How do you calculate the pressure equilibrium constant?

The pressure equilibrium constant, Kp, is calculated using the partial pressures of the reactants and products at equilibrium, following a specific formula based on the balanced chemical equation.

Here's a breakdown:

1. Understand the Balanced Chemical Equation:

First, you need the balanced chemical equation for the reversible reaction. For example:

aA + bB ⇌ cC + dD

Where A and B are reactants, C and D are products, and a, b, c, and d are their respective stoichiometric coefficients.

2. Determine the Partial Pressures at Equilibrium:

You must know the partial pressures of all reactants and products at equilibrium. Partial pressure is the pressure exerted by an individual gas in a mixture of gases.

3. Apply the Kp Formula:

The pressure equilibrium constant (Kp) is calculated using the following formula:

Kp = (P_C^c P_D^d) / (P_A^a P_B^b)

Where:

• P_A, P_B, P_C, and P_D are the partial pressures of reactants A, B, and products C, D at equilibrium, respectively.
• a, b, c, and d are the stoichiometric coefficients from the balanced chemical equation.

Example:

Consider the reaction:

PCl₅(g) ⇌ PCl₃(g) + Cl₂(g)

Kp = (P_PCl3 * P_Cl2) / P_PCl5

To calculate Kp, you would:

1. Measure or be given the partial pressures of PCl₅, PCl₃, and Cl₂ at equilibrium.
2. Plug those values into the Kp expression: For example, if P_PCl5 = 0.5 atm, P_PCl3 = 0.2 atm, and P_Cl2 = 0.3 atm, then:

Kp = (0.2 atm * 0.3 atm) / 0.5 atm = 0.12 atm

Key Considerations:

• Units: Kp is often expressed without units, but it's important to be aware of the pressure units used in the calculation (e.g., atm, kPa).
• Equilibrium: The partial pressures must be at equilibrium.
• Gases: Kp only applies to reactions involving gases.
• Temperature: Kp is temperature-dependent. A change in temperature will change the value of Kp.

In summary, to calculate Kp, you need the balanced chemical equation and the partial pressures of all gaseous reactants and products at equilibrium. Then, simply apply the formula, raising each partial pressure to the power of its stoichiometric coefficient.