For a gas-phase reaction, which statement correctly describes Kp?

For a gas-phase reaction, which statement correctly describes Kp?

• A. Kp uses the partial pressures of gases in the equilibrium expression.
• B. Kp uses concentrations in molarity.
• C. Kp uses temperature only.
• D. Kp uses activity of solids.

Answer: (A)

Gas-phase equilibria use Kp, defined from the partial pressures of the gaseous species involved. In the expression, each gas appears to the power of its stoichiometric coefficient, while solids or liquids are omitted because their activities are effectively 1. For a general gas-phase reaction, the equilibrium constant is written as the product of the product gases' partial pressures raised to their coefficients divided by the product of the reactant gases' partial pressures raised to their coefficients. This shows that Kp is based on partial pressures, not molar concentrations. Temperature does affect the numerical value of Kp, but the form of the expression is still about partial pressures for gases. The activity of solids (and liquids) is not included in Kp, since their activities are taken as 1.

Gas-phase equilibria use Kp, defined from the partial pressures of the gaseous species involved. In the expression, each gas appears to the power of its stoichiometric coefficient, while solids or liquids are omitted because their activities are effectively 1. For a general gas-phase reaction, the equilibrium constant is written as the product of the product gases' partial pressures raised to their coefficients divided by the product of the reactant gases' partial pressures raised to their coefficients. This shows that Kp is based on partial pressures, not molar concentrations.

Temperature does affect the numerical value of Kp, but the form of the expression is still about partial pressures for gases. The activity of solids (and liquids) is not included in Kp, since their activities are taken as 1.