Part 1 - Solutions, solubility and supersaturation

Question 1

Give an example of the solubility constant in simple dissolution (sugar in water)

Answer 1

K_s = [C12H22O11(aq)] / [C12H22O11(s)]

Question 2

Give an example of the solubility constant in dissolution with disassociation?

Answer 2

PbCl2 (s) -> Pb2+ 2Cl-

K_sp = [Pb2+][Cl-]^2

Question 3

Which regions do we have in a supersaturation diagram? What characterizes them?

Answer 3

Stable: solution is stable, no crystallization
Meta-stable: crystallization, but not spontaneous
Labile: spontaneous crystallization

Question 4

How is the supersaturation ratio defined thermodynamically?

Answer 4

S = Activity product / solubility product

Question 5

How does the supersaturation definition change when considering electrolyte solutions?

Answer 5

S = (Ionic activity product / solubility product)^ny

Question 6

What is the concentration based supersaturation?

Answer 6

S = c / c, where c is the concentration and c is the critical concentration where the solution is saturated.

Question 7

What is the relative supersaturation?

Answer 7

sigma = ∆c / c* = (c - c) / c = S - 1

Question 8

In the chemical industry, what type of supersaturation is mostly used?

Answer 8

The concentration based supersaturation.

Question 9

Give some reasons why the supersaturation doesn’t linearly increase as the concentration is increased.

Answer 9

• High concentration doesn’t necessarily mean that the free concentration is equally high.
• Liquid complexes can bind up active solute compounds.
• At very high concentrations, the precipitate may be amorphous, creating a temporary equilibrium with the solution, leaving a supersaturation corresponding to the solubility of the amorphous compound, which can be lower than the initial mixed concentration.

Question 10

What are the different crystals CaCO3 can form?

Answer 10

Vaterite, aragonite, calcite. Also amorphous form and two hydrated forms.

Question 11

What are the relative stability of the different forms of CaCO3?

Answer 11

Calcite > Aragonite > Vaterite > amorphous

Question 12

What is the abundance of polymorphs of CaCO3 determined by?

Answer 12

• Degree of supersaturation
• Temperature (high T prefers aragonite, intermediate vaterite and low calcite)
• Impurities / additives (divalent cations stabilize aragonite)