1. Chemical Systems & Equilibrium
Unit 4

2. Calculating the Equilibrium Constant
The equilbrium constant, Keq, is the ratio of equilibrium concentrations at a particular temp
Kc for solution-phase systems or Kp for gas- phase systems
Keq = [C]c[D]d for the eqn
[A]a[B]b aA+bB cC+dD
Note: The equilibrium constant depends ONLY on the concentration of gases (not liquids/solids)

3. Questions: Equilibrium Law Expression
1. Write the equilibrium law expression for the following:
a) 2NO2(g) ↔ N2O4(g)
b) 2HI(g) ↔ H2(g) + I2(g)
2. A reaction vessel contains NH3, N2 and H2 gas at equilibrium at a certain temperature. The equilibrium concentrations are [NH3] = 0.25mol/L, [N2] = 0.11mol/L and [H2] = 1.91 mol/L. Calculate the equilibrium constant for the decomposition of ammonia.
K =
[N2O4(g)]
[NO2(g) ]2
K =
[H2(g)] [I2(g) ]
[HI(g) ]2
2NH3(g) ↔ N2(g) + 3H2(g)
K =
[N2(g)] [H2(g) ]3
[NH3(g) ]2
K =
[0.11] [1.91 ]3
[0.25 ]2
K = 12.3

4. Questions: Equilibrium Law Expression
3. Nitryl chloride gas, NO2Cl, is in equilibrium at a certain temperature in a closed container with NO2 and Cl2 gases. At equilibrium, [NO2Cl] = mol/L and [NO2] = mol/L. If K = 0.558, what is the equilibrium concentration of Cl2? 4. Write a balanced equation for the reaction with the following equilibrium law expression:
K =
[NO2(g)]2
[NO (g) ]2 [O2 (g) ]

5. Heterogeneous Equilibria
homogeneous equilibria = equilibria in which all entities are in the same phase
Reactants and products are all gas or all aqueous
heterogeneous equilibria = equilibria in which reactants and products are in more than one phase
Reactants and products are in different phases

6. Homogenous equilibrium applies to reactions in which all reacting species are in the same phase.
N2O4 (g) NO2 (g)
Kp =
NO2 P2
N2O4 P
Kc =
[NO2]2
[N2O4]

7. Heterogenous equilibrium applies to reactions in which reactants and products are in different phases.
CaCO3 (s) CaO (s) + CO2 (g)
Kc =
[CaO(s) ][CO2(g)]
[CaCO3(s)]
[CaCO3(s)] = constant
[CaO(s) ] = constant Kc
[CaO(s)]
[CaCO3(s)]
= [CO2(g)]
Constant values are incorporated into the equilibrium constant and are NOT included in the equilibrium expression
Kc = [CO2(g)]
The concentration of solids and pure liquids are considered to be constant and are not included in the expression for the equilibrium constant.

8. PCO2 = Kp PCO does not depend on the amount of CaCO3 or CaO
CaCO3 (s) CaO (s) + CO2 (g)
PCO2
= Kp
PCO 2
does not depend on the amount of CaCO3 or CaO

9. Equilibrium favors the reactant side
N2O4 (g) NO2 (g)
equilibrium
equilibrium
equilibrium
Start with NO2
Start with N2O4
Start with NO2
& N2O4
Equilibrium favors the reactant side

10. CHECKPOINT
The reaction at 200C between ethanol and ethanoic acid produces ___________________ and __________________.
Write the equation for this reaction
Determine the equilibrium constant expression for the reaction

11. Calculating Equilibrium Concentrations (when given one concentration)
Sample Problem:
When ammonia is heated it decomposes:
2NH3(g)↔ N2(g) + 3H2(g)
When 4.0 mol of ammonia is introduced in a 2.0L container and heated. The
equilibrium amount of ammonia is 2 0 mol. Determine the equilibrium concentrations of the other two entities.
STEP 1: Determine the concentration (initial and equilibrium) for known values
STEP 2: Setup an ICE Table
STEP 3: Determine the value of X
STEP 4: Use x value to determine the other quantities

12. Determine the concentrations
[NH3]initial = 4.0mol/2.0L = 2.0mol/L
[NH3]equilibrium = 2.0mol/2.0L = 1.0mol/L
Setup ICE Table

13. Determine the value of X
[NH3](g)equil = 2 0mol / L - 2x
[NH3](g)equil = 1.0mol/L (from calculations in Step 1)
2.0mol/L – 2x = 1.0mol/L
-2x = - 1.0mol/L
x = 0.5mol/L
Use X to determine other quantities

14. constant

15. Reversible Reactions
For a given overall system composition, the same equilibrium concentrations are reached whether equilibrium is approached in the forward or the reverse direction
What about Keq will it be the same in fwd/rev?