Equilibrium Pressure If the values at equilibrium are given in partial pressure, then solving for the constant is the same, but use Kp instead of Kc. What makes them different: Kc = equilibrium constant based on molarity and concentration. Kp = equilibrium constant based on partial pressures in atm.

The mole ratio in a mixture gas is proportional to the partial pressures of each of the gases. Dalton’s Law of Partial Pressures – the total pressure in a system is equal to the sum of the individual pressures. Total = Pressure of + Pressure of …. Pressure Gas #1 Gas #2

Kp = 0.0404 Kp = [NO]2 [O2] [NO2]2 Kp = [0.270]2 [0.100] [0.425]2 2 NO2 (g) 2 NO (g) + O2 (g) Determine the Kp for the above reaction when at equilibrium, the pressure of nitrogen dioxide is 0.425 atm, nitrogen monoxide is 0.270 atm, and oxygen is 0.100 atm. Kp = [NO]2 [O2] [NO2]2 Kp = [0.270]2 [0.100] [0.425]2 Kp = 0.0404

Relationship between Kc and Kp We use the Ideal Gas Law, and derive the relationship between Kc and Kp. Kp = Kc (RT)n Kp = Equilibrium constant in partial pressure Kc = Equilibrium constant in molar concentrations R = Ideal Gas constant (always 0.082) T = Temperature (in Kelvin) n = Change in moles of GASES ONLY (moles of gas products) – (moles of gas reactants)

2 NO2 (g) 2 NO (g) + O2 (g) At equilibrium, the pressure of nitrogen dioxide is 0.425 atm, nitrogen monoxide is 0.270 atm, and oxygen is 0.100 atm. Determine the Kp for the above reaction.

CO(g) + Cl2(g)  COCl2(g) [CO][Cl2] [0.012][0.054] At equilibrium the concentrations at 740C are [CO] = 0.012 M, [Cl2] = 0.054 M, and [COCl2] = 0.14 M. Calculate the equilibrium constants Kc and Kp. Start with the equilibrium expression for Kc: Kc = [COCl2] [CO][Cl2] Kc = [0.14 M] = 216 [0.012][0.054]

CO(g) + Cl2(g)  COCl2(g) Kp = 216 x (.082 x 347)(1-2) Kp = 7.6 At equilibrium the concentrations at 740C are [CO] = 0.012 M, [Cl2] = 0.054 M, and [COCl2] = 0.14 M. Calculate the equilibrium constants Kc and Kp. To solve for Kp we use the Ideal gas law eq. Kp = Kc (RT)n Kp = 216 x (.082 x 347)(1-2) Kp = 7.6