Calculate Kp for The Following Equilibrium

The equilibrium constant Kp is a fundamental concept in chemical equilibrium that describes the ratio of product concentrations to reactant concentrations at equilibrium. This calculator helps you determine Kp for any given equilibrium reaction.

What is Kp?

Kp is the equilibrium constant expressed in terms of partial pressures of gases. It's particularly useful for reactions involving gaseous species. The value of Kp indicates the position of equilibrium:

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 follows the van't Hoff equation. For a reaction:

aA + bB ⇌ cC + dD

The equilibrium expression is:

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

How to Calculate Kp

To calculate Kp, you need:

The balanced chemical equation
Partial pressures of all gaseous species at equilibrium
Stoichiometric coefficients from the balanced equation

The calculation involves:

Raising each partial pressure to the power of its stoichiometric coefficient
Multiplying the product pressures
Dividing by the product of reactant pressures

Note: Kp is unitless because it's a ratio of pressures. For reactions involving solids or liquids, their partial pressures are considered constant and included in the equilibrium constant.

Kp vs Ka

Both Kp and Ka (equilibrium constant in terms of concentrations) describe equilibrium, but they differ in how they express concentrations:

Property	Kp	Ka
Units	Unitless	M^-Δn (where Δn is change in moles of gas)
Applicability	Gaseous systems	All systems
Relation	Kp = Ka × (RT)^Δn	Ka = Kp / (RT)^Δn

Example Calculation

Consider the reaction: 2NO(g) + O₂(g) ⇌ 2NO₂(g)

At equilibrium, the partial pressures are:

P_NO = 0.20 atm
P_O₂ = 0.15 atm
P_NO₂ = 0.45 atm

The calculation would be:

Kp = (P_NO₂²) / (P_NO² × P_O₂)

Kp = (0.45²) / (0.20² × 0.15)

Kp = (0.2025) / (0.06)

Kp = 3.375

This indicates the reaction strongly favors the formation of NO₂.

FAQ

What does a Kp value of 1 mean?

A Kp value of 1 means the reaction is at equilibrium, with equal concentrations of reactants and products.

How does temperature affect Kp?

Kp is temperature-dependent. According to the van't Hoff equation, Kp increases with temperature for exothermic reactions and decreases for endothermic reactions.

Can Kp be negative?

No, Kp cannot be negative because it's a ratio of positive pressures. However, the change in Kp (ΔKp) can be negative when the reaction shifts left with increasing temperature.