Download Unit 8: Equilibrium Content Outline: Shifting Equilibrium and Le

Unit 8: Equilibrium
Content Outline: Shifting Equilibrium and Le Châtelier’s Principle (8.2)
I.
Dynamic Equilibrium (“Dynamic” means “constant movement”; “equilibrium” means “state of being
equal”.)
A. This is a “state” where opposite reactions/processes are occurring at the same rate and at the same
time in the same space.
B. Equilibrium is affected by changes in concentrations, pressure, and temperature.
1. These changes will cause the re-established equilibrium to favor the reactants or products.
2. When a stress is applied to a system at equilibrium the Q,, reaction quotient, (same equation as
equilibrium constant) will not equal K. Thus there will be a shift in the forward or reverse
reaction to re-establish equilibrium.
II. Le Châtelier’s Principle
A. This scientific observation was proposed by the French Chemist Henri Louis Le Châtelier in 1888.
B. This principle is a method of predicting a “shift” in equilibrium for a chemical reaction.
1. The “shift” (Direction) is influenced by “stress factors”, such as concentration, pressure, and
temperature.
C. The principle states “If a system, at equilibrium, is subjected to a “stress”, the equilibrium will shift
in the direction that tends to relieve the stress on the system.
D. When a stress is applied the Reaction Quotient (Q), does not equal the Equilibrium Constant (K).
1. If K < Q, the reverse reaction is favored to re-establish equilibrium.
2. If K > Q, the forward reaction is favored to re-establish equilibrium.
III. Shift due to Change in Concentration:
A. Increase in a [reactant]:
1. The stress (change) shifts the equilibrium to the right (forward), so products are favored.
a. The [product] increases, and the second [reactant] drops.
For example: A + B  C + D
So, let [A] increase. [B]will decrease because of combining with A to make products… so the
[C] and [D] will increase.
OR
For example: A + B  AB
So, let [A] increase… [B]will decrease because of combining with A to make products… so
the [AB] will increase.
B. Increase in a [product]:
1. The stress (change) shifts the equilibrium to the left (reverse), so the reactants are favored.
a. The [reactants] increases, and the second [product] decrease, if one exists.
For example: A + B  C + D
So, let [C] increase. [D]will decrease because of combining with C to re-make reactants… so
the [A] and [B] will increase.
OR
For example: A + B  AB
So, let [AB] increase… so the [A] and [B] will increase.
C. Decrease in a [product]:
1. The stress (change) shifts the equilibrium to the right (forward), so products are favored.
2. By removing the product as it is made, we can prevent the forward reaction from reaching
equilibrium… so more product keeps being made… more product = more money/profit in the
real world.
3. If a product is an insoluble precipitate, and falls out (decreases a product) of the solution, then
the right is favored and will run to completion. (All of a reactant is used up.)
4. If a product is water from an acid/base neutralization reaction, it too will also decrease product
as water has no concentration, so the reaction favors the right, and will run to completion.
D. Remember, the Equilibrium Constant (K) is not affected by concentration for a particular
chemical reaction.
1. The ratios of reactant to product are constant and are shown by a balanced chemical equation.
Simply, the basic chemical reaction did not change, regardless of how much you give it.
E. Pure Substances (solid or liquid) do not change in concentration… as they are pure.
1. They would not be shown in the Equilibrium Constant (K) calculation.
For example: SnO2(s) + 2 CO(g)  Sn(s) + 2 CO2(g)
As SnO2 and Sn are pure solids, so the equilibrium expression becomes:
So K = [products] = [CO2]2
[reactants]
[CO]2
F. Water is also not included in Equilibrium Constant (K) calculations, as water has no concentration.
For example: H2O(l) + CO32-(aq)  OH- + HCO3-(aq)
As H2O is pure and has no concentration, so is not present in the equilibrium expression.
So K = [products] = [OH-] [HCO3-]
[reactants]
[CO32-]
IV. Common Ion Effect
A. This is an equilibrium system achieved by adding a solution containing common ions with an
original reactant or product.
1. Involves a weak electrolyte being combined with a strong electrolyte together in solution.
a. The weak ionizes less than the strong, so the major species is the weak molecule.
b. The strong electrolyte dissociates 100% and thus the major species are ions.
For example:
Weak electrolyte: Acetic Acid = CH3COOH(aq)  H+(aq) + CH3COO- (Both products in small[ ] as it
is weak acid.)
Strong electrolyte: Sodium Acetate = CH3COONa(aq)  Na+(aq) + CH3COO- ( Both have large [ ].)
When combined together, the solution would have a large concentration of both the weak acid,
CH3COOH(aq) and its conjugate base CH3COO- .This solution is known as a buffer.
Na+ is essentially a spectator ion. So it is left out of the calculation, so the overall reaction is:
CH3COOH(aq)  H+(aq) + CH3COO-