ions + solutions - colligative properties

Question 1

colligative properties definition

Answer 1

group of generic properties of the way water behaves when it has solutes, regardless of the specific type of solute

Question 2

what are the 4 colligative properties?

Answer 2

when water has solutes, the solution has:
- lower solvent vapour pressure
- lower freezing point
- higher boiling point
- greater osmotic pressure

Question 2

what are the 3 types of solution?

Answer 2

colligative (generic)
additive (e.g. mass)
constitutional (specific to solute)

Question 3

why are colligative properties different?

Answer 3

colligative properties are generic properties resulting from the dilution of the solvent by the solute, therefore for ideal solutions they only depend on the concentration of dissolved particles, not what type of particle is dissolved

Question 4

explain why the type of particle dissolved does not affect the colligative properties

Answer 4

this is explained through free energy
dissolution only happens when ΔG<0 for the reaction - consider enthalpy/entropy for the different states of water:
solid = very strongly bound and regularly ordered, lowest H and smallest S
liquid = less strongly bound and more disordered, H/S are inbetween solid + gas
gas = very weakly bound and very disordered, highest H and largest S

consider graphically, H = intercept and S = gradient, solid, liquid and gas are 3 separate lines - when the liquid and solid lines cross this is the temperature at which liquid has become more stable than solid, as Gliquid < Gsolid
when the liquid and gas lines cross is the temperature at which gas has become the most stable state, as Ggas < Gliquid
- this is true for pure water with no dissolved substances
now if a substance/salt is added only thr liquid line is affected as salt only dissolves in liquid, reaction is favourable as Gsolution < G liquid, and this changes the properties of the liquid state

Question 5

what is the vapour pressure of a salt?

Answer 5

0 vapour pressure, as it is a non volatile solid

Question 6

what is the vapour pressure of water in solution + link to colligative properties?

Answer 6

Pvap = P0*xw
where P0 = pure water vapour pressure
xw = mole fraction of water, which is now <1 as not 100% of the solution = water

therefore vapour pressure decreases as a result of solutes

Question 7

what is the impact of lower vapour pressure?

Answer 7

this means b.p. will have increased as this is the point at which the vapour pressure of the solvent = 1atm, if solvent has a lower vapour pressure, we need to rise to a higher temperature for this to be true/to reach boiling point

Question 8

chemical potential, μ definition

Answer 8

gibbs free energy per mole

Question 9

how does chemical potential μ link to ΔG?

Answer 9

making molecules from less stable phase X to more stable phase Y releases free energy, as ΔG<0
therefore - ΔG = μy - μx per mole

Question 10

what is the chemical potential of a solution/solvent at phase transition?

Answer 10

at phase transition the system is in equilibrium, meaning ΔG = 0 therefore μ are equal for the 2 phases

Question 11

what is ΔGmix in regards to μ?

Answer 11

ΔGmix = the change in μ between Gwater and Gsolution

Question 12

how does ΔGmix relate to temperature?

Answer 12

when ΔGvap between pure liquid/gas = ΔGmix, the gap between the increased boiling point and the initial boiling point = ΔT

Question 13

give the equation for μ of substance a for an ideal solution

Answer 13

μxa = μa,pure + RTlnxa
we can go further:
xwater = 1-xsolute
so lnxw = -xs in dilute conditions
meaning Δμwater = -RTxs

Question 14

ebullioscopy definition

Answer 14

observance/relating to the boiling of substances

Question 15

what is Kb + how is it found?

Answer 15

linking ΔHvap and ΔGvap, we find that
ΔT = (xsRTb^2)/(ΔHvap)
however mole fraction of solute can be tedius to find, so molality is used often, bs
ΔT = approx. Kbbs where Kb = (MwR*Tb^2)/Hvap
Kb = ebullioscopic constant

Question 16

how does ebullioscopic constant Kb describe the properties of a solution?

Answer 16

solvents, e.g. water, with very low Kb also have very high Hvap due to hydrogen bonds
wheras solvents with higher Kb that don’t have the anomalously high Hvap are much more sensitive for ebullioscopy

Question 17

how can Kb be used to find molality?

Answer 17

molality of a substance can be found by observing ΔT in a solvent with known Kb - the van’t hoff index must also be known

Question 18

cryoscopy definition

Answer 18

observance/relating to freezing of substances

Question 19

what is Kf + how is it found?

Answer 19

ΔT = approx. Kfbs where Kf = (MwR*Tf^2)/Hfus
Kf = cryoscopic constant

Question 20

give 1 real life application of the colligative property of freezing point

Answer 20

this is why roads are salted in the winter, as the dissolution of the salt lowers the freezing point making it harder for roads to freeze over

Question 21

how does Kb compare to Kf?

Answer 21

usually Kf > Kb due to the slope of μ

Question 22

what is osmotic pressure?

Answer 22

the measurable and observable pressure exerted as a result of the difference in concentration of 2 solutions separated by a semi-permeable membrane through which water can pass through far more easily than the solute molecules
this pressure can build up ahead of liquid on one side

Question 23

how can le chatelier be applied to osmotic pressure?

Answer 23

water molecules are free to move between solutions and therefore will move down the concentration gradient by osmosis to develop an equilibrium of movement + make both solutions have equal concentration

Question 24

using ideas about chemical equilibrium, explain how osmotic pressure relates to chemical potential + how it is affected by solute concentration

Answer 24

solvent/water molecules are at a constant concentration in dilute solution at lower pressure, and in the concentrated solution at higher pressure - moving dn moles of solvent with volume dv to the higher pressure side gives a free energy change (Δμ dn) but also does work (P dv - these cancel out at equilibirum because the net free energy change must be 0 - this tells us the osmotic pressure is exactly enough to balance the chemical potential difference therefore Δμ depends on solute concentration, so osmotic pressure increases with concentration

Question 25

give the van't hoff expression for osmotic pressure

Answer 25

Π = icRT where Π = osmotic pressure i = van't hoff index c = molar concentration of solute (therefore ic = total concentration of dissolved ions)

Question 26

how does the van't hoff expression behave similarly to the ideal gas law?

Answer 26

just like pv=nRT => p = (n/v)*RT Π = c*RT

Question 27

osmometry definition

Answer 27

a method to find the Mr of any unknown substance using ideas of osmotic pressure, 'measuring the pushing' this is done by setting up a cell for osmometry, using polymers for the semi-permeable membrane

Question 28

why are larger solutes easier to use in osmometry?

Answer 28

the larger the solute molecules the easier it is to make a semi-permeable membrane for it - this also applies to individually solvated ions as the solvation shell of water molecules gives ions a larger hydrodynamic radius

Question 29

reverse osmosis definition

Answer 29

a method to produce very pure water

Question 30

how does reverse osmosis work?

Answer 30

a mechanical pressure which is greater than the osmotic pressure is exerted on the more concentrated side, this pushes the position of equilibrium to the other side, driving solvent to the less dilute side - this technique is used in desalinating technology to convert seawater to drinkable water