Equilibrium Homework
· I can write an equilibrium expression given a chemical equation
· I can identify the strength of an acid or base by its equilibrium constant, Ka or Kb
· I can identify how soluble a substance is by its equilibrium constant, Ksp
· I can identify how an reaction will shift if reactants, products, a catalyst, or if temperature /pressure is changed
· I can solve for the pH for a weak acid or weak base solution given its concentration and Ka or Kb value
· I can solve for the molar solubility of a salt given its chemical formula and dissociation equation and Ksp value and vice versa
1. Write the equilibrium expression for the following chemical reactions
SAMPLE: 2 NO2 (g) ßà N2O4 (g)
SOLVING: The general equilibrium expression is written as follows:
aA + bB ßà cC + dD Then, = Kequilibrium
So, in this case Kequilibrium = would be the equilibrium expression
NOTE: Only gases and aqueous concentrations are included because their concentrations can change. The concentration of a solid or liquid does not change appreciably.
a. The production of ammonia from nitrogen and hydrogen gases.
b. The thermal decomposition of calcium carbonate.
c. The oxidation-reduction reaction occurring between iron (III) chloride and tin (II) chloride.
d. The replacement of silver ions by copper.
e. The interaction of acetic acid with water.
f. The reaction of the weak base ammonia in water
NH3 (aq) + H2O (l) NH4+ (aq) + OH- (aq)
g. The dissociation of table salt in water
NaCl (s) à Na+ (aq) + Cl- (aq)
h. The dissociation of lead (II) chloride in water
PbCl2 (s) à Pb2+ (aq) + 2 Cl- (aq)
i. The dissociation of sodium phopshate in water
Na3PO4 (s) à 3 Na+ (aq) + PO43- (aq)
j. If the equilibrium constant, K, is larger than 1, are the products or reactants favored?
k. If the equilibrium constant, K, is smaller than 1, are the products or reactants favored?
Brief Ka Table of Acids
Acid / Ka / Acid / KaHydrochloric
HCl / 1 x 107 / Carbonic
H2CO3 / 4.4 x 10-7
Phosphoric
H3PO4 / 7.5 x 10-3 / Dihydrogen phosphate
H2PO4-1 / 6.2 x 10-8
Hydrofluoric
HF / 6.6 x 10-4 / Ammonium
NH4+ / 5.6 x 10-10
Acetic
HC2H3O2 / 1.8 x 10-5 / Hydrocyanic
HCN / 4.9 x 10-10
2. Use the given Ka table (above) to answer the following questions
a. What does the value of Ka tell us about the strength of an acid? What does a large value and small value mean to us?
b. Of the listed acids which is the strongest acid?
c. Write the equilibrium expression for HF in water.
d. If you have a 1.0 M HF solution, what would be the concentration of hydronium? Use your equilibrium expression to solve for hydronium. For more on how to do this, consult the attached ICE table.
e. What would be the pH of a 1.0 M HF solution?
f. Write the equilibrium expression for Ammonium in water.
g. If you have a 1.0 M NH4+ solution, what would be the concentration of hydronium? Use your equilibrium expression to solve for hydronium.
h. What would be the pH of a 1.0 M NH4+ solution?
i. Write the equilibrium expression for Phosphoric acid in water.
j. If you have a 2.0 M H3PO4 solution, what would be the concentration of hydronium?
k. What would be the pH of a 2.0 M H3PO4 solution?
l. Ammonia, NH3, is a weak base. Write the equilibrium expression for ammonia in water.
m. If you have a 0.50 M NH3 solution, what would be the concentration of hydroxide?
n. What would be the pH of a 0.50 M NH3 solution? Reminder to subtract your pOH from 14.
Brief Ksp Table of Salts
Salt / Ksp / Dissociates IntoAgCl / 1.8 x 10-10 / AgCl (s) ßà Ag+ (aq) + Cl- (aq)
Ag2CO3 / 8.4 x 10-12 / Ag2CO3 (s) ßà 2 Ag+ (aq) + CO3-2 (aq)
Al(OH)3 / 1.3 x 10-33 / Al(OH)3 (s) ßà Al+3 (aq) + 3 OH- (aq)
CaCO3 / 2.8 x 10-9 / CaCO3 (s) ßà Ca+2 (aq) + CO3-2 (aq)
3. Use the above Ksp table to answer the following questions.
a. Which of the salts is the most soluble in water? Which of the salts is most insoluble in water?
b. Write the equilibrium expression for AgCl (s) in water.
c. Solve for the molar solubility of AgCl (s) in water.
d. How many grams of AgCl (s) are needed to saturate 1.0 L of water?
e. Write the equilibrium expression for Al(OH)3 (s) in water.
f. Solve for the molar solubility of Al(OH)3 (s) in water.
g. How many grams of Al(OH)3 (s) are needed to saturate 0.50 L of water?
Not a question, but a note: When water treatment centers want to ‘soften’ water, they make the solution very basic. They do this because the hydoxide ion is not very soluble with aluminum, calcium, and magnesium. The salt forms and is easily separated from the solution. If the solution becomes acidic, the ions become very soluble since the acid will react to eliminate the hydroxide.
h. Write the equilibrium expression for Ag2CO3 (s) in water.
i. Solve for the molar solubility of Ag2CO3 (s) in water.
j. How many grams of Ag2CO3 (s) are needed to saturated 1.25 L of water?
k. Copper (I) Bromide is dissolved into solution until the solution is saturated. It dissociates as follows: CuBr (s) ßà Cu+1 (aq) + Br-1 (aq). If the concentration of the copper ion is found to be 7.9 x 10-5 M, what is the Ksp value for Copper (I) Bromide?
l. Iron (II) Sulfide is dissolved into solution until the solution is saturated. It dissociates as follows: Fe2S (s) ßà 2 Fe+1 (aq) + S2- (aq). If the concentration of the sulfide ion is found to be 2.0 x 10-6 M, what is the Ksp value for Iron (II) Sulfide?
4. Coral reefs are composed of Calcium Carbonate (see chart above). The solubility of calcium carbonate changes quite a bit with changing pH (see chart below). This is because Calcium Carbonate in acidic solution reacts to form soluble Calcium Bicarbonate as follows: CaCO3 (s) + H2O (l) + CO2 (g) ßà Ca(HCO3)2 (aq). Answer the following questions
a. If more carbon dioxide dissolves into the ocean, does the reaction above (in paragraph) shift left or right?
b. How does this shift change the amount of solid calcium carbonate present?
c. Explain how the pH of the ocean influences the size and amount of coral reefs around the world.
Solubility of Calcium Carbonate at different pH’s
pH of 2 = 170 Molar solubility
pH of 4 = 1.70 Molar solubility
pH of 6 = 0.020 Molar Solubility
pH of 8 = 0.0011 Molar Solubility
pH of 10 = 1.4 x 10-4 Molar Solubility
pH of 12 = 7.8 x 10-5 Molar Solubility
5. Consider the following equilibrium reaction and how it will shift if the following happen.
[Cu(H2O)4]+2 (aq) + 4 NH3 (aq) [Cu(NH3)4]+2 (aq) + 4 H2O (l)
a. Adding more [Cu(H2O)4]+2 to solution.
b. Removing NH3 from solution.
c. Adding water
d. Removing [Cu(NH3)4]+2 from solution.
6. Consider the following equilibrium expression and how it will shift if the following happen. The value of K for the equilibrium is 8.0
N2 (g) + 3 H2 (g) ßà 2 NH3 (g)
a. Addition of hydrogen.
b. Removal of nitrogen.
c. Increasing pressure
d. If the concentration of Nitrogen is 0.20 M, Hydrogen is 0.30 M, and Ammonia is 0.10, is the reaction at equlibrium? If not, which way will it shift?
e. If the concentration of Nitrogen is 0.40 M, Hydrogen is 0.60 M, and Ammonia is 0.70, is the reaction at equlibrium? If not, which way will it shift?
f. If the concentration of Nitrogen is 0.60 M, Hydrogen is 0.60 M, and Ammonia is 0.40, is the reaction at equlibrium? If not, which way will it shift?
7. Consider an unopened can of pop and the following equilibrium expression and how it will shift if the following happens.
H3O+ (aq)+ HCO3- (aq)2 H2O (l) + CO2 (g) + Energy
a. Adding Hydronium H3O+
b. Increasing Pressure
c. Increasing Temperature