Chapter 4 - Intermolecular Forces

Question 1

What are intermolecular forces?

Answer 1

Intermolecular forces are forces(push or pull) which mediate interaction between molecules, including forces of attraction or repulsion which act between molecules and other types of neighboring particles

Question 2

Why is it important whether a molecule is polar or non-polar and the intermolecular forces it has on other molecules?

Answer 2

These are important in explaining the properties of substances

Question 3

What are the different types of intermolecular forces and explain each one?

Answer 3

Explanations will not be provided below as they are too long.
Different types of intermolecular forces:
*Ion dipole forces
*Ion-induced forces
*Dipole-dipole forces
*Induced dipole forces
*Dipole-induced-dipole forces

Question 4

Draw the graph with all the different types of intermolecular forces interacting with one another

Answer 4

Done?

Question 5

Name the types of intermolecular forces that are considered van der Waals’ forces

Answer 5

Dipole-dipole forces
Induced dipole forces
Dipole-induces-dipole forces

Question 6

What is different between intermolecular and intramolecular forces ? Explain with regard to the properties of each

Answer 6

Intermolecular forces:

• forces between molecules
• strength of bonds are relatively weak
• Large distance between each bond

Intramolecular forces:

• forces between atoms
• Strength of the bonds are strong
• short distance between the atoms

Question 7

What is solubility?

Answer 7

Solubility is the ability to be dissolved in water

Question 8

Name the intermolecular forces in order of strength from strongest to weakest

Answer 8

1. Dipole - dipole forces (hydrogen bonding dipole-dipole forces are stronger than dipole-dipole forces without hydrogen bonding)
2. Ion-dipole forces
   3&4: Depends on the chemicals involved, but it is ion-induced dipole forces and dipole-induced-dipole forces
3. Induced dipole forces (weakest) aka London forces/ Momentary dipole forces/dispersion forces

Question 9

What is the relation between a weak intermolecular force and surface tension, boiling points and evaporation

Answer 9

A weak intermolecular force will get evaporated quickly. It will also have a weak surface tension and a low boiling point

Question 10

What are the factors that affect boiling and melting points in a molecule? And how?

Answer 10

The boiling and melting point of a molecule is determined by its molecular structure and surface area.
How: As the electron density of a dipole increases, so does the intermolecular forces which would increase its boiling and melting points. The round the molecule, the less polar it can become

Question 11

What are alkanes?

Answer 11

Alkanes are organic compounds containing carbon and hydrogen bonded together. The carbon atoms link together to form chains of varying lengths

Question 12

How does boiling point and melting point change in different alkanes?

Answer 12

The more carbon atoms there are in an alkane, the greater the surface area and the greater it’s melting and boiling points. The melting and boiling point increases as the number of carbon atoms increase.

Question 13

What is the general formula for alkanes?

Answer 13

CnH2n+2

Question 14

What is the simplest alkane in terms of simplicity and what is its formula?

Answer 14

Methane. CH4

Question 15

Name 5 different alkanes and give their chemical formulas

Answer 15

Methane = CH4
Ethane = C2H6
Pentane= C5H12
Hexane= C6H14
Icosane= C20H42

Question 16

How does the phase of an alkane change as molecular mass increases or decreases?

Answer 16

When molecular mass is low(I.e. there are few carbon atoms), the alkane’s phase is gas as the intermolecular forces are weak. As this molecular mass increases (the more carbon atoms), the alkane’s phase starts to change from gas to liquid and even solid as the intermolecular forces get stronger

Question 17

How does the size of molecules impact the strength of its intermolecular forces?

Answer 17

As the size of a molecule increases, so does the intermolecular force

Question 18

What is viscosity?

Answer 18

Viscosity is the resistance to flow of a liquid

Question 19

What is the relationship between viscosity and intermolecular forces?

Answer 19

A substance with stronger substance Intermolecular forces will be more viscous than substances with weaker molecular forces

Question 20

What is the relationship between viscosity and molecular size?

Answer 20

The more viscous a substance is, the larger the molecular size

Question 21

What is density?

Answer 21

Density is mass per unit of volume

Question 22

What is the relationship between density and the number of molecules in a unit volume? Explain?

Answer 22

The more molecules there are in a unit volume, the higher the substance’s density. This is because the strong intermolecular forces in a solid(most dense phase) pull the molecules together which results in a higher density

Question 23

Explain thermal expansion

Answer 23

As substances are heated, the particles gain more kinetic energy and move around faster which causes the substance to expand

Question 24

Explain thermal conductivity is metals and covalent molecular compounds

Answer 24

In metals, there are some free, delocalized electrons which help transfer the heat energy through the metal. In covalent molecular compounds, there are no free, delocalized electrons which help transfer heat through the substance

Question 25

Explain why the melting point of oxygen(O2) is much lower than the melting point of Hydrogen Chloride(HCl)

Answer 25

Oxygen is non-polar and, and therefore, the intermolecular forces are induced dipole forces. Hydrogen chloride, however, is polar and has dipole-dipole forces. The intermolecular forces in hydrogen chloride is stronger than that of oxygen and since melting points increase as the intermolecular forces get stronger, oxygen has a melting point lower than Hydrogen Chloride

Question 26

Explain why the boiling point of F2 is much lower than the boiling point of NH3

Answer 26

Boiling point is directly proportional to intermolecular forces. F2 is non-polar and therefore has induced dipole forces. These are very weak intermolecular forces. On the other hand, ammonia is polar and has hydrogen bonding which are the strongest intermolecular force. Since ammonia has a stronger intermolecular force, it’s boiling point will be higher

Question 27

Explain why Sodium chloride is likely to dissolve in methanol(CH3OH)

Answer 27

Sodium chloride is an ionic compound and, therefore, has ionic bonds. Methanol is a polar molecule. As Sodium chloride is submerged in methanol, there are ion dipole forces. This intermolecular force cause the sodium cation to be attracted to the negative pole of methanol and the chlorine anions are attracted to the positive pole of methanol. This breaks the ionic bond between sodium and chlorine and causes it to dissolve in methanol

Question 28

What properties of water cause it to behave differently from other compounds?

Answer 28

The properties of water that cause it to behave differently from other compounds include: microscopic structure of water, the shape of the molecule, it’s polar nature and the intermolecular forces that hold water molecules together

Question 29

What type of bonds hold water molecules together?

Answer 29

Hydrogen bonds

Question 30

What are the unique properties of water and explain each one?

Answer 30

1.Specific heat:
Water has a high specific heat meaning that a lot of energy must be absorbed by water before it’s temperature changes. Usually increasing the temperature of a substance means that the particles will move more quickly, but before this can be done, the intermolecular forces have to be disrupted and because hydrogen bonding is involved in water, a lot of energy is required to break this bond before heating up the molecule

2. Absorption of infra-red radiation:
   Water is able to absorb infra-red radiation from the sun. As a result of this, the oceans and other water bodies act as heat reservoirs and are able to help moderate the Earth’s climate.
3. Melting and boiling point:
   The melting point of water is 0C and it’s boiling point is 100C. This large range is very important as water can exist as a liquid over a large range of temperature.
4. High heat of vaporization:
   The strength of the hydrogen bonds between water molecules means that it has a high heat of vaporization as a lot of energy is required to break the hydrogen bonds, before it can change phase into a gas.
5. Less dense solid phase:
   Another unusual property of water is that it’s solid phase is less dense than its liquid phase. While other materials contract when they solidify, water expands.

Question 31

What is specific heat?

Answer 31

Specific heat is the amount of heat energy that is needed to increase the temperature of a unit mass of a substance by 1 degree.

Question 32

What is heat of vaporization?

Answer 32

Heat of vaporization is the energy that is needed to change a given quantity of substance into a gas

Question 33

What is the heat of vaporization for water?

Answer 33

40.65 KJ•mol^-1