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.

*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: (Answers will be at the end)

~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.
*If you add more
reactants to the equation, the equation will lean toward the left (reactant) side. To keep it balanced, the equation will shift to the right.
*If you add more
products to the equation, the equation iwill tilt toward the right (product) side. To keep it balanced, the equation will shift to the left.

Pressure Change:

*If pressure is increased, then 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.

Practice Problems:

~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)

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.

Practice Problem:

~What is the common ion in the following situation: 50.0mL of 1.0M NaCH3COO is added to 1.0M CH3COOH.
~What is the common ion if 5.0g of NaCl is added to a 2.0M solution of HCl?


~Keq = [N2O4]
This is because NO2 is the reactant, so it goes on the bottom. N2O4 is the product, so it goes on the top.
~Keq= [NH3]2
N2 and H2 are the reactants. There are three hydrogens needed for the equation to work, so it is to the third power. NH3 is the product, and there are two needed in the equation.
~The equation moves to the left.
(Pressure change rules.)
~The equation will move to the right.
(Pressure change rules)
~The common ion is CH3COO. It is the only ion that is found in both compounds.
~Cl is the common ion. It is the only ion common between the two compounds.

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