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

Equilibrium Constant Expression Equilibrium Constant Expression – Equation showing the ratio of the concentrations of products to reactants at equilibrium Concentration is symbolized with brackets “[A]”

Example: Write the equilibrium constant expression for the following: 2 H 2 (g) + O 2 (g)  2 H 2 O (g) Write the concentration of products on the top— take each one to a power of the coefficient in the balanced equation. Write the concentration of reactants on the bottom—also take each to the power of the balanced equation coefficient. 1 2 Writing Equilibrium Constant Expressions

Example: Write the equilibrium constant expression for the following: 2 H 2 (g) + O 2 (g)  2 H 2 O (g) [H 2 O] [H 2 ][O 2 ] Write the concentration of products on the top— take each one to a power of the coefficient in the balanced equation. Write the concentration of reactants on the bottom—also take each to the power of the balanced equation coefficient. 1 2 K = 22 Writing Equilibrium Constant Expressions

Homogeneous Equilibrium – All of the species are the same state of matter Heterogeneous Equilibrium – There are at least 2 states of matter 2 H 2 (g) + O 2 (g)  2 H 2 O (g) 2 H 2 (g) + O 2 (g)  2 H 2 O (l) Heterogeneous Equilibrium

Concentrations of Solids and Liquids Concentration (Molarity) is the ratio of moles / liters The density (mass / liter) of a pure solid or a pure liquid is constant (density is an intensive property) The mass per molecule (molecular mass) is also intensive The number of molecules (moles) and therefore the mass per liter won’t change For pure solids and liquids, the concentration is a constant!

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 H 2 (g) + O 2 (g)  2 H 2 O (g) 2 H 2 (g) + O 2 (g)  2 H 2 O (l) H 2 O is not included in this “K” expression because it’s a liquid. Only gases and solutions are included in “K” expressions! “K” Expressions with Solids or Liquids

Example #1—Writing K expression Example: Write the equilibrium constant expression for Fe 2 O 3 (s) + 3 H 2 (g)  2 Fe (s) + 3 H 2 O (g)

Fe 2 O 3 and Fe were not included in the K expression as they are solids! Example #1—Writing K expression Example: Write the equilibrium constant expression for Fe 2 O 3 (s) + 3 H 2 (g)  2 Fe (s) + 3 H 2 O (g)

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

Example #2—Calculating K Example: Solve for the equilibrium constant for Fe 2 O 3 (s) + 3 H 2 (g)  2 Fe (s) + 3 H 2 O (g) If at equilibrium [H 2 ] = 0.45 M and [H 2 O] = 0.18 M

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

In general…. If K is very large… [Products] [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 is very small… [Products] [Reactants] There is a much smaller ratio of products to reactants at equilibrium The reaction is said to “lie to the left” Meaning of Equilibrium Constant

Let’s Practice #1 Write the equilibrium constant expression for N 2 (g) + O 2 (g)  2 NO (g)

Let’s Practice #1 Write the equilibrium constant expression for N 2 (g) + O 2 (g)  2 NO (g)

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

Let’s Practice #2 If the equilibrium constant for N 2 (g) + O 2 (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.

Let’s Practice #3 If the equilibrium constant for N 2 (g) + O 2 (g)  2 NO (g) is 1.24 × and the equilibrium concentration of [N 2 ] = M and [O 2 ] = M, what is the equilibrium concentration of NO?

[NO] eq = M Let’s Practice #3 If the equilibrium constant for N 2 (g) + O 2 (g)  2 NO (g) is 1.24 × and the equilibrium concentration of [N 2 ] = M and [O 2 ] = M, what is the equilibrium concentration of NO?