1. Lesson # 2 Equilibrium Law & Constants
Chemical Equilibrium
Lesson # 2
Equilibrium Law & Constants

2. Developing the Equilibrium Constant
In equilibrium, we know that the rate of forward reaction is equal to the rate of the reverse reaction.
The equilibrium law is the mathematical description of a chemical system at equilibrium.
Consider reaction A + B = 2 C:
*Note that we can only convert molar coefficients to powers if the reaction is a simple one-step reaction (elementary process).*

3. Dynamic Equilibrium In general for: aA + bB = cC + dD. K = [C]c [D]d
K is unitless (unlike rate values of k), but varies with temperature.

4. Example 1
Write the equilibrium equation for the reaction described by the following balanced equation:
4 NH3 (g) + 7 O2 (g) = 4 NO2 (g) + 6 H2O (g) Assume this is a one-step reaction.

5. Example 2
In a closed vessel at 500°C, nitrogen and hydrogen gases combine in an equilibrium reaction to form ammonia gas. N2 (g) + 3 H2 (g) = 2 NH3 (g). The equilibrium concentration of nitrogen is 1.50 x 10-5 mol/L, hydrogen is 3.45 x 10-1 mol/L and ammonia is 2.00 x 10-4 mol/L. Calculate the equilibrium constant, K, for this chemical reaction under these conditions. Assume this is a one-step reaction.

6. Heterogeneous Equilibria
So far all examples discussed have been ones of homogeneous equilibrium, in which all reactants and products in the system were in the same state of matter.
When the reactants and products exist in more than one state, the system is a heterogeneous equilibrium.
It is important to note that equilibrium position of a heterogeneous equilibrium does not depend on the quantities of pure solids or liquids – only aqueous or gaseous entities.
This is because the concentration of pure solids and liquids cannot change.

7. Example 3
a) Consider the reaction: CaCO3 (s) = CaO (s) + CO2 (g). Determine the equilibrium law (equation) for the reaction.
b) Solid phosphorus pentachloride can undergo a reversible reaction to form liquid phosphorus trichloride and chlorine gas. Write the equilibrium law for this reaction:

8. The Magnitude of K
The value of K (its magnitude) can help scientists predict the equilibrium position between reactants and products.
When K >>> 1, the equilibrium favours the products – the equilibrium lies to the right
When K = 1, the equilibrium concentration of the products is similar to that of the reactants – the equilibrium lies in the middle
When K <<< 1, the equilibrium favours the reactants – the equilibrium lies to the left.