2. Section 8.2—Equilibrium Constant
How can we describe a reaction at equilibrium?

3. Equilibrium Constant Expression
Equilibrium Constant Expression – Equation showing the ratio of the concentrations of products to reactants at equilibrium
We use brackets, [ ], to symbolize concentration!!!

5. Writing Equilibrium Constant Expressions
Write the product of the product concentrations on the top—take each one to a power of the coefficient from the balanced equation. 1
Write the product of the reactant concentrations on the bottom—also take each to the power of the balanced equation coefficient. 2
Example:
Write the equilibrium constant expression for the following:
2 H2 (g) + O2 (g)  2 H2O (g)

7. Writing Equilibrium Constant Expressions
Write the product of the product concentrations on the top—take each one to a power of the coefficient from the balanced equation. 1
Write the product of the reactant concentrations on the bottom—also take each to the power of the balanced equation coefficient. 2
Example:
Write the equilibrium constant expression for the following:
2 H2 (g) + O2 (g)  2 H2O (g)
[H2O] 2
K = 2
[O2]
[H2]

8. Heterogeneous Equilibrium
Homogeneous Equilibrium – All of the species are the same state of matter
2 H2 (g) + O2 (g)  2 H2O (g)
Heterogeneous Equilibrium – There are at least 2 states of matter
2 H2 (g) + O2 (g)  2 H2O (l)

9. Concentrations of Solids and Liquids
Solids and Liquids are PURE – they are not described using concentration terms.
Concentration means a part (solute) in the whole (solvent) If a substance is pure it can’t be a part of a whole 
THEREFORE – WE DO NOT INCLUDE SOLIDS OR LIQUIDS IN THE EXPRESSION!!!!

10. “K” Expressions with Solids or Liquids
If the “concentration” of a pure solid or liquid is constant, then it will not change during equilibrium and it is not written in the “K” expression.
2 H2 (g) + O2 (g)  2 H2O (g)
2 H2 (g) + O2 (g)  2 H2O (l)
H2O is not included in this “K” expression because it’s a liquid.
Only gases and aqueous solutions are included in “K” expressions!

11. Example #1—Writing K expression
Write the equilibrium constant expression for
Fe2O3 (s) + 3 H2 (g)  2 Fe (s) + 3 H2O (g)

12. Example #1—Writing K expression
Write the equilibrium constant expression for
Fe2O3 (s) + 3 H2 (g)  2 Fe (s) + 3 H2O (g)
Fe2O3 and Fe were not included in the K expression as they are solids!

13. Equilibrium Constant
Equilibrium Constant (K)– The number calculated from the equilibrium constant expression
“K” is different for every reaction at every temperature!

14. Example #2—Calculating K
Solve for the equilibrium constant for
Fe2O3 (s) + 3 H2 (g)  2 Fe (s) + 3 H2O (g)
If at equilibrium [H2] = 0.45 M and [H2O] = 0.18 M

15. Example #2—Calculating K
Solve for the equilibrium constant for
Fe2O3 (s) + 3 H2 (g)  2 Fe (s) + 3 H2O (g)
If at equilibrium [H2] = 0.45 M and [H2O] = 0.18 M
K = 0.064
Most instructors and textbooks do not require units for “K” as each one would be different

16. Meaning of Equilibrium Constant
In general….
[Products]
If K >1 or large…
[Reactants]
There is a much larger ratio of products to reactants at equilibrium
The reaction is said to “lie to the right” (products are on the right)
If K < 1 or small….
[Products]
[Reactants]
There is a much smaller ratio of products to reactants at equilibrium
The reaction is said to “lie to the left”
If K = 1 it means we have similar amounts of reactants and products at equililbrium.

17. Write the equilibrium constant expression for
Let’s Practice #1
Write the equilibrium constant expression for
N2 (g) + O2 (g)  2 NO (g)

18. Write the equilibrium constant expression for
Let’s Practice #1
Write the equilibrium constant expression for
N2 (g) + O2 (g)  2 NO (g)

19. If the equilibrium constant for
Let’s Practice #2
If the equilibrium constant for
N2 (g) + O2 (g)  2 NO (g) is 1.24 × 10-4, what can be said in general about this reaction at equilibrium?

20. If the equilibrium constant for
Let’s Practice #2
If the equilibrium constant for
N2 (g) + O2 (g)  2 NO (g) is 1.24 × 10-4, what can be said in general about this reaction at equilibrium?
The equilibrium constant is very small, so at equilibrium the concentration of products is much lower than reactants. The reaction lies to the left.

21. If the equilibrium constant for
Let’s Practice #3
If the equilibrium constant for
N2 (g) + O2 (g)  2 NO (g) is 1.24 × 10-4 and the equilibrium concentration of [N2] = M and [O2] = M, what is the equilibrium concentration of NO?

22. If the equilibrium constant for
Let’s Practice #3
If the equilibrium constant for
N2 (g) + O2 (g)  2 NO (g) is 1.24 × 10-4 and the equilibrium concentration of [N2] = M and [O2] = M, what is the equilibrium concentration of NO?
[NO]eq = M