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  Reactants on the left and products on the right  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 3H 2 (g) + N 2 (g)  2NH 3 (g)  Write the equilibrium expression for the reaction above:  Keq = [NH 3 ] 2 [H 2 ] 3 [N 2 ] 1  C 3 H 8 (g) + 5 O 2 (g)  3 CO 2 (g) + 4 H 2 O(l)  Write the equilibrium expression for the reaction above:  The [H 2 O] is not used because it’s a liquid.  Keq = [CO 2 ] 3 [C 3 H 8 ] [O 2 ] 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 H 2 O and solid C are placed in a large container and allowed to come to equilibrium according to the following reaction: C (s) + H 2 O (g)  CO (g) + H 2 (g)  The equilibrium concentrations of the species are as follows: [H 2 O] = 0.56 M, [CO] = 0.33 M, and [H 2 ] = 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] [H 2 ] [H 2 O]  Keq = [0.33][0.040] [0.56]  =  There are more reactants; the value of Keq is smaller than 1

Example 2  Given: 2 SO 2 (g) + O 2 (g)  2 SO 3 (g)  The K eq = 6.67  Write the equilibrium expression  Calculate the equilibrium concentration of [SO 3 ] when…  [SO 2 ] = 0.25 M and [O 2 ] = 0.20 M

Example 3  2 SO 2 (g) + O 2 (g)  2 SO 3 (g)Keq = 6.67  C (s) + H 2 O (g)  CO (g) + H 2 (g)Keq = Which reaction favors the products most? WHY