A.P. Chemistry
Chapter Fourteen Outline
Chemical Equilibrium
Reactants↔Products
 Chemical Equilibrium: when the concentrations of reactants and products in a
chemical reaction remain constant
 Product favored reactions: when products predominate
 Reactant favored reactions: when reactants predominate
 Dynamic Equilibrium: when the rate of the forward reaction is equal to the rate of
the reverse reaction
 Although catalysts speed up reaction, the same equilibrium will be reached
I.
The equilibrium constant
 Kc = the equilibrium constant, which is a quotient of equilibrium
concentrations of reactants and products for a given reactions at a given
temperature
 The equilibrium constant expression can be obtained by multiplying
together the rate constants for the forward reactions in all steps and then
dividing by the rate constants for the reverse reaction in all steps
II.
Writing Equilibrium Constant Expressions
 The only concentrations that appear in an equilibrium constant expression
are those of gases and of solutes in dilute solutions
 Example: AX(g) + BY(g) → 2CZ(g)
Kc = [CZ]2/[AX][BY]
 Note that the coefficients become powers and the products are on the
numerator and reactants on denominator
 Whenever the stoichiometric coefficients of a balanced equation are
multiplied by some factor, the equilibrium constant for the new
equation is the original equilibrium constant raised to the power of the
multiplication factor
Example: N2(g) + 3H2(g) → 2NH3(g)
Kc = [NH3]2/[N2][H2]3 if the reaction is doubled than the new K would
be the square of the original
 The equilibrium constant for a reaction and that for its reverse are
reciprocals of one another
 If two chemical equations can be summed to give a third, than the
equilibrium constant for the overall equation equals the product of
the two equilibrium constants for the equations that were summed
III.
Determining Equilibrium Constants
Write the balanced equation for the equilibrium reaction
Write the equilibrium constant expression
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Set up a table containing initial concentrations, change in concentrations, and
equilibrium concentrations for each substance included in the equilibrium
constant expression. Enter all known info into table
Use x to symbolize the change in concentration of one substance.
Use the stoichiometric coefficients in the balanced equation to calculate the
others in terms of x
From initial concentrations and the changes in concentrations calculate the
equilibrium concentrations in terms of x and enter it into table
Use the simplest equation to solve for x, then use x to calculate what you are
looking for
Check your answer for reasonableness
The Meaning of the Equilibrium Constant
If Kc>>1 than the reaction is strongly product favored
If Kc<<1, than the reaction is strongly reactant favored
Remember that the reverse reaction K is the inverse of the forward reaction K
When Kc approximates 1 than calculations are needed to determine
equilibrium concentrations
If a reaction has a large tendency to occur in one direction than the reverse
reaction has little tendency to occur
Using Equilibrium Constants
To predict the direction of a reaction, first determine the reaction quotient,
which has the same mathematical form as the equilibrium expression but is a
ratio of actual concentrations in the mixture
If Qc= Kc than the reaction is at equilibrium
If Qc is less than Kc than the reaction will proceed towards product until
equilibrium is reached
If Qc > Kc than the reverse reaction will proceed until equilibrium is reached
Shifting a Chemical Equilibrium: Le Chatelier’s Principle
Le Chatelier’s Principle: Of a system is at equilibrium and the conditions are
changed so that it no longer is at equilibrium; the system will react in a way
that partially counteracts the change
Changing pressure by changing the volume shifts an equilibrium only if the
sum of the coefficients for gas phase reactants is different from the sum of the
coefficients of the gas phase products
For an endothermic reaction, an increase in temperature always means an
increase in Kc, the reaction will become more product favored at higher
temperatures
For an exothermic reaction, an increase in temperature always means a
decrease in Kc, the reaction will become less product favored at higher
temperatures
Entropy is a measure of randomness or disorder. Reaction favor increases in
Entropy