The Properties Of Mixtures

Question 1

What is the partial molar property?

Answer 1

It is the contribution - per mole - that a substance makes to an overall property of a mixture

Question 2

What is the relationship between the chemical potential of a liquid and its vapour at equilibrium?

Answer 2

They are the same at equilibrium.

A system is at equilibrium when the chemical potential of each substance has the same value in every phase in which it occurs

Question 3

What is chemical potential?

Answer 3

It is the partial molar Gibbs energy, this is an indication of the potential of a substance to be chemically active.

Question 4

How does a solution deviate from an ideal solution?

Answer 4

By increasing the concentration of solute

Question 5

What is the chemical potential for both solvent and solute (by definition) at all concentrations for a real solution?

Answer 5

µj = µj˚ + RT lnaj

Question 6

What is Raoult’s law?

Answer 6

The partial vapour pressure of a substance in a liquid mixture is proportional to its mole fraction in the mixture and its vapour pressure when pure.

Pj = Xj Pj˚

Pj = partial pressure in solution
Xj = mole fraction
Pj˚ = vapour pressure of pure substance

Question 7

What is vapour pressure?

Answer 7

Vapour pressure of a pure solvent represents the tendency of the system and its surroundings to reach a higher entropy.

The entropy of the solution is higher than that of the pure solvent, so it has a lower tendency to acquire an even higher entropy.

The vapour pressure of the solvent in a solution is lower than that of the pure solvent.

Question 8

What are ideal-dilute solutions?

Answer 8

Ideal solutions are those that are dilute enough for the solute to obey Henry’s law.

Question 9

What is Henry’s law?

Answer 9

The vapour pressure of a volatile solute B is proportional to its mole fraction in a solution

Pb = xb Kh

Or Pb = Kh [J]

Where Kh is Henry’s law constant, this constant is characteristic of the solute.

Question 10

What are ideal solutions?

Answer 10

An ideal solution comprises a pair or more of liquids that obey Raoult’s law throughout the composition range.

Question 11

What happens as Xb gets closer to 1 (almost pure)?

Answer 11

It behaves in accordance with Raoult’s law and a slope of Pj˚.

Question 12

How are partial pressure of a gas and chemical potential in a mixture related?

Answer 12

µj = µj˚ + RT ln Pj/P˚

This equation shows us that the higher the partial pressure of a gas, the higher its chemical potential is.

Question 13

What equation gives the chemical potential of pure A?

Answer 13

µa = µa˚+ RT ln xa

Question 14

What happens to the chemical potential of A when solvent A becomes more pure?

Answer 14

µa = µa˚+ RT ln xa

Because xa < 1, so lnxa < 0, hence making the chemical potential of a solvent lower in a solution than when it is pure.

In other words, the more pure a solvent, the more chemically active it is.

Question 15

How does enthalpy change when ideal gases are mixed at constant temperature and pressure? Why?

Answer 15

There is no change as ideal gases ignore intermolecular interactions

Because ∆U = 0 as a result of mixing, hence with ∆H = ∆U +p(∆V), p(∆V) = 0 as no change in volume.

Therefore ∆H=∆U, which = 0.

Question 16

How does entropy change when ideal gases are mixed at constant temperature and pressure?

Answer 16

There is an increase in entropy as the mixed gas is more disordered than the unmixed gases

Question 17

What happens to internal energy as a result of mixing when two ideal gases are mixed?

Answer 17

No change in internal energy due to mixing of gases

Question 18

What is ∆G for spontaneous mixing of ideal gases at constant temperature and pressure?

Answer 18

∆G = nRT {Xa lnXa + Xb lnXb}

Question 19

How are ∆G and ∆S related in the spontaneous mixing of ideal gases at constant temperature and pressure?

Answer 19

∆G = -T∆S

Question 20

What does ∆H = 0 mean for ideal solutions?

Answer 20

Although there are interactions between molecules in an ideal solution, the average intermolecular interactions are the same, and so there change is zero

Question 21

What effect does adding an ideal solute to an ideal solute have on enthalpy of the solution?

Answer 21

It has no effect on the enthalpy of the solution

Question 22

How does the addition of a solute change the physical properties of a solution?

Answer 22

Lowers the vapour pressure of the solution

Raise the boiling point

Lowers the melting/freezing point

Gives rise to osmotic pressure

Question 23

What are the physical changes due to in a solution when a solute is added to a solvent?

Answer 23

The changes in physical properties are due to changes in the disorder in the solvent

Question 24

What are the changes in physical properties of a solution dependent on?

Answer 24

Only dependent on the number of solute particles present, not their chemical identity.

Question 25

What are colligative properties?

Answer 25

Colligative properties are the physical changes that result from adding solute to a solvent

Question 26

What are the changes of the boiling and freezing point proportional to?

Answer 26

They are both proportional to the molarity, bB, of the solute - with Kb and Kf being the boiling/freezing point constant.

∆Tb = Kb bB

∆Tf = Kf bB

Question 27

What two assumptions are made when investigating colligative properties (boiling/freezing points)?

Answer 27

Question 28

Where does colligative properties originate from?

Answer 28

They originate from the lowering of chemical potential of the solvent by the presence of solute

Question 29

What is the ‘tie line’ in phase diagrams?

Answer 29

The tie line is the equilibrium of the solution and its vapour at constant temperature

Question 30

What is an azeotrope? Why are they significant?

Answer 30

It is where the vapour of a boiling mixture has an identical composition to that of the liquid mixture (azeotrope).

Mixtures that are azeotropes cannot be separated by distillation.

Question 31

What do favourable intermolecular forces mean for boiling point?

Answer 31

High boiling point