Equilibrium Expressions

Equilibrium When the rate of the forward and reverse reactions are equal. The equilibrium constant K eq is a value that shows the balance between the [reactants] and the [products]. K eq can help you determine which chemicals are in higher concentration

K eq

Example Write the equilibrium expression and calculate the Kc value for N 2 (g) + 3 H 2 (g)  2 NH 3 (g) when [N 2 ]= 2.0 M, [H 2 ]= 1.00 M and [NH 3 ]= 2.5M at equilibrium.

Pressure and K

Example Write the equilibrium expression from the previous example in terms of pressure and calculate K p if the pressures of each gas at equilibrium are 2.00 atm.

Rules for K eq Only aqueous (aq) or gaseous (g) chemicals can be entered into the equation. – Solids and liquids are left out because they are pure and do not have a concentration or pressure. We can write the expression for the reverse reaction by placing the reactants on the top and products on the bottom

Example Write the equilibrium expression for the reverse reaction in terms of concentration. N 2 (g) + 3 H 2 (g)  2 NH 3 (g)

Other equilibrium constants K eq for general equilibrium constant K c concentration K p pressure K a acid K b base K sp solubility product (for salts dissolving) The only reason there are so many is to help clarify what type of chemicals are being dealt with in the equilibrium

What does K tell us? When K >>1 then the products are favored. Equilibrium lies to the right (products side) When K<<1 then the reactants are favored. Equilibrium lies to the left (reactants side) When K ≈ 1 then the concentration of products and reactants is about equal.

Examples For the first example are the products or reactants favored? Which side of the reaction does the equilibrium lie on? For the second example are the products or reactants favored? Which side of the reaction does the equilibrium lie on?