14.4-14.7 Equilibrium Constant in Terms of Pressure, Heterogeneous Equilibria, and the Reaction Quotient Jack Dykeman

Reference Table Page 3

Equilibrium Constants In gaseous reactions the partial pressure of a gas is proportional to its concentration so the equilibrium constant can also express equilibrium in terms of partial pressure in atmospheres. Let’s practice writing the Kc expression for the following reaction: This can be rewritten as the Kp expression because it consists of gases:

Kc vs Kp Kc and Kp are not always the same; however, using the ideal gas law and the law of mass action the following relationship can be derived when gases are acting ideally. Δn is the difference between the number of gaseous moles of products and reactants. If the number of moles reacting equals the number of moles produced then Δn=0 and Kc = Kp

Let’s Try a Practice Problem! Calculate Kp for the following reaction: at 298 K

Heterogeneous Equilibria: Reactions Involving Solids and Liquids Pure solids and pure liquids (denoted by (s) and (l) in chemical reactions) are excluded from equilibrium expressions because as the amount of solid or liquid is increased, its concentration doesn’t change. Gaseous and aqueous reactants and products are therefore the only ones considered in equilibrium expressions. Write the Kc expression for the following reaction: * Water is omitted because it is a pure liquid (l )

For which reaction does Kp = Kc A.) B.) C.) Practice Problem: For which reaction does Kp = Kc A.) B.) C.) There are three moles of reactants and products because Fe2O3 and Fe are both solids and therefore is not included.

Using Measured Equilibrium Concentrations to Calculate Equilibrium Constants If the concentrations of the products and reactants are given when a reaction reaches equilibrium, these values can be plugged into the equilibrium expression and a value for Kc can be determined. If this information isn’t given it is still possible to determine the equilibrium concentrations for a reaction if the initial concentrations of the reactants and the equilibrium concentration of one reactant or product is given. To do this we can set up an ICE table and determine the unknown concentrations.

ICE Tables [A] [B] Initial 1.00 0.00 Change Equilibrium 0.75 -0.25   [A] [B] Initial 1.00 0.00 Change Equilibrium 0.75 As the concentration of the reactants decrease the concentration of the products increases -0.25 +0.50 0.50 If we know that the concentration of a is 1.00 M and the concentration of B is 0.00 M and that at equilibrium the concentration of A is 0.75 M we can fill in the ice table for the unknown values. Because B has a coefficient of 2 we have to multiply the Change by 2 for B.

ICE Table Practice! Consider the following reaction: [CO] [H2] [CH3OH] A reaction mixture is at 730oC initially contains [CO] = 0.500 M and [H2] = 1.00 M. At equilibrium, the CO concentration is found to be 0.15 M. what is the value of Kc?   [CO] [H2] [CH3OH] Initial 0.500 1.000 0.000 Change Equilibrium 0.150  -0.35  -0.70 +0.35  0.30  0.35 Why is the concentration of CH3OH 0.000 M? The reaction hasn’t started yet which means there are no products.

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The Reaction Quotient: Predicting the Direction of Change To gauge the progress of a reaction relative to equilibrium, a quantity called the reaction quotient (Q) is used. The reaction quotient takes the same form as the equilibrium constant but the reaction doesn’t need to be at equilibrium. At a given temperature Kc has one value but Qc can have many. In a reaction containing only reactants Qc=0 In a reaction containing only products Qc=∞ In a reaction containing both with a molarity of one Qc=1 Q relative to K is a measure of the progress of a reaction towards equilibrium.

The Reaction Quotient: Predicting the Direction of Change Relationship Direction of Shift Q=K No Shift Q=0 Reaction Shifts Right Q=∞ Reaction Shifts Left Q>K Q<K

Q<K so the reaction proceeds to the right. Practice Problem Consider the following reaction: Kp=81.9 If PI = 0.114 atm, PCl = 0.102 atm, and PICl = 0.355 atm, determine which way the reaction will proceed. 2 2 Q<K so the reaction proceeds to the right.

AP Exam Question

ICE Table Practice! Consider the following reaction: [CO] [H2] [CH3OH] A reaction mixture is at 1700oC initially contains [CH4] = 0.115 M. At equilibrium, the mixture contains [C2H2] = 0.035 M. What is the value of the equilibrium constant?   [CO] [H2] [CH3OH] Initial 0.115 0.00 Change Equilibrium -0.070 + 0.035 +0.105  0.045  0.035  0.105

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