Calculate Kp for The Following Reaction at 298 K

What is Kp?

Kp (equilibrium constant in terms of partial pressures) is a measure of the equilibrium position of a chemical reaction when expressed in terms of partial pressures of gases. It's particularly useful for gas-phase reactions where concentrations can be expressed as partial pressures.

The value of Kp indicates the extent to which a reaction proceeds to completion:

• If Kp > 1, the reaction favors products
• If Kp = 1, the reaction is at equilibrium
• If Kp < 1, the reaction favors reactants

Kp is temperature-dependent and must be calculated at a specific temperature, typically 298 K (25°C) unless otherwise specified.

How to Calculate Kp

To calculate Kp, you need to know the partial pressures of the products and reactants at equilibrium. The general formula is:

Where:

• P represents the partial pressure of each gas
• a, b, c, d are the stoichiometric coefficients from the balanced chemical equation

For reactions with multiple products or reactants, you multiply the partial pressures of each product and divide by the partial pressures of each reactant, each raised to the power of their respective stoichiometric coefficients.

Example Calculation

Consider the reaction: 2H₂(g) + O₂(g) → 2H₂O(g)

At equilibrium, the partial pressures are:

• H₂: 0.2 atm
• O₂: 0.3 atm
• H₂O: 0.8 atm

Using the formula:

This result indicates the reaction strongly favors the formation of water.

Interpretation of Results

Interpreting Kp values requires understanding the context of the reaction:

• Large Kp values (>100) indicate reactions that proceed almost to completion
• Moderate Kp values (1-100) show reactions that reach equilibrium with significant amounts of both products and reactants
• Small Kp values (<0.01) suggest reactions that barely proceed

Remember that Kp is temperature-dependent. For reactions not at 298 K, you would need to use the van't Hoff equation to adjust the value.