

Answer 2

London Dispersion forces, dipole-dipole forces, hydrogen bonding

Question 3

London (dispersion) forces

Answer 3

At some instant, more of the electron cloud happens to be at one end of the molecule; the molecule has an instantaneous dipole (eg: Cl2-Cl2)

Question 4

Dipole-dipole forces

Answer 4

Dipole-dipole forces are attractive forces between the positive end of one polar molecule and the negative end of another polar molecule. (HCl)

Question 5

Hydrogen bond

Answer 5

An electrostatic force of attraction between a hydrogen atom which must be bonded to a highly electronegative element that is nitrogen (N), fluorine (F) and oxygen (O).(eg: H2O)

Question 6

Strength of intermolecular bonds (weakest to strongest)

Answer 6

LDF->Dipole-dipole forces->Hydrogen bonding

Question 7

Why is ice less dense than water at atmospheric pressure?

Answer 7

Water is at its densest at 4ºC and becomes less dense as that water molecules begin into the open, hexagonal structure present in ice crystals at its freezing point.

Question 8

Meaning of intramolecular forces

Answer 8

The bond within the molecule

Question 9

Van der Waal’s forces

Answer 9

LDF and dipole-dipole forces

Question 10

What do the intermolecular forces between molecules tell us?

Answer 10

The physical properties of the molecules (ie: boiling point, melting point, and solubility of a substance)

Question 11

Temporary dipole

Answer 11

Caused by changes of electron density within an atom or molecule, giving it a slight partial positive or negative charge

Question 12

Induced dipole

Answer 12

When a molecule with a temporary dipole induces a neighbouring dipole

Question 13

What do the strength of LDF depend on?

Answer 13

1. polarisability of the molecule (increases with increasing molar mass)
2. Surface area of the molecule

Question 14

Put these molecules in terms of lowest boiling point to highest

Answer 14

Br2, I2, Cl2, F2

Question 15

Which molecules and atoms have LDF?

Answer 15

All

Question 16

How do you know if a molecule is dipole-dipole?

Answer 16

Only exists between polar molecules that have a permanent dipole due difference in electronegativity, giving it a partial negative/positive charge (eg: HCl)

Question 17

Hydrogen bonding occurs between

Answer 17

Molecules that have an electronegative nitrogen, oxygen, or fluorine atom directly bonded to a hydrogen atom

Question 18

The effects of a hydrogen bond on boiling points (give examples)

Answer 18

Higher boiling points (Eg: H2O, HF and NH3 all have higher boiling points than other molecules with similar molar masses)

Question 19

Why is there a gradual increase from H2S to H2Te?

Answer 19

The molar mass increases, creating a stronger LDF

Question 20

Why do group 14 hydrides have a lower boiling point? (eg: CH4)

Answer 20

They are non-polar, so they only have LDF forces between them

Question 21

What does ‘like dissolves like’ mean? Give examples

Answer 21

Polar substances dissolve in polar solvents (eg: ethanol and water), non-polar substances dissolve in non-polar solvents (eg: carbon dioxide and hexane)

Question 22

boiling points of the halogens down a group?

Answer 22

The boiling points increase down a group because of the increasing London dispersion forces between the molecules

Question 23

why is the boiling point of PH3 is lower than that of NH3?

Answer 23

PH3 is not hydrogen bonded whereas NH3 is hydrogen bonded