lecture 5

Question 1

What causes the vapour phase?

Answer 1

in a liquid, most molecules are in the liquid phase but some have enough kinetic energy to escape into the vapour phase

when molecules escape into the vapour phase, this causes a vapour pressure

Question 2

Whatis the total vapor pressure equal to?

Answer 2

the partial vapour pressure of one component + the partial vapour pressure of the other component

Question 3

What is the partial vapour pressure equal to?

Answer 3

the vapour pressure of the pure component multipled by the mole fraction

Question 4

What are the attractive forces in an ideal solution?

Answer 4

attractive forces are the same between all molecules

i.e. forces between solute and solvent = forces between solute-solute and solvent-solvent

Question 5

What are solutions in reality like?

Answer 5

slight deviations so we see different interactions between different molecules

Question 6

What happens if the solvent and solute molecules are structurally similar?

Answer 6

the interactions between these will be similar to the interacions betwen the same molecules

Question 7

what are cohesive forces?

Answer 7

forces between molecules that are the same

Question 8

what are adhesive forces?

Answer 8

forces between molecules that are different

Question 9

What happens to the cohesive and adhesive forces when we have molecules which are structurally similar?

Answer 9

the cohesive and adhesive forces are similar

Question 10

WHat happens if the molecules are structurally different?

Answer 10

then the interactions will be different

Question 11

What is Raoult’s law more likely to hold true for?

Answer 11

solutions with low solute concentrations

Question 12

What is the vapour pressure of a solution like in reality and why?

Answer 12

we often have a greater vapour pressure positive deviation or lower negative deivation which occur due to cohesive and adhesive forces

Question 13

What causes a positive deviation?

Answer 13

More molecules in the vapour phase, due to more molecules having escaped into the vapour phase.

this indicates that cohesive forces are stronger than adhesive forces.

we see a positive deviation

Question 14

What causes a negative deviation?

Answer 14

If there is not much vapour pressure after mixing the two components together

Adhesive forces are stronger than cohesive forces, so not many molecules escape into the vapour phase.

This causes a negative deviation

Question 15

what are the limitations of Raoult’s law?

Answer 15

Doesn;t explain behaviour of solutes especially as solutes must be present in low concentrations
At low concentrations, solute is surrounded by solvent. So there is no opportunity for any cohesive forces
in reality we may not get that.
Raoults law Does not explain what the solute is doing at low concentrations

Question 16

What is Henry’s law used for?

Answer 16

cases where the solute is present at low concentrations.

Question 17

What is henry’s law?

Answer 17

partial pressure of the solute = K solute solute

talks about the partial pressure due to the solute.

Question 18

What are the differences between Henry’s law and Raoult’s law?

Answer 18

Henry’s law describes the behaviour of the solute at low concentrations for real liquid pairs
The K solute does NOT equal the partial pressure of the pure substance

Raoult’s law is restricted to the solvent.
The law applies over the entire concentration range

Question 19

What are colligative properties?

Answer 19

changes which depend on the number of things dissolved, not the nature of the things dissolved

Question 20

What are the important colligative properties?

Answer 20

lowering of vapour pressure
elevation of boiling point
lowering of freezing point
osmotic pressure

Question 21

How is vapour pressure lowered?

Answer 21

If the solute molecule is non volatile e.g. sugar or salt, it will not enter the vapour phase.
This means the overall vapour pressure of the system is lowered.

Lowering is proportional to the number of molecules as the number of molecules occupying the surface prevents the water molecules from escaping into the vapour phase.

Question 22

What is the partial pressure of the non volatile solute and why?

Answer 22

The partial vapour pressure of the non volatile solute is equal to the vapour pressure of the solution because the solute does not contribute anything, it just takes up space.

Question 23

What is the end result?

Answer 23

we cant measure vapour pressure easily, but it does allow the elevation of the boiling point

Question 24

What is boilin?

Answer 24

when vapour pressure = the atmospheric pressure

Question 25

What happens to the boiling time when we add a solute?

Answer 25

we have increased the boiling time as we have lowered the vapour pressure by adding a non volatile solute

Question 26

What happens to boiling when atmospheric pressure is decreased e.g. on Mt Everest?

Answer 26

Things will boil at a lower temperature

Question 27

What happens to the boiling point when you add solute at sea level?

Answer 27

You’ll need to go a bit above 100degrees for it to boil

Question 28

Why does boiling point rise when you add in a solute?

Answer 28

when you add in a non volatile solute it takes longer for the vapour pressure to reach the atmospheric pressure so a higher temperature will be required before the vapour pressure = atmospheric pressure

Question 29

How is boiling point elevation applicable to a pressure cooker?

Answer 29

inside the pressure cooker the pressure is higher so it cooks faster

Question 30

In the equation of elevation of BP, what is m?

Answer 30

Molality

Question 31

Where is the depression of freezing point practically used?

Answer 31

in black ice

Question 32

How can freezing point be reduced?

Answer 32

if salt is added to water it may freeze at -1 or -2 degrees as oppossed to 0 degrees

This means that by 0 degrees, it has melted.

Question 33

Are the constants Kb and Kf the same for boiling and freezing points

Answer 33

no

Question 34

Which property (boiling point or freezing point) has a larger effect?

Answer 34

freezing point because the constant number is bigger

Question 35

What is osmosis?

Answer 35

the passage of water from a low solute concentration to a high solute concentration

Question 36

What is created if the two regions are separated by a semipermeable membrane?

Answer 36

pressure is created which is separated by an osmotic pressure

Question 37

What is the van hoff equation?

Answer 37

pixV = nRT where
pi = osmotic pressure
n = number of moles
T = absolute temp in kelvin
V = volume in L

Question 38

WHat is interesting about the van hoff equation?

Answer 38

it has related osmotic pressure to a gas molecule occupying the same volume.

Question 39

What is C?

Answer 39

The concentration of the solute in moles/L i.e. molarity

Question 40

What happens to the osmotic pressure when you increase the concentration?

Answer 40

there is more osmotic pressure

Question 41

What is the morse equation?

Answer 41

pi = nRT

where instead of concentration, you put in molality.