What is Kp?

What is Kp?

• A. The equilibrium constant expressed in terms of pressure
• B. The equilibrium constant expressed in terms of molarity
• C. The rate constant for gas-phase reactions
• D. The ratio of products to reactants at any moment

Answer: (A)

Kp is the equilibrium constant expressed in terms of the partial pressures of the gaseous species. For reactions involving gases, using partial pressures as the basis for the equilibrium expression is natural because, for ideal gases, the activity of a gas is proportional to its partial pressure. This makes Kp a convenient way to describe how the balance of products and reactants shifts with pressure and temperature. The relationship to the concentration-based constant Kc is Kp = Kc (RT)^(Δn_gas), where Δn_gas is the change in moles of gas on forming products minus reactants. This is distinct from a rate constant, which governs the speed of a reaction rather than its position at equilibrium, and from the reaction quotient Q, which gives the ratio of products to reactants at any moment and only equals the equilibrium constant when the system is at equilibrium.

Kp is the equilibrium constant expressed in terms of the partial pressures of the gaseous species. For reactions involving gases, using partial pressures as the basis for the equilibrium expression is natural because, for ideal gases, the activity of a gas is proportional to its partial pressure. This makes Kp a convenient way to describe how the balance of products and reactants shifts with pressure and temperature. The relationship to the concentration-based constant Kc is Kp = Kc (RT)^(Δn_gas), where Δn_gas is the change in moles of gas on forming products minus reactants.

This is distinct from a rate constant, which governs the speed of a reaction rather than its position at equilibrium, and from the reaction quotient Q, which gives the ratio of products to reactants at any moment and only equals the equilibrium constant when the system is at equilibrium.