The Equilibrium Constant, Heterogeneous Equilibria, The Equilibrium Constant, and The Reaction Constant 14.4 - 14.7 Pages 658 - 667 John Jackson

Expressing The Equilibrium Constant in Terms of Pressure 14.4 Expressing The Equilibrium Constant in Terms of Pressure In gaseous reactions, the partial pressure of a gas (the pressure that would be exerted by one of the gases in a mixture if it occupied the same volume on its own) is proportional to its concentration Therefore: Concentration Partial Pressure

14.4 If the gas is acting like an ideal gas, eventually one can derive this formula in case Kp and Kc aren’t equal (concentration in molarity and partial pressure clearly aren’t always equal) R is the constant R from PV=nRT (use the one that says .08206 for these) Δn is the sum of all the coefficients of the gaseous products minus the sum of all the coefficients of the gaseous reactants

Practice :) 14.4 Find Kp for this reaction: N2(g) + 3H2(g) ⇌ 2NH3(g) Kc = 3.7 x 108 at 298 K Kp = 3.7 x 108 (0.08206 x 298)-2 Kp = 6.2 x 105

Heterogeneous Equilibria 14.5 Heterogeneous Equilibria When reactions involve solids or liquids, you don’t include them in the equilibrium expression Solids and liquids don’t have a concentration that’s subject to change, only a density, which is already accounted for in K (don’t dig into it too much it just sorta works itself out) Ex: (Notice water is excluded)

14.5 Practice :) Write an Equilibrium constant for the following reaction: 2CO (g) ⇌ CO2 (g) + C (s) CO2 KC = ________ [ CO ]2

14.6 Calculating the Equilibrium Constant from Measured Equilibrium Concentrations One can calculate the actual value of Kc if they are given the concentrations of the products and the reactants when the reaction is at equilibrium by plugging the values into the equilibrium expression This can also be done with only the initial concentrations of the reactants and the concentration of only one product

To do so….. We need an ICE Table Initial Change Equilibrium 14.6 To do so….. We need an ICE Table For Example: If we are given a starting concentration of 1.00M A and 0.00M B and are told A equals 0.75M when equilibrium is reached, we can deduce the other values. We can then use these values to deduce K

The Reaction Quotient: Predicting the Direction of Change 14.7 The Reaction Quotient: Predicting the Direction of Change When a reaction mixture not at equilibrium contains both reactants and products, we use the reaction quotient ( Qc ) A reaction quotient ( Qc ) is similar to an equilibrium constant (K) but does not need to be at equilibrium, and can have many values, whereas (K) can only have one for a given temperature Example: a mixture with only products would have ( Qc ) = ∞ A mixture with only reactants would have ( Qc ) = 0 A mixture of reactants and products, both at a concentration of 1.0 M would have ( Qc ) = 1

Q < K reaction goes to the right (toward products) 14.7 Q < K reaction goes to the right (toward products) Q > K reaction goes to the left (toward reactants) Q = K reaction is at equilibrium

Practice :) 14.7 Is the reaction going left, right, or in equilibrium? Q = 12.5 K = 18 K = 34 Q = 6 K = 12.56 Q = 12.57 K = 2 Q = 2 K = -4 Q = 3 Right Right Left Equilibrium TRICK QUESTION (values of K and Q can’t be negative)