1. Reversible Reactions
Some reactions may be reversible –the conversion of reactants to products and the conversion of products to reactants occur simultaneously
Forward: 2SO2(g) + O2(g) → 2SO3(g)
Reverse: 2SO2(g) + O2(g) ← 2SO3(g)

2. Reversible Reactions
The two equations can be combined into one, by using a double arrow, 2SO2(g) + O2(g) ↔ 2SO3(g)
A chemical equilibrium occurs

3. Le Chatelier’s Principle
Le Chatelier’s principle: If stress is applied to a system in equilibrium, the system changes in a way that relieves the stress
ONLY changed by gasses!

4. Le Chatelier’s Principle
Concentration
Temperature
Pressure
Concentration – adding to one side pushes equilibrium to the other side. Taking away from one side then favors that side.
Adding reactants makes more products

5. Le Chatelier’s Principle
Temperature – increasing the temperature causes the equilibrium position to shift away from the side with heat on it
Decreasing temperature will favor the side with heat on it.
N2(g) + 3 H2(g) ↔ 2 NH3(g) + heat
If temp increases, the equilibrium shifts left and produces more reactants while using some products.

6. Le Chatelier’s Principle
Pressure – changes in pressure will only effect gaseous substances
Increasing the pressure will usually favor the direction that has fewer molecules
N2(g) + 3H2(g) ↔ 2NH3(g)
If pressure increases the reaction favors the right because there are only 2 atoms, vs 4 on the left.

7. Equilibrium Constants: Keq
Chemists generally express the position of equilibrium in terms of numerical values
These values relate to the amounts (Molarity) of reactants and products at equilibrium
This is called the equilibrium constant, and abbreviated Keq

8. Equilibrium Constants
consider this reaction (the capital letters are the chemical, and the lower case letters are the balancing coefficient):
aA + bB  cC + dD
The equilibrium constant (Keq) is the ratio of product concentration to the reactant concentration at equilibrium, with each concentration raised to a power

9. Equilibrium Constants
consider this reaction:
aA + bB  cC + dD
Thus, the “equilibrium constant expression” has this general form:
[C]c x [D]d
[A]a x [B]b
(brackets: [ ] = molarity concentration)
Note that Keq has no units on the answer; it is only a number because it is a ratio
Keq =

10. Equilibrium Constants
the equilibrium constants provide valuable information, such as whether products or reactants are favored:
if Keq > 1, products favored at equilibrium
if Keq < 1, reactants favored at equilibrium
if Keq = 1, the amounts of product and reactants will be approximately equal

11. Summarize
How do the 3 types of stresses affect the equilibrium of an equation?