Flashcards in lecture 5 Deck (41)
What causes the vapour phase?
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
Whatis the total vapor pressure equal to?
the partial vapour pressure of one component + the partial vapour pressure of the other component
What is the partial vapour pressure equal to?
the vapour pressure of the pure component multipled by the mole fraction
What are the attractive forces in an ideal solution?
attractive forces are the same between all molecules
i.e. forces between solute and solvent = forces between solute-solute and solvent-solvent
What are solutions in reality like?
slight deviations so we see different interactions between different molecules
What happens if the solvent and solute molecules are structurally similar?
the interactions between these will be similar to the interacions betwen the same molecules
what are cohesive forces?
forces between molecules that are the same
what are adhesive forces?
forces between molecules that are different
What happens to the cohesive and adhesive forces when we have molecules which are structurally similar?
the cohesive and adhesive forces are similar
WHat happens if the molecules are structurally different?
then the interactions will be different
What is Raoult's law more likely to hold true for?
solutions with low solute concentrations
What is the vapour pressure of a solution like in reality and why?
we often have a greater vapour pressure positive deviation or lower negative deivation which occur due to cohesive and adhesive forces
What causes a positive deviation?
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
What causes a negative deviation?
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
what are the limitations of Raoult's law?
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
What is Henry's law used for?
cases where the solute is present at low concentrations.
What is henry's law?
partial pressure of the solute = K solute solute
talks about the partial pressure due to the solute.
What are the differences between Henry's law and Raoult's law?
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
What are colligative properties?
changes which depend on the number of things dissolved, not the nature of the things dissolved
What are the important colligative properties?
lowering of vapour pressure
elevation of boiling point
lowering of freezing point
How is vapour pressure lowered?
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.
What is the partial pressure of the non volatile solute and why?
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.
What is the end result?
we cant measure vapour pressure easily, but it does allow the elevation of the boiling point
What is boilin?
when vapour pressure = the atmospheric pressure
What happens to the boiling time when we add a solute?
we have increased the boiling time as we have lowered the vapour pressure by adding a non volatile solute
What happens to boiling when atmospheric pressure is decreased e.g. on Mt Everest?
Things will boil at a lower temperature
What happens to the boiling point when you add solute at sea level?
You'll need to go a bit above 100degrees for it to boil
Why does boiling point rise when you add in a solute?
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
How is boiling point elevation applicable to a pressure cooker?
inside the pressure cooker the pressure is higher so it cooks faster
In the equation of elevation of BP, what is m?
Where is the depression of freezing point practically used?
in black ice
How can freezing point be reduced?
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.
Are the constants Kb and Kf the same for boiling and freezing points
Which property (boiling point or freezing point) has a larger effect?
freezing point because the constant number is bigger
What is osmosis?
the passage of water from a low solute concentration to a high solute concentration
What is created if the two regions are separated by a semipermeable membrane?
pressure is created which is separated by an osmotic pressure
What is the van hoff equation?
pixV = nRT where
pi = osmotic pressure
n = number of moles
T = absolute temp in kelvin
V = volume in L
WHat is interesting about the van hoff equation?
it has related osmotic pressure to a gas molecule occupying the same volume.
What is C?
The concentration of the solute in moles/L i.e. molarity
What happens to the osmotic pressure when you increase the concentration?
there is more osmotic pressure