Chapter 18: Chemical Equilibrium
*Some reactions are reversible.
When these reactions have the same reaction forwards and reverse, the reaction is at equilibrium.

*This does not mean that there are 50/50 mix of products and reactants.

Mass Action Expression
pA+qB <=> rC+sD
Keq= [products] = [C]r [D]s
[reactants] [B]q [A]p

*If Keq = 1, there is an equal concentration of products and reactants.
Equation at Equilibrium
Equation at Equilibrium

*If Keq>1, there are more products than reactants. The equilibrium "favors" the products.
*If Keq<1, there are more reactants than products. The equilibrium "favors" the reactants.
*Keq equations only include GAS and AQUEOUS compounds. If a compound is solid, do not include it in the equation.

Practice problems:
~2NO2 (g) <=> N2O4(g)
Give the Keq.

~N2 +3H2 <=> 2NH3
Give the Keq.

LeChatelier's Principle (LCP):
If an equilibrium is stressed, the reaction will shift in such a way as to minimize the stress.
Reactants-------------------Products
Reactants-------------------Products
*If you add more reactants to the equation, then the equation will shift to the right.
*If you add more product to the equation, then the equation will shift to the left.

Pressure Change:
*If pressure is increased, thne the reaction moves to the smaller side (the side with fewer moles).
*If the pressure is decreased, then the reaction moves to the side with more moles.
Example:
~CH3OH <=> 2H2 +CO. Increase the pressure. Which direction does the equation move?
~3O2 <=> 2O3 What happens if you decrease the pressure?

Solubility Product:
NaCl(s) <=> Na+(aq) + Cl- (aq)
Ksp=[Na+][Cl-]
What will happen if CaCl2 is added?
CaCl2 <=> Ca2+ +2Cl-
Cl is the common ion, because it is the ion common between the two equations.


Answers:
~Keq = [N2O4]
[NO2]
~Keq= [NH3]2
[N2][H2]3
~The equation moves to the left.
~The equation will move to the right.