Equilibrium

Equilibrium Constant (K Values) The equilibrium constant (Keq) is a number showing the relationship between the concentration of the products and the reactants. Each raised to the power of its coefficient in a balanced equation. True for reversible reactions (go both forwards and backwards).

Equilibrium Constant Equation Keq = [Products]coefficients [Reactants]coefficients Products on the top and reactants on the bottom Concentration (molarity) is represented by putting a [bracket] around a formula. Only use [GASES (g)] and [AQUEOUS (aq)] compounds in Keq expressions because SOLIDS (s) and LIQUIDS (l) have no concentrations, so they do not need to be included.

Example 3H2(g) + N2(g)  2NH3(g) Write the equilibrium expression for the reaction above: Keq = [NH3]2 [H2]3 [N2]1 C3H8(g) + 5 O2(g)  3 CO2(g) + 4 H2O(l) Write the equilibrium expression for the reaction above: The [H2O] is not used because it’s a liquid. Keq = [CO2]3 [C3H8] [O2]5

What does the calculated value for Keq tell us? A. Keq = 1 means that AT EQUILIBRIUM the [products] = [reactants] B. Keq > 1 means that AT EQUILIBRIUM the [products] > [reactants] C. Keq < 1 means that AT EQUILIBRIUM the [products] < [reactants]

Example 1 A mixture of plenty of gaseous H2O and solid C are placed in a large container and allowed to come to equilibrium according to the following reaction: C (s) + H2O (g)  CO (g) + H2 (g) The equilibrium concentrations of the species are as follows: [H2O] = 0.56 M, [CO] = 0.33 M, and [H2] = 0.040 M. What is the value of the equilibrium constant for this reaction? Does this equilibrium constant show that there are more products, more reactants, or equal amounts of the products and reactants? AND, how do you know?

Answer Keq = [CO] [H2] [H2O] Keq = [0.33][0.040] [0.56] = 0.024 There are more reactants; the value of Keq is smaller than 1

Example 2 The Keq = 6.67 Write the equilibrium expression Given: 2 SO2 (g) + O2 (g)  2 SO3 (g) The Keq = 6.67 Write the equilibrium expression Calculate the equilibrium concentration of [SO3] when… [SO2] = 0.25 M and [O2] = 0.20 M

Example 3 C (s) + H2O (g)  CO (g) + H2 (g) Keq = 0.024 2 SO2 (g) + O2 (g)  2 SO3 (g) Keq = 6.67 C (s) + H2O (g)  CO (g) + H2 (g) Keq = 0.024 Which reaction favors the products most? WHY