Sections
The Equilibrium State
The Reaction Quotient
Le Chatelier's Principle
Chemical Thermodynamics: Enthalpy
Chemical Thermodynamics: Entropy
Chemical Thermodynamics: Gibbs Free Energy
Solubilty Product Constant
Protic Acids and Bases
The pH Scale
Acid Strength

Under an equilibrium state the rates of the reverse and forward reaction are equal. 

The Equilibrium State

Concept #1: Most chemical reactions do not go to completion and instead reach a state of chemical equilibrium. 

Equilibrium is reached when the rates of the forward and reverse reaction are equal. 

The magnitude of K determines which direction is favored at equilibrium. If K is greater than 1 then products are favored over reactants. 

If K is less than 1 then reactants are favored over products. 

If K is equal to 1 then both reactants and products are favored. 

Equilibrium State Calculations

Example #1: For the following chemical reaction N2 (g) + O2 (g)  2 NO (g), Kc = 3.7 x 10-5, kf = 2.5 x 10-3 and kr = 67.57. Addition of a catalyst increases the forward rate constant 1.8 x 10-1. What is the new reverse rate constant after the addition of the catalyst? 

Example #2: Consider the following reactions at 25°C:

Reaction                                                                      Kc
2 NO (g) ⇌ N(g) + O(g)                             1 × 1030
2 H2O (g) ⇌ 2 H(g) + O(g)                       5 × 10−82
2 CO (g) + O(g) ⇌ 2 CO(g)                      3 × 1091

Which compound is most likely to dissociate and give O(g) at 25°C?

a) CO2                          b) NO                           c) CO                           d) H2O

Practice: Write the equilibrium expression for the following reaction.

4 NH(g) + 3 O(g) ⇌  2 N(g) + 6 H2O (l)

Example #3: When reaction 1 and 2 below are added together, the result is reaction 3.

      1)  H2O (l) + HNO(aq)  H3O+ (aq) + NO2 (aq)              K1 = 4.50 × 104

     

      2)  H3O+ (aq) + OH (aq)  2 H2O (l)                                        K2 = 1.00 × 1014

     

      3)  HNO(aq) + OH (aq)  NO2 (aq) + H2O (l)                 K3 = ?

 

Find the equilibrium constant, K3.           

A) 4.50 × 1018        B) 2.22 × 1017​        C) 4.50 × 1010​        D) 1.00 × 1014