Table 9

South Pasadena  AP Chemistry
Name _________________________________
Period ___ Date ___/___/___
16  Chemical Equilibrium
Examples of Equilibrium:
In chapter 5, we tested HCl and HC2H3O2 as
electrolytes and found that HC2H3O2 was a weak
electrolyte. We developed the model that HC2H3O2
dissociates into ions, but the ions also reform the
acid molecule.
16.1
Ratef = Rater
NOTES
The equation is:
HC2H3O2(aq)  H+(aq) + C2H3O2-(aq)
HCl is essentially “one-way” and dissociates 100%.
In chapter 12, we talked about the vapor pressure of
water inside the eudiometer tube. This is officially
called the equilibrium vapor pressure.
The equation is: H2O(l)  H2O(g)
Rateforward = Ratereverse:
In any equilibrium, the macroscopic (observable)
properties become constant. What is constant in the
above two examples?



These properties are constant because for every
forward change, there is an equal reverse change.
That is, the Rate of reaction forward = the Rate of
reaction for the reverse reaction.
Think back to Ch 15. Write the Rate Law for the
following reaction: A + B  C + D
Ratef =
kf/kr is renamed, Keq, the Equilibrium Constant.
This ratio of concentrations is called the Mass
Action Expression or the Equilibrium
Expression.
Write the equilibrium expression for the generalized
equation: aA + bB  cC + dD
Write the equilibrium expression for the
dissociation equation of HC2H3O2.
Solids and Liquids:
What is the concentration of a solid such as Copper,
Cu? (density = 8.96 g/mL) (mass = 63.55 g/mol)
Write the Rate Law for the reverse of this reaction:
C+DA+B
Rater =
We are saying that Ratef = Rater. Substitute in the
two rate laws and put like terms together as your
instructor does on the board.
This value is constant.
We could calculate the concentration of H2O(l) in a
similar way. [H2O(l)] is also constant.
Manipulating Kc values:
Reverse the reaction:
Solids and liquid are left out of the mass action
expression. Example: H2O(l)  H2O(g)
Double the reaction:
Reactant-favored / Product-favored:
The value of the Keq indicates whether a reaction at
equilibrium is mostly reactants or products.
Halve the reaction:
HF
 H+ + F-
The Keq for this reaction will be ______ (>1, <1, 1)
This reaction is ____________-favored.
Keq >> 1 = _________________________
Keq << 1 = _________________________
Add two reactions together:
Example:
H2S  H+ + HS-
Keq 0.1 … 10 = _____________________
Keq, Kc, Kp:
Example: N2(g) + 3H2(g)  2NH3(g)
HS-  H+ + S2-
H2S  2H+ + S2-

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