Unit 8: Equilibrium
Content Outline: Solubility Equilibrium and Q (8.7)
I. Dynamic Equilibrium (“Dynamic” means “constant movement”; “equilibrium” means “state of being equal”)
A. This is a “state” where opposite reactions/processes are occurring at the same rate and at the same time in the same space.
B. Equilibrium is affected by changes in concentrations, pressure, and temperature.
1. These changes will cause the re-established equilibrium to favor the reactants or products.
II. Solubility Product Constant (KSP)
A. It is the same equation as for all other K’s. This equation involves insoluble salts (solids), so the reactant is not a part of the equilibrium expression.
KSP = [Products] = [Products] = [Products]
[Reactants] 1
B. As the pure solids, pure liquids, and water would not be included in the calculation and we are only looking at products that are dissolved in solution, we can think of it as K = [Products] which are almost always ions.
For example, H2O(l) + AgCl(s) Û [Ag+](aq) + [Cl-](aq)
As water is pure and AgCl is too, they are left out of the calculation…so [reactants] equals 1.
So then KSP = [Ag+](aq) x [Cl-](aq)
C. It is important not to confuse the solubility product constant (KSP) with the term solubility.
1. Solubility is the amount of a solid required to create a saturated solution with a defined amount of solvent, such as 100mL or 1L.
2. KSP is an equilibrium representation of ions in solution and a specified temperature, usually 25OC.
D. KSP is used mostly with solids that generally have very small ionizations in solvent.
1. A table of KSP is very helpful for quick calculations. Remember it is temperature dependent, so most times the table is set at 25OC.
III. Reaction Quotient (QSP)
A. This mathematical calculation is useful in predicting if a precipitate will form when adding two solutions.
B. QSP is calculated just like KSP.
QSP = [Products] See above.
C. Comparing QSP and KSP allows us to predict if a precipitate will form in the mixing of solutions.
1. If KSP is greater that QSP, the system is not at equilibrium. It is not saturated and no precipitant will form.
a. More Ions can be added to the solution.
2. If QSP = KSP, the system is at equilibrium. It is saturated.
3. If KSP is less than QSP, the solution is not at equilibrium. It is supersaturated. A precipitant will form.
a. Since the solution is supersaturated, a precipitate will form to relieve the stress, by reducing ion concentration.
D. KSP is dependent on temperature; but QSP is not…it can change.
IV. Solubility Tables
A. This are graphs usually that show a comparison between temperature and the amount in grams of a solute that will dissolve in a specified amount of solvent, such as 100 g of H2O.
1. As the solvent is measured in grams, these would be molality…not molarity.
V. Calculations with KSP and QSP
A. For example, Does a precipitate form when 0.10L of 8.0 x10-3 M Pb(NO3)2 is added to 0.40 L of 5.0 x 10-3 M Na2SO4? Do not get intimidated by the numbers or molecules, just think logically. You are
adding two different solutions together…so we have to account for a volume change.
B. Step 1: Write the balanced chemical reaction. It looks like a double replacement reaction.
Pb(NO3)2(aq) + Na2SO4(aq) Û PbSO4 + 2 NaNO3
Step 2: Look at your products and use your solubility rules (Unit 6 outline 5)
NaNO3 is soluble. (Rule #6… Yes on NaCates.)
PbSO4 is not very soluble. (Rule #5… Yes on sates, EXCEPT BLSCaHg…Sounds like “Blues
Catalog”)
You need to look at those again as they are needed for this section!
Step 3: Look up your KSP value for the molecule that is not soluble…PbSO4.
KSP for PbSO4 Û [Pb2+](aq) + [SO42-](aq) = 6.3 x 10-7 It is a very small amount…-7
Step 4: Add the quantities of fluid together.
0.10 L + 0.40 L = 0.50 L together when mixed
Step 5: Determine the number of moles Pb2+ there were initially, before mixing.
0.10 L X 8.0 x 10-3M = 8.0 x 10-4 moles of Pb2+
1 L
Remember Molarity (concentration) is mol/L… we did not have 1 L so we had to
make the conversion by multiplying the quantity we did have…0.10L.
Step 6: Repeat step 5 but for the other ion… SO42-.
0.40 L X 5.0 x 10-3 = 2.0 x 10-3moles
1 L
Step 7: Now take your initial concentrations of each ion and adjust the value for the mixed quantity.
[Pb2+] = 8.0 x 10-4 moles = 1.6 x 10-3 M
0.50 L
[SO42-] = 2.0 x 10-3 moles = 4.0 x 10-3 M
0.50 L
Step 8: Take your values for the mixed solution and calculate QSP.
QSP= (1.6 x 10-3 M) x (4.0 x 10-3 M)
= 6.4 x 10-6 (Remember, we are multiplying exponents…so we add them after the
multiplying)
Step 9: Now compare QSP and KSP.
6.4 x 10-6 is larger than 6.3 x 10-7 (so KSP< QSP ) so we get a precipitate to form.