Unit 1 - Intermolecular forces

Question 1

What are intermolecular forces?

Answer 1

Forces of attraction between molecules

Question 2

What is intramolecular bonding?

Answer 2

Bonding within molecules

Eg Covalent / Ionic / Metallic

Question 3

What is intermolecular bonding?

Answer 3

Forces of attraction between molecules

Eg Van Der Waals forces

Question 4

What are London dispersion forces?

Answer 4

London dispersion forces are forces of attraction that can operate between all atoms and molecules.

These forces are much weaker than all other types of bonding.

Question 5

How do London Dispersion Forces arise?

Answer 5

London Dispersion Forces are formed as a result of electrostatic attraction between temporary dipoles and induced dipoles caused by movement of electrons in atoms and molecules.

Question 6

What is overcome when liquids become gases?

Answer 6

Intermolecular forces of attraction

Van Der Waals forces

Question 7

What type of bonding is found in the noble gases?

Answer 7

Intermolecular forces of attraction

Weak London Dispersion Forces

Question 8

Why does sulfur have a higher boiling point than chlorine?

Answer 8

Sulfur is a larger molecule (S₈ vs Cl₂)

More electrons are present in a sulfur molecule than a chlorine molecule. London Dispersion Forces are stronger therefore there is greater attraction between molecules. Higher boiling point.

Question 9

Why does iodine have a higher boiling point than fluorine?

Answer 9

As you go down a group on the periodic table, atoms get larger so have more electrons.

This leads to larger temporary and induced dipoles being formed. Greater temporary dipoles mean stronger London Dispersion Forces. Higher boiling point.

Question 10

Why do the boiling points of the noble gases increase as you go down the group?

Answer 10

As you go down a group on the periodic table, atoms get larger so have more electron shells.

More shells mean more electrons. This leads to larger temporary and induced dipoles being formed. Greater temporary dipoles mean stronger London Dispersion Forces. Higher boiling point.

Question 11

Name the three different types of Van Der Waals forces?

Answer 11

London Dispersion Forces

Permanent dipole-permanent dipole interactions

Hydrogen bonding

Question 12

How do polar molecules arise?

Answer 12

A molecule is described as polar if it has a permanent dipole.

The spatial arrangement of polar covalent bonds can result in a molecule being polar.

Question 13

What evidence is there for intermolecular forces of attraction?

Answer 13

All molecular elements and compounds and monatomic elements condense and freeze at sufficiently low temperatures.

For this to occur, some attractive forces must exist between the molecules or discrete atoms.

Question 14

What are Permanent dipole-permanent dipole interactions?

Answer 14

Permanent dipole-permanent dipole interactions are additional electrostatic forces of attraction between polar molecules.

Permanent dipole-permanent dipole interactions are stronger than London dispersion forces for molecules with similar numbers of electrons.

Question 15

Is phosphine, PH₃, a polar or a non polar molecule?

Answer 15

Non polar

∆E between P and H is less than 0.5

Question 16

Is CH₃F a polar or a non polar molecule?

Answer 16

Polar

The molecule is tetrahedral. The molecule contains 3 non polar C-H bonds. The molecule contains one polar C-F bond. There is a temporary dipole ς⁻ on the F atom in the C-F bond.

Question 17

Is carbon dioxide (O=C=O) a polar or a non polar molecule?

Answer 17

Non Polar

The molecule is linear. The molecule contains 2 polar C=O bonds. There is a temporary dipole ς⁻ on each of the O atoms in the C=O bonds. Symmetry cancels out the opposing ς⁻ dipoles on the O=C=O molecule.

Question 18

Is H-H a polar or a non polar molecule?

Answer 18

Non polar

∆E between H atoms is 0

Question 19

Is H-Cl a polar or a non polar molecule?

Answer 19

Polar

Linear molecule. ∆E between more than 0.5. ς⁻ dipole on the Cl atom. ς⁺ dipole on the H atom.

Question 20

Is CH₂Cl₂ a polar or a non polar molecule?

Answer 20

Polar

The molecule is tetrahedral. The molecule contains 2 non polar C-H bonds. The molecule contains 2 polar C-Cl bonds. There is a temporary dipole ς⁻ on each of the Cl atoms in the C-Cl bonds.

Question 21

Is CH₄ a polar or a non polar molecule?

Answer 21

Non polar

∆E between C and H is less than 0.5

Question 22

Is NH₃ a polar or a non polar molecule?

Answer 22

Polar

The molecule is trigonal pyramidal. The molecule contains 3 polar N-H bonds. There is a temporary dipole ς⁺ on each of the H atoms in the N-H bonds. There is a temporary dipole ς- on the N atom in the N-H bonds.

Question 23

Is H₂O a polar or a non polar molecule?

Answer 23

Polar

The molecule is angular. The molecule contains 2 polar O-H bonds. There is a temporary dipole ς⁺ on each of the H atoms in the O-H bonds. There is a temporary dipole ς- on the O atom in the O-H bonds.

Question 24

What type of molecules have London Dispersion Forces as significant interactions?

Answer 24

Non polar molecules

Noble gases

Question 25

When comparing Van Der Waals forces, why should you compare molecules of a similar size?

Answer 25

As molecules get larger, the London Dispersion Forces become stronger.

Question 26

What is a polar molecule?

Answer 26

A molecule is described as polar if it has a permanent dipole.

The spatial arrangement of polar covalent bonds can result in a molecule being polar.

Question 27

What effect does a permanent dipole have on boiling point?

Answer 27

A permanent dipole will dramatically increase boiling point

A permanent dipole is always there and the molecules arrange themselves around the dipoles.

Question 28

What is hydrogen bonding?

Answer 28

Bonds consisting of a hydrogen atom bonded to an atom of a strongly electronegative element such as fluorine, oxygen or nitrogen are highly polar.

Hydrogen bonds are electrostatic forces of attraction between molecules that contain these highly polar bonds. A hydrogen bond is stronger than other forms of permanent dipole-permanent dipole interaction but weaker than a covalent bond.

Question 29

Name three molecules which exhibit hydrogen bonding

Answer 29

Ammonia

Water

Hydrogen Fluoride

Methanol

Ethanol

Question 30

Give an example which proves the strength of hydrogen bonding

Answer 30

The anomalously high boiling points of ammonia, water and hydrogen fluoride

(Graph in class notes)

Question 31

Clearly explain the differences in boiling point between methylpropane (-12°C), propanone (57°C) and propan-2-ol (82°C)

Answer 31

Different Van Der Waals forces between molecules

The molecules can be compared as they have similar molecular masses. Methylpropane has the lowest boiling point as only London Dispersion Forces between molecules. Propanone has the next highest boiling point as it has permanent dipole - permanent dipole interactions between molecules. Propan-2-ol has the highest boiling point as it exhibits hydrogen bonding between its molecules.

Question 32

What types of solutes are soluble in water?

Answer 32

Ionic compounds

Polar solutes

Question 33

What types of solutes dissolve in non polar solvents?

Answer 33

Non polar solutes

Question 34

Explain the process of hydration when an ionic compound dissolves

Answer 34

Water molecules are highly polar so are attracted to and surround the ions.

Question 35

Explain how Van Der Waals forces affect viscosity

Answer 35

The greater the Van Der Waals force, the more viscous the liquid will be.

Question 36

Why is ice less dense than water?

Answer 36

When water freezes, hydrogen bonding causes the molecules to arrange themselves in a lattice structure with hexagonal symmetry.

The molecules are further apart in the solid than they are when moving randomly in the liquid so less dense.

Question 37

Why is hydrogen bonding essential to life on Earth?

Answer 37

DNA has a double helix structure due to hydrogen bonding between base pairs.

The boiling point of water is estimated to be -60°C without hydrogen bonding so all water on the planet would be water vapour.