1. Kp When the reactants and products are gases, we can determine the equilibrium constant in terms of partial pressures. 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 …. PressureGas #1Gas #2

2. Equilibrium Pressure Pressure is measured in atmospheres. If the values at equilibrium are given in atmospheres, then solving for the constant is the same, but use K p instead of K c. What makes them different: Kc = equilibrium constant based on molarity and concentration. Kp = equilibrium constant based on partial pressures. Note: These two values are usually different even for the same reaction!

3. 2 NO 2 (g) 2 NO (g) + O 2 (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. Use the law of mass action to solve for Kp and similar steps because problem gives the pressures at equilibrium

4. 2 NO 2 (g) 2 NO (g) + O 2 (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. K p = [NO] 2 [O 2 ] [NO 2 ] 2 K p = [0.270] 2 [0.100] [0.425] 2 Kp = 0.0404

5. Relationship between K c and K p If values at equilibrium are not in atmospheres, We use the Ideal Gas Law, and derive the relationship between K c and K p. K p = Equilibrium constant in partial pressure K c = 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) K p = K c (RT)  n

6. CO(g) + Cl 2 (g)  COCl 2 (g) At equilibrium the concentrations at 74 0 C are [CO] = 0.012 M, [Cl 2 ] = 0.054 M, and [COCl 2 ] = 0.14 M. Calculate the equilibrium constants K c and K p. Start with the equilibrium expression for K c : K c = [COCl 2 ] [CO][Cl 2 ] K c = [0.14 M] = 216 [0.012][0.054]

7. CO(g) + Cl 2 (g)  COCl 2 (g) At equilibrium the concentrations at 74 0 C are [CO] = 0.012 M, [Cl 2 ] = 0.054 M, and [COCl 2 ] = 0.14 M. Calculate the equilibrium constants K c and K p. To solve for K p we use the Ideal gas law eq. K p = K c (RT)  n K p = 216 x (.082 x 347) (1-2) Kp = 7.6

8. Practice Problems Kp