## Chem – College: Pressure and the Equilibrium Constant (Kp)

What is the equilibrium constant Kp and how does it relate to pressure?

You can also talk about a chemical equation and its equilibrium in the form of pressure. Most books and teachers will call this equilibrium constant Kp. It works the same as any other equilibrium constant. All the same rules apply as in the equilibrium equations we have been solving for so far. The only difference is you can use the values of pressure (usually in atm, torr, mmHg, or Pa). For the examples I give I will use the units of atm. Because Kp does not directly involve concentration the Kp equation for the equilibrium constant is slightly different. You don’t use brackets in the equation so the equation looks like the one below.

For the chemical equation: N2(g) + 3 H2(g) <—-> 2 NH3(g)

 Kp = ( NH3 )2 ( N2 ) ( H2 )3

VIDEO Solving the Equilibrium Equation (Kp) Demonstrated Example 1: What is the equilibrium constant of the equation below when the pressure at equilibrium of C4 is 1atm, O2 is 4atm, and CO2 is 5atm?

C4(s) + 4 O2(g) <—-> 4 CO2(g)

What is the equilibrium equation look like with chemicals?

 Kp = ( CO2 )4 ( O2 )4

C4 is not shown above because it is a solid. Replace the chemicals with their corresponding pressure (numbers).

 Kp = ( 5atm )4 ( 4atm )4

Calculate in steps. First the exponents.

 Kp = ( 625atm4 ) ( 256atm4 )

Then divide top and bottom

 Kp = 2.44 1

COMPLETE ANSWER: Kp = 2.44

PRACTICE PROBLEMS: Solve the problems below for the Kp or different pressures.

What is the Kp of the equation below when the pressure at equilibrium of N2O3 is 20atm, N2 is 5atm, and O2 is 9atm?

2 N2O3(g) <——> 2 N2(g) + 3 O2(g)

Answer: Kp = 45 atm3

What is the equilibrium constant of the equation below when the pressure at equilibrium of Ba(OH)2 is 0.1atm, HCl is 0.2 atm, and BaCl2 is 0.4atm?

Ba(OH)2(aq) + 2 HCl(aq) <—-> 2 H2O(l) + BaCl2(aq)

Answer: Kp = 100 atm-2

What is the pressure of O2 in the equation below when the pressure at equilibrium of H2O is 0.7atm, H2 is 40atm, and Kp is 65atm?

2 H2O(g) <—-> 2 H2(g) + O2(g)

Answer: pressure of O2 = 0.02 atm

What is the pressure of NH3 in the equation below when the pressure at equilibrium of OF2 is 0.5 atm, N2F4 is 1.3atm, O2 is 2.5atm, H2 is 0.8atm, and the Kp is 2.7 * 102atm?

2 OF2(g) + 2 NH3(g) <—> N2F4(g) + O2(g) + 3 H2(g)

Answer: pressure of NH3 = 0.157 atm  