Chemistry 181

1
Chemistry 18-1

• The Nature of Chemical Equilibrium

2
Equilibrium Overview

• At the beginning of a reaction, the
  concentrations of the reactants are at a maximum,
  and the concentrations of the products are at
  zero
• As the reaction progresses, the concs of the
  reactants increase, decrease, while the concs
  of the products increase, decrease
• DECREASE, INCREASE
• If the reaction is reversible, EQUILIBRIUM is
  established when the rates of the forward and
  reverse reactions become equal

3
Equilibrium Constant

• Once equilibrium is established, the
  concentrations of the reactants and products will
  remain constant
• Therefore, the RATIO of the concentrations should
  also remain constant!
• Ex) nA mB ? xC yD
• The equilibrium expression for this rxn is Keq
  Cx Dy An Bm

4
Comparing Keq and Rate Laws

• Keq involves K as the whole expression
• (K something)
• Rate Laws involve k as part of the expression
• (R k something)

• Keq deals with BOTH products AND reactants
• Rate Laws deal with __________ only!
• REACTANTS

5
Equilibrium Constant, cont.

• The numerical value of Keq for a particular
  reaction depends on temperature and must be
  experimentally determined
• If Keq is approx. equal to 1, then at equilibrium
  there must be approx. equal concentrations of
  reactants and products present

6
Equilibrium Constant, cont.

• If Keq is very small (much less than 1), then the
  concentration of the products at equilibrium must
  be much greater, smaller than the concentration
  of reactants
• Therefore, the forward, reverse reaction is
  favored
• SMALLER, REVERSE
• Similarly, if Keq is very large (much greater
  than 1), then the concentration of the products
  at equilibrium must be much greater than the
  concentration of reactants
• Therefore, the forward reaction is favored

7
Practice ?p559 6

• Write the equilibrium expression for the
  following reaction4HCl(g) O2(g) ? 2Cl2(g)
  2H2O(g)
• Answer Keq Cl22 H2O2 HCl4 O2

8
More Practice ?p559 8

• Calculate the equilibrium constant (Keq) for the
  reaction H2(g) I2(g) ? 2HI(g)if, at 425oC,
  H2 1.83x10-3 M, I2 3.13x10-3 M, and HI
  1.77x10-2 M.
• Answer Keq HI2 54.7 H2 I2

9
Chemistry 18-2

• Shifting Equilibrium

10
LeChateliers Principle (Review)

• If a system at equilibrium is subjected to a
  stress, the system will shift to reduce the
  stress.
• Effect of pressure changes?
• Pressure is only a concern for reactions
  involving gases!
• Effect of temperature changes?
• The direction of shift depends on whether the
  reaction is endothermic or exothermic!

11
LeChateliers Principle Review Problem ?

• 556 kJ CaCO3(s)?CaO(s) CO2(g)
• Which way will the system shift to reduce the
  following disturbances?
• Increase temp
• Decrease pressure
• Add CaO
• Remove CO2

12
LeChateliers Principle, cont.

• Effects of concentration changes
• Changing the concentration does not change the
  equilibrium constant, Keq! (Why not??)
• Because the system will always shift to
  reestablish equilibrium!
• Pure solids and pure liquids are always defined
  to have a concentration of 1, so they do not
  affect the equilibrium constant!
• This is only a concern for problems involving
  solubility (Section 18-4)

13
Rxns that Go To Completion

• Not all chemical reactions result in
  equilibrium!!!
• Under what conditions will the forward reaction
  go to completion?
• (This means that reactants are fully converted to
  products, and the reverse reaction does not
  occur!)

14
Rxns that Go To Completion, cont.

• Conditions favoring a complete reaction
• 1) Formation of a gas
• A gas will escape from the reaction. According
  to LeChateliers Principle, removal of a product
  (the gas) will cause the reaction to shift to the
  _______, and so
• RIGHT, and so THE REACTION RUNS AWAY!

15
Rxns that Go To Completion, cont.

• 2) Formation of a precipitate
• A precipitate is soluble, insoluble.
  Therefore
• INSOLUBLE. It cannot break apart to reform the
  reactants and the reverse reaction cannot occur!

• 3) Formation of a slightly ionized product
• If the product only ionizes minimally (like
  liquid water), then
• Very little of the reactants reform, and the
  reverse reaction essentially does not occur!

16
Common Ion Effect

• Suppose we have a saturated solution of sodium
  chloride in equilibrium
• NaCl(s) ? Na1(aq) Cl-1(aq)
• Now suppose we add some hydrochloric acid to the
  salt system
• HCl(g) H2O(l) ? H3O 1(aq) Cl-1(aq)
• (Recall, how do we know the HCl will fully
  dissociate in water?)
• HCl is a STRONG ACID!

17
Common Ion Effect, cont.

• According to LeChateliers Principle, how will
  the NaCl equilibrium adjust to compensate for the
  addition of HCl? NaCl(s) ? Na1(aq) Cl-1(aq)
• Answer Because of the added Cl-1 ion, the
  system will shift to the _______, causing more
  NaCl to _______!
• LEFT, PRECIPITATE
• This phenomenon is known as the common ion effect
  (where a common ion causes precipitation to
  occur).

18
An Application of the Common Ion Effect (18-3)

• Buffers solutions that can resist changes in
  pH.
• Formed from a weak acid and its salt (or, from a
  weak base and its salt).
• Ex) acetic acid and sodium acetate
• HC2H3O2 H2O ? H3O1 C2H3O2-1
• NaC2H3O2 ? Na1 C2H3O2-1
• This buffer system only works because both the
  acid (HC2H3O2) and its conjugate base (which
  is?) are present in large amounts.

19
Buffers (Common Ion Effect), cont.

• Recall, conjugates come from whose definition of
  acids and bases??
• BRONSTED-LOWRY! ACID p donor, BASE p
  acceptor
• So, what is the conjugate base of
  HC2H3O2? HC2H3O2 H2O ? H3O1 C2H3O2-1
• CONJUGATE BASE C2H3O2-1
• Why is it necessary to include the salt
  (NaC2H3O2) in the buffer system?
• Because a weak acid only dissociates slightly, so
  the concentration of the conjugate base would be
  very small without the salt (which dissociates
  fully).

20
Buffers (Common Ion Effect), cont.

• What would happen if an ACID (like HCl) was added
  to this buffer system? HC2H3O2 H2O ? H3O1
  C2H3O2-1
• Equilibrium will shift to the right, left.
• LEFT
• This converts the added acid to a much weaker
  acid (acetic).
• The pH of the system changes only a little, and
  we say that the pH has been buffered.

21
Buffers (Common Ion Effect), cont.

• What would happen if a BASE (like KOH) was added
  to this buffer system? HC2H3O2 H2O ? H3O1
  C2H3O2-1
• HINT What would the added K1 ion do to the
  system? What about the added OH-1 ion?
• K1 reacts with C2H3O2-1, OH-1 reacts with H3O1
• This is why we need a good quantity of conjugate
  base present!
• So, equilibrium will shift to the right, left.
• RIGHT
• This converts the added base to a much weaker
  base (the conjugate).
• Again, the pH of the system has been buffered!

22
What good are buffers?

• The blood in your body in naturally buffered to
  maintain a neutral pH (between 7.3 and 7.5)
• Aspirin is often buffered to prevent sharp
  changes to the pH of your stomach acid (between
  1.0 and 3.0)