Chemical Equilibrium

1
Chapter 14

• Chemical Equilibrium

2
14.1 Equilibrium Constant, Keq

• Objective
• To write the equilibrium constant expression for
  a chemical reaction.

3
Reversible Reactions and Equilibrium

• Reversible Reaction A chemical reactions in
  which products re-form the original reactants.
• Arrows that point in opposite directions are used
  to indicate a reaction is reversible.
• Example H2(g) I2(g)
  2HI(g)
• Chemical Equilibrium A state of balance in which
  the rate of a forward reaction equals the rate of
  the reverse reactions and the concentrations of
  products and reactants remain unchanged.

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Equilibrium Constant, Keq

• Equilibrium Constant, Keq a number that relates
  that concentrations of starting materials and
  products of a reversible chemical reaction to one
  another at a given temperature.
• aA bB ? cC dD

coefficient
concentration
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Writing an Equilibrium Constant Expression

• Step 1 Balance the chemical equation.
• Step 2 Set up your Keq expression with the
  products on the top of a fraction and the
  reactants on the bottom of a fraction.
• Step 3 Raise each substance's concentration to
  the power equal to the substances coefficient in
  the balanced equation.
• Note Solids (s) and pure liquids (l) are not
  used in the expression because their
  concentrations do not change.

6
Example

• Write the equilibrium constant expression for the
  following reaction
• CaCO3(s) CO2(aq) H2O(l) Ca2(aq)
  2HCO3-(aq)

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Practice

• Write the equilibrium constant expression for the
  following chemical reactions at equilibrium
  (dont forget to balance the equation)
• 1.) H2CO3(aq) H2O(l) HCO3-(aq)
  H3O (aq)
• 2.) COCl2 (g) CO(g) Cl2 (g)
• 3.) CO(g) C(s) CO2 (g)

8
Answers

• 1.) H2CO3(aq) H2O(l) HCO3-(aq)
  H3O (aq)
• 2.) COCl2 (g) CO(g) Cl2 (g)
• 3.) 2CO(g) C(s) CO2 (g)

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14.1 Equilibrium Constant, Keq

• Objective
• To calculate the equilibrium constant.

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What does the Keq tell us?

• Keq lt 1 Favors Reactants
• Keq 1 Same amount of Reactants and Products
• Keq gt 1 Favors Products
• Practice Determine if the following Keq values
  favor the reactants, products, or neither.
• 1.) Keq 0.02 2.) Keq 1 3.) Keq 50

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Calculating Keq

• Step 1 Write the balanced chemical equation.
• Step 2 Set up your Keq expression.
• Step 3 Substitute concentrations.
• Step 4 Calculate!

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Example

• An aqueous solution of carbonic acid reacts to
  reach equilibrium as described below
• H2CO3(aq) H2O(l) HCO3-(aq) H3O
  (aq)
• The solution contains the following solute
  concentrations H2CO3 3.3 x 10-2 M HCO3-
  1.19 x 10-4 M H3O 1.19 x 10-4 M. Determine
  the Keq.

Note Keq does not have units!
13
Practice

• 1.a. Calculate the equilibrium constant for the
  following reaction
• COCl2(g) CO(g) Cl2(g)
• CO 0.0178 M
• Cl2 0.0178 M
• COCl2 0.00740 M
• b. Are the reactants for products favored?

14
Practice

• 2.a. For the system involving dinitrogen
  tetraoxide and nitrogen dioxide at equilibrium at
  a temperature of 100C, the product concentration
  of N2O4 is 4.0 x 10-2 M and the reactant
  concentration of NO2 is 1.4x 10-1 M. What is the
  Keq value for this reaction?
• NO2(g) N2O4(g)
• b. Are the reactants or products favored?

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Practice

• 3.a. An equilibrium mixture at 852 K is found to
  contain 3.61 x 10-3 M of SO2, 6.11 x 10-4 M of
  O2, and 1.01 x 10-2 M of SO3. Calculate the
  equilibrium constant for the reaction.
• SO2 (g) O2 (g) SO3 (g)
• b. Are the reactants or products favored?

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Calculating Concentrations from Keq

• 4. Keq for the equilibrium below is 1.8 x 10-5 at
  a temperature of 25C. Calculate NH3 when
  NH4 and OH- are 3.5 x 10-4 M.
• NH3(aq) H2O(l) NH4 (aq) OH-(aq)

NH3 6.8 x10-3 M
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Practice

• 5. a. If the equilibrium constant is 1.65 x 10-3
  at 2027C for the reaction below, what is the
  equilibrium concentration of NO when N2 1.8 x
  10-3 M and O2 4.2 x 10-3 M.
• N2(g) O2(g) NO(g)
• b. Are the reactants for products favored?

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Practice

• 6.a. At 600C, the Keq for the reaction below is
  4.32 when SO3 0.260 M and O2 0.045 M.
  Calculate the equilibrium concentration for
  sulfur dioxide.
• SO2(g) O2(g) SO3(g)
• b. Are the reactants or products favored?

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14.2 Solubility Product Constant, Ksp

• Objective
• (1) To calculate the solubility product constant,
  Ksp.

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Solubility

• The maximum concentration of a salt in an aqueous
  solution is called the solubility of the salt in
  water.

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Solubility Product Constant, Ksp

• Solubility Product Constant, Ksp the equilibrium
  constant for a solid that is in equilibrium with
  the solids dissolved ions.
• How much of a partially soluble salt will
  dissolve?
• AaBb (s) aA (aq) bB (aq)

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Calculating Ksp

• The lower the value of Ksp, the less soluble the
  substance.
• Practice
• Rank the following substances from least soluble
  to most soluble

Salt Ksp
Ag2CO3 8.4 x 10-12
BaSO4 1.1 x 10-10
Ca3(PO4)2 2.1 x 10-33
CuS 1.3 x 10-36
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Calculating Ksp

• The lower the value of Ksp, the less soluble the
  substance.
• Practice
• Rank the following substances from least soluble
  to most soluble

Salt Ksp
Ag2CO3 8.4 x 10-12
BaSO4 1.1 x 10-10
Ca3(PO4)2 2.1 x 10-33
CuS 1.3 x 10-36
CuS Least soluble Ca3(PO4)2 Ag2CO3 BaSO4 Most
soluble
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Calculating Ksp

• Step 1 Write and Balance the equation.
• Step 2 Determine the concentration of the ions.
• Step 3 Write the solubility product expression.
• Step 4 Substitute values and calculate.

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Example

• Most parts of oceans are nearly saturated with
  calcium fluoride. A saturated solution of CaF2
  at 25C has a solubility of 3.4 x 10-4 M.
  Calculate the solubility product constant for
  CaF2.
• CaF2(s) Ca2(aq) F-(aq)

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• Solution
• Balance equation CaF2(s) Ca2(aq)
  2F-(aq)
• Determine Concentrations
• CaF2(s) Ca2(aq) 2F-(aq)
• 3.4 x 10-4 3.4 x 10-4
  6.8 x 10-4
• 3. Write solubility product expression
• 4. Substitute values and calculate

Note Ksp does not have units!
27
Practice

• 1. Copper(I) bromide is dissolved in water to
  saturation at 25C. The concentration of Cu and
  Br- ions in solution is 7.9 x 10-5 M. Calculate
  the Ksp for copper(I) bromide at this temperature.

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Practice

• 2. What is the Ksp value for calcium phosphate at
  298 K if the concentrations in a solution at
  equilibrium with excess solid are 3.42 x 10-7 M
  for Ca2 and 2.28 x 10-7 M for PO43- ions?

29
Practice

• 3. If a saturated solution of silver chloride
  contains an AgCl concentration of 1.34 x 10-5 M,
  what is the solubility product constant?

30
Practice

• 4. A saturated solution of magnesium fluoride
  contains a MgCl2 concentration of 1.19x10-3 M.
  What is the Ksp for magnesium fluoride?

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Calculating Concentration from Ksp

• 5. What is the concentration of Ca2 in a
  saturated solution of CaF2 if the concentration
  of F- is 2.20 x 10-3M and Ksp 5.30 x 10-9.

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Practice

• 6. What is the concentration of Al3 in a
  saturated solution of Al(OH)3 if the OH-
  concentration is 7.90 x 10-9 M. Ksp 1.30 x
  10-33.

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Practice Chem 331

• 7. The Ksp for lead(II) iodide is 7.08 x 10-9 at
  25C. What is the molar concentration of PbI2 in
  a saturated solution?

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• The Ksp for lead(II) iodide is 7.08 x 10-9 at
  25C. What is the molar concentration of PbI2 in
  a saturated solution?
• Step 1 Write and Balance Equation
• PbI2 (s) Pb2 (aq) 2I-
  (aq)

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• The Ksp for lead(II) iodide is 7.08 x 10-9 at
  25C. What is the molar concentration of PbI2 in
  a saturated solution?
• Step 1 Write and Balance Equation
• PbI2 (s) Pb2 (aq) 2I-
  (aq)
• Step 2 Write the Ksp expression

36

• The Ksp for lead(II) iodide is 7.08 x 10-9 at
  25C. What is the molar concentration of PbI2 in
  a saturated solution?
• Step 1 Write and Balance Equation
• PbI2 (s) Pb2 (aq) 2I-
  (aq)
• Step 2 Write the Ksp expression
• Step 3 Assign x values to concentrations
• PbI2 (s) Pb2 (aq) 2I-
  (aq)
• x x
  2x

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• The Ksp for lead(II) iodide is 7.08 x 10-9 at
  25C. What is the molar concentration of PbI2 in
  a saturated solution?
• Step 1 Write and Balance Equation
• PbI2 (s) Pb2 (aq) 2I-
  (aq)
• Step 2 Write the Ksp expression
• Step 3 Assign x values to concentrations
• PbI2 (s) Pb2 (aq) 2I-
  (aq)
• x x
  2x
• Step 4 Solve

x PbI2 1.21 x 10-3 M
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Practice

• 8. The Ksp of calcium sulfate is 9.1 x 10-6.
  What is the molar concentration of calcium
  sulfate in a saturated solution?

39
Practice

• 9. The Ksp of CdF2 is 6.4 x 10-3. What is the
  molar concentration of cadmium fluoride in a
  saturated solution?

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14.3 LeChateliers Principle

• Objective
• (1) To use LeChateliers Principle to determine
  how a system at equilibrium will respond to an
  external stress.

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LeChateliers Principle

• LeChateliers Principle When a system at
  equilibrium is disturbed, the system adjusts in
  a way to reduce the change.
• There are 3 possible disturbances
• Change in (1) concentration, (2) temperature, or
  (3) pressure

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1. Change in Concentration

• Increase concentration of reactant ? Equilibrium
  shifts toward products
• Decrease concentration of reactant ? Equilibrium
  shifts toward reactants
• Increase concentration of product ? Equilibrium
  shifts toward reactants
• Decrease concentration of product ? Equilibrium
  shifts toward products

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• Use the following reaction to answer the
  questions below
• H2 (g) I2 (g) 2HI (g)
• In which direction (left or right) does the
  equilibrium shift in each of the following
  situations
• 1.) Increase H2
• 2.) Decrease I2
• 3.) Increase HI
• 4.) Decrease HI

44

• Use the following reaction to answer the
  questions below
• H2 (g) I2 (g) 2HI (g)
• In which direction (left or right) does the
  equilibrium shift in each of the following
  situations
• 1.) Increase H2 RIGHT
• 2.) Decrease I2 LEFT
• 3.) Increase HI LEFT
• 4.) Decrease HI RIGHT

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2. Change in Temperature

• Think of heat as a reactant or product
• Exothermic heat is a product
• Endothermic heat is a reactant
• For an exothermic reaction
• Increasing temperature ? equilibrium favors
  reactants
• Decreasing temperature ? equilibrium favors
  products
• For an endothermic reaction
• Increasing temperature ? equilibrium favors
  products
• Decreasing temperature ? equilibrium favors
  reactants

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• Use the following reaction to answer the
  questions below
• 2SO3(g) CO2 (g) heat CS2 (g)
  4O2(g)
• In which direction (left or right) does the
  equilibrium shift in each of the following
  situations
• 1.) Increase the temperature
• 2.) Decrease the temperature

47

• Use the following reaction to answer the
  questions below
• 2SO3(g) CO2 (g) heat CS2 (g)
  4O2(g)
• In which direction (left or right) does the
  equilibrium shift in each of the following
  situations
• 1.) Increase the temperature RIGHT
• 2.) Decrease the temperature LEFT

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3. Change in Pressure

• Only affects gases!
• Increasing pressure ? Equilibrium shifts toward
  the side with fewer moles of gas
• Decreasing pressure ? Equilibrium shifts toward
  the side with more moles of gas

49

• Use the following reaction to answer the
  questions below
• 2SO3(g) CO2 (g) heat CS2 (g)
  4O2(g)
• In which direction (left or right) does the
  equilibrium shift in each of the following
  situations
• 1.) Increase the pressure
• 2.) Decrease the pressure

50

• Use the following reaction to answer the
  questions below
• 2SO3(g) CO2 (g) heat CS2 (g)
  4O2(g)
• In which direction (left or right) does the
  equilibrium shift in each of the following
  situations
• 1.) Increase the pressure LEFT (3 moles gas)
• 2.) Decrease the pressure RIGHT (5 moles gas)

51

• Use the following reaction to answer the
  questions below
• H2 (g) I2 (g) 2HI (g)
• In which direction (left or right) does the
  equilibrium shift in each of the following
  situations
• 1.) Increase Pressure
• 2.) Decrease Pressure

52

• Use the following reaction to answer the
  questions below
• H2 (g) I2 (g) 2HI (g)
• In which direction (left or right) does the
  equilibrium shift in each of the following
  situations
• 1.) Increase Pressure NO CHANGE
• 2.) Decrease Pressure NO CHANGE

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Practice

• What direction will the equilibrium shift (left
  or right) in the reaction
• ___POCl3(g) ___PCl3(g) ___O2 (g)
  heat
• 1.) Add PCl3
• 2.) Increase Pressure
• 3.) Increase Temperature

54
Practice

• What direction will the equilibrium shift (left
  or right) in the reaction
• _2_POCl3(g) _2_PCl3(g) _1_O2 (g)
  heat
• 1.) Add PCl3 LEFT
• 2.) Increase Pressure LEFT
• 3.) Increase Temperature LEFT