Lecture 4 Flashcards
What is a colligative property? What are some examples? What are the assumptions made for those examples?
it is a physical property that depends on the relative number of solute particles present but not their chemical identity.
Examples of such properties are:
1. Lowering of vapor pressure
2. The elevation of the boiling point
3. The depression of the freezing point
4. the osmotic pressure arising from the presence of a solute.
The assumptions made:
1. The solute is not volatile, so it contributes to the vapour
2. The solute doesn’t dissolve in the solid form of the solvent.
Note: these properties depend only on the number of solute particles present IN DILUTE SOLUTIONS
What is the relation between the colligative properties and the Chemical potential? How does this affect the b.p and f.p?
All of the colligative properties stem from the decrease of the liquid phase’s chemical potential due to the addition of the solute.
For an ideal solution with a solute present, the solvent’s chemical potential is represented via μ = μ* + RT ln xa, whereas the more solute is added the lower the value of Xa and the lower the values of the liquid solvent chemical potential.
Due to the solute not appearing in the solid or vapor phase of the solvent only the chemical potential of the liquid decreases, this causes the b.p to increase and the F.p to be depressed as illustrated by a chemical potential against temperature graph in notes on page 1.
What causes the decrease in chemical potential when a solute is added to a solvent?
The decrease stems from the increase of entropy in the solvent due to the addition of solute particles. This increase in randomness results in a weaker tendency for the solvent to release vapor, therefore decreasing the vapor pressure, which means the boiling point increases. Similarly, this addition of randomness causes opposition to freezing, therefore a lower temperature must be reached before equilibrium between solid and solution is achieved. Hence, the freezing point is lowered.
What is the derivation for the boiling point elevation?
Found in notes on pages 1-2.
What is the derivation for the freezing point depression?
Found in notes on page 3
What is the derivation of the relationship between the solubility and temperature? What is the basis that makes this derivation possible?
The basis that makes it possible is that even though solubility isn’t a colligative property when the solute is left in the solvent, it dissolves until it reaches saturation. Saturation is a state of equilibrium represented by the undissolved solid solute in a solution and the dissolved solute, and since in equilibrium, their potentials can equal each other.
The derivation is found in notes on pages 3-4.
What is osmosis? What is a semipermable membrane? What is osmotic pressure? What osmometry?
It is the spontaneous passage of pure solvent into a solution separated by a semi-permeable membrane, which is a membrane that only allows the passage of the solvent and not the solute.
Osmotic pressure is the pressure that must be applied to teh solution to stop the influx of solvent.
osmometry is an example of osmosis and it is used to determine the molar mass of macromolecules
What complications follow the derivation of the relation between osmotic pressure and the concentration of a solute?
We require a complicated system when it comes to deriving the relation between osmotic pressure and the concentration, since due to the inflow of the solvent into the solution the concentration of the solvent drops because of the solute molecules which causes complications to our calculations
Therefore we use a system with no flow and constant concentration, which is achieved with a semi-membrane that separates the solvent with pressure P applied to it (therefore it has its pure chemical potential) and a solution with pressure P + the osmotic pressure applied it, allowing for both systems to be at equilibrium therefore we can equate their chemical potentials and solve.
This shows that when the solution is dilute the extra pressure applied to it is proportional to the molar concentration of the solute in the solution.
The step-up is illustrated in notes on page 4.
what is the derivation of the relation between osmotic pressure and the concentration of a solute? (Van’t Hoff equation)
found in notes on pages 4-5
What is the osmotic viral expansion?
found in notes on page 5.
