Chemistry 12: Solution Equilibrium Flashcards Preview

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Flashcards in Chemistry 12: Solution Equilibrium Deck (10):
1

What is a solution equilibrium?

an equilibrium involving saturated solutions of slightly soluble ionic compounds
-low solubility (precipitates)

-equilibrium between solid and dissociated ions

The rate of dissolving = rate of precipitation
*undissolved solute enter the solute = dissolved solute precipitates from the solution

*PHYSICAL SYSTEM (no chemical changes)
-->therefore can say dissolving

Macroscopic constant, lots going on microscopically

-number of ions in solution and number of crystals undissolved (precipitate) is constant
-temperature change will offset and shift the equilibrium (change Ksp value)

2

What is the solubility of a substance for a given temperature?

The concentration of the ions in solution is the solubility of the substance for a given temperature

3

How to write a solution/solubility equilibrium equation?

The equation is the dissociated equation of a solid
-subscripts in solid compound became the coefficient in the ions
-include states and charges
-must balance (charges in ions)

ex: Ag2S(s) 2Ag+(ag) + S^2-(aq)

4

How to write a Ksp expression?

Solubility Product Constant (Ksp): product of the ion concentrations present

-must be in concentration mol/L

-do not include solid solute (concentration is constant)

-coefficient in balanced equation become the power that the ion concentration is raised to

*No units for Ksp
**Ksp is for a solute/compound ex: the Ksp for AgBr

5

What two equations do you always need for solution equilibrium problems/calculations?

Solution Equilibrium Equations & Ksp expression

6

Ksp values and the relationship to solubility

All compounds studied have a low solubility because they saturate quickly in pure water
-Therefore must now rank the compounds with same mole ratio

Low Ksp value = low solubility substance (low ion concentrations, meaning more precipitate and fewer ions dissolved in the solution)

High Ksp value = higher solubility substance

7

Calculate Ksp from Ion concentrations

Solubility (concentration) of a solid compound (solute that dissociates) is stoichiometrically related to the ion concentrations

1) write solution equilibrium equation
2) identify mole ratio -->individual concentrations
3) write Ksp expression
4) sub in ion concentrations

**differentiate between solubility and ion concentrations by looking at the charges (ions)

8

Calculating Ion Concentrations/Solubility from Ksp

1) write solution equilibrium equation
2) identify mole ratio
3) Write Ksp expression, using 'x' for individual ion concentrations
4) solve for x (concentration)
5) concluding statement with units (mol/L)

1:1 Ksp = x^2
1:2 Ksp = 4x^3
1:3 Ksp = 27x^4

9

Solubility Conversions
between mol/L and g/L, g/100ml, g/ml etc.

*Ksp must have concentrations and solubility in Mol/L

Beginning of Ksp problem:
If given solubility/ion concentration in g/L -->want mol/L
**mathematically solve by cancelling units

g/L divide by mm (xmol/g) = mol/L

End of Ksp problem:
Solubility/ion concentration is in mol/L, but required in g/L

mol/L x mm (g/mol) --> g/L

Required in g/100 ml
g/1000mL divide by 10 top and bottom--> xg/100ml

10

The common ion effect

A reduction in the solubility of an ionic compound due to the presence of a common ion

When a SOLUBLE substance with a COMMON ION is added to a saturated solution at equilibrium, the ion concentration will increase and cause the system to shift reverse. This results in more solid forming (precipitating), leading to less solid dissolving and a lower solubility than in pure water.

**If there is no common ion, then there is no common ion effect (solubility same as in pure water).
***If the concentration of the common ion substance increases, there is even lower solubility due to increased common ion effect

calculations:
-need I(on)CE table
-can eliminate the original concentration for the ion with the common ion because the value is so small (due to little dissociation from low solubility substance) in comparison to the common ion concentration.