Chemical Equilibrium

1
Chemical Equilibrium
2
Complete and Reversible Reactions

• Complete Forms a precipitate or evolves gas,
  all reactants are used up
• Reversible - When products formed in a chemical
  reaction decompose back to the original reactants

3
Reversible Reactions

• The arrows go in both directions
• ? forward reaction
• ? reverse reaction
• Must be in a closed system where nothing can
  escape

4
Equilibrium

• Occurs when the forward and reverse reactions
  happen at an equal rate there is no net change
• Based on a specific temperature and pressure
• The total amount of particles remains the same
  and therefore so does the concentration
• The concentration of a substance is denoted by
  the use of brackets around the formula H2
• The reaction is dynamic - in constant motion

5
(No Transcript)
6
Equilibrium Constant

• For the reaction aA bB cC dD,
• Keq CcDd
• AaBb
• Keq equilibrium constant
• concentration in M (mol/L)
• Do not include any solids or liquids in the Keq
  expression
• Both solids and liquids are pure substances,
  their concentration cannot change by definition

7
Write the formula for the equilibrium constant
for each of the following reactions

• H2 (g) I2 (g) 2HI (g)
• As4O6 (aq) 6C (s) As4 (g) 6CO (g)
• 3. Hg (l) Hg (g)
• NH3 (aq) H2O (l) NH4 (aq) OH- (aq)

8
Equilibrium Constant Calculations

• At a temp of 25C, the following concentrations
  of the reactants and products for the reaction
  involving carbonic acid and water are present
  H2CO3 3.3 x 10-2M H3O 1.1 x 10-6M and
  HCO3- 7.1 x 10-1M. What is the Keq value for
  the following reaction at equilibrium in a dilute
  aqueous solution?
• H2CO3 (aq) H2O (l) H3O (aq) HCO3- (aq)

9
Equilibrium Constant Calculations

• What is the equilibrium constant of formic acid,
  HCHO2? In water, the equilibrium concentrations
  are HCHO2 2.00M H3O 6.00 x 10-6M and
  CHO2- 6.00 x 10-6M.
• HCHO2 (aq) H2O(l) H3O (aq) CHO2- (aq)

10
System Equilibria

• Equilibria can favor the formation of reactants
  or products
• Keq can determine which direction is favored in a
  rxn
• Keq gt 1 means forward rxn favored
• Keq lt 1 means reverse rxn favored
• Keq 1 means neither is favored
• If conditions of the reaction are changed, the
  reaction will shift from its original equilibrium
  state to compensate for the change

11
Le Chateliers Principle

• When a system at equilibrium is disturbed it
  attains a new equilibrium position to accommodate
  the change
• Used in industry to increase efficiency

12
System Equilibria

• Factors that alter chemical equilibrium
• Concentration of reactants or products
• Pressure
• Temperature

13
Concentration

• Adding a substance to a system at equilibrium
  drives the system to consume that substance
• Removing a substance from a system at equilibrium
  drives the system to produce more of that
  substance

14
Temperature

• Only factor that affects the value of the
  equilibrium constant
• Affects how completely a reaction proceeds to
  products
• Remember
• exothermic releases heat
• endothermic absorbs heat

15
Temperature

• Heat can be treated as a product or a reactant
• If the reaction is exothermic, heat is written on
  the product side of the equation
• If the reaction is endothermic, heat is written
  on the reactant side of the equation
• Adding heat to an exothermic reaction will shift
  the equilibrium towards the reactants
• Adding heat to an endothermic reaction will shift
  the equilibrium towards the products

16
Pressure

• Increase system pressure - the system will shift
  to reduce that pressure by proceeding in the
  direction that produces fewer molecules of gas
• An equilibrium reaction that has the same of
  moles of gas on both sides of the equation will
  not be affected by changes in pressure

17
The Solubility Product Constant
18
Dissolution and Precipitation

• Remember ionic substances separate into their
  ions in solution and become uniformly distributed
  in the soln
• Dissolution- the process in which an ionic solid
  dissolves in a polar liquid
• Can write an equation for dissociation
• Only dissociated substances are written as ions
  in equations
• Must balance numerically and electrically

19
Precipitation

• Precipitation- the process in which ions leave a
  soln and regenerate an ionic solid
• Precipitate- insoluble solid formed
• Dissolution and precipitation are opposite
  process
• Solubility equilibrium- rate of dissolution rate
  of precipitation

20
What is a solubility product constant, and what
is it used for?

• An equilibrium constant for slightly soluble
  ionic substances
• symbolized Ksp
• Used to determine solubility of sparingly soluble
  compounds
• Cannot be applied successfully to salts that are
  more soluble

21
How is a solubility constant written?

• The equation for a slightly soluble ionic
  substance in a saturated soln can be written in
  the following general form
• AaBb (s) ? aA(aq) bB-(aq)
• The solubility product constant is
• Ksp AaB-b

22
Association Equations and Solubility Product
Constants

• Write the dissociation equation and solubility
  product constant for each of the following
  substances.
• Strontium arsenite
• Calcium oxalate
• Barium sulfide
• Magnesium hydroxide

23
Solubility Product

• At 25C, the concentration of Pb2 ions in a
  saturated soln of PbF2 is 1.9 x 10-3M. What
  is the value of Ksp for PbF2?

24

• PbF2 (s) ? Pb2 (aq) 2F- (aq)
• For every molecule of PbF2, there will be one
  Pb2 ion and two F- ions. If PbF2 x, then
  Pb2 x and F- 2x
• Ksp Pb2 F-2
• Ksp x (2x)2
• Ksp 4x3

25
Solubility Product

• A sample of Cd(OH)2 (s) is added to distilled
  water and allowed to come to equilibrium at 25C.
  The concentration of Cd2 is 1.7 x 10-5M at
  equilibrium. What is the value of Ksp for
  Cd(OH)2?

26
Solubility

• What will be the equilibrium concentrations of
  lithium and phosphate ions in a saturated
  solution of lithium phosphate? (Ksp 3.2 x 10-9)

27
Solubility

• What will be the equilibrium concentrations of
  strontium and phosphate ions in a saturated
  solution of strontium phosphate?
  (Ksp 1.0 x 10-31)

28
Precipitates

• Supersaturated solutions are unstable
• Non equilibrium state achieved by manipulating
  conditions
• Precipitates will form in a supersaturated
  solution
• To determine supersaturated solution calculate Q,
  the ion product
• Ksp lt Q Supersaturated
• Ksp gt Q Unsaturated
• Ksp Q Saturated

29
Precipitation Reactions

• Reaction in which 2 solutions are mixed and a
  precipitate is formed
• Described by a chemical equation
• Remember ionic substances dissociate in solution
• The precipitate that forma is a combination of
  ions present
• The precipitate formed can be identified by using
  solubility rules but can only be truly confirmed
  experimentally

30
What is the common ion effect?

• Common ion an ion that comes from two or more
  substances making up a chemical reaction
• example BaSO4 and Na2SO4 common ion is SO4-2
• Common ion effect a process in which an ionic
  compound becomes less soluble upon the addition
  of one of its ions by adding another compound

31
Why does the common ion effect work?

• The common ion effect is an example of Le
  Chateliers principle
• When a product is added to a system in
  equilibrium, it will cause the equilibrium to
  shift to the left, making more insoluble reactant

32
a) Saturated silver sulfate solution, Ag2SO4
(aq), is colorless. A schematic of the solution
is shown above, omitting the water for
simplicity.(b) Following the addition of Na2SO4
(aq), most of the Ag ions originally present
(about 7 of 8 shown) have precipitated. The
schematic shows the only remaining silver ion as
a silver ball.