The Properties Of Mixtures Flashcards
What is the partial molar property?
It is the contribution - per mole - that a substance makes to an overall property of a mixture
What is the relationship between the chemical potential of a liquid and its vapour at equilibrium?
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
What is chemical potential?
It is the partial molar Gibbs energy, this is an indication of the potential of a substance to be chemically active.
How does a solution deviate from an ideal solution?
By increasing the concentration of solute
What is the chemical potential for both solvent and solute (by definition) at all concentrations for a real solution?
µj = µj˚ + RT lnaj
What is Raoult’s law?
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
What is vapour pressure?
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.
What are ideal-dilute solutions?
Ideal solutions are those that are dilute enough for the solute to obey Henry’s law.
What is Henry’s law?
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.
What are ideal solutions?
An ideal solution comprises a pair or more of liquids that obey Raoult’s law throughout the composition range.
What happens as Xb gets closer to 1 (almost pure)?
It behaves in accordance with Raoult’s law and a slope of Pj˚.
How are partial pressure of a gas and chemical potential in a mixture related?
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.
What equation gives the chemical potential of pure A?
µa = µa˚+ RT ln xa
What happens to the chemical potential of A when solvent A becomes more pure?
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.
How does enthalpy change when ideal gases are mixed at constant temperature and pressure? Why?
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.
How does entropy change when ideal gases are mixed at constant temperature and pressure?
There is an increase in entropy as the mixed gas is more disordered than the unmixed gases
What happens to internal energy as a result of mixing when two ideal gases are mixed?
No change in internal energy due to mixing of gases
What is ∆G for spontaneous mixing of ideal gases at constant temperature and pressure?
∆G = nRT {Xa lnXa + Xb lnXb}
How are ∆G and ∆S related in the spontaneous mixing of ideal gases at constant temperature and pressure?
∆G = -T∆S
What does ∆H = 0 mean for ideal solutions?
Although there are interactions between molecules in an ideal solution, the average intermolecular interactions are the same, and so there change is zero
What effect does adding an ideal solute to an ideal solute have on enthalpy of the solution?
It has no effect on the enthalpy of the solution
How does the addition of a solute change the physical properties of a solution?
Lowers the vapour pressure of the solution
Raise the boiling point
Lowers the melting/freezing point
Gives rise to osmotic pressure
What are the physical changes due to in a solution when a solute is added to a solvent?
The changes in physical properties are due to changes in the disorder in the solvent
What are the changes in physical properties of a solution dependent on?
Only dependent on the number of solute particles present, not their chemical identity.
What are colligative properties?
Colligative properties are the physical changes that result from adding solute to a solvent
What are the changes of the boiling and freezing point proportional to?
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
What two assumptions are made when investigating colligative properties (boiling/freezing points)?
Where does colligative properties originate from?
They originate from the lowering of chemical potential of the solvent by the presence of solute
What is the ‘tie line’ in phase diagrams?
The tie line is the equilibrium of the solution and its vapour at constant temperature
What is an azeotrope? Why are they significant?
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.
What do favourable intermolecular forces mean for boiling point?
High boiling point
