Kp
Calculate Kp equilibrium constant from partial pressures for gas-phase reactions aA + bB ⇌ cC + dD. Free chemistry Kp calculator with optional Kc conversion.
Kc Conversion (optional)
About This Calculator
The Kp Calculator computes the equilibrium constant in terms of partial pressures for gas-phase reversible chemical reactions of the form aA(g) + bB(g) ⇌ cC(g) + dD(g). This tool is essential for chemistry students studying chemical equilibrium, researchers in physical chemistry, and professionals in industrial gas-phase processes who need to determine equilibrium positions from experimentally measured partial pressures.
The equilibrium constant Kp is calculated using the formula Kp = (P_C^c × P_D^d) / (P_A^a × P_B^b), where P_A, P_B are the equilibrium partial pressures of the gaseous reactants, P_C, P_D are the equilibrium partial pressures of the gaseous products, and a, b, c, d are their respective stoichiometric coefficients. The calculator also displays log₁₀(Kp), the change in moles of gas (Δn), and includes an optional Kc conversion section using the relationship Kp = Kc(RT)^Δn with R = 0.082057 L·atm/mol·K.
If Kp > 1, the equilibrium mixture contains more products than reactants, meaning the forward reaction is favored. If Kp < 1, the equilibrium favors the reactants. The Δn value indicates the change in the number of gas molecules — when Δn = 0, Kp equals Kc directly. The calculator provides a visual bar chart comparing product and reactant partial pressure contributions, plus a pie chart showing the equilibrium balance.
Regional Notes
India (IN): Kp and Kc calculations are covered extensively in CBSE and NCERT chemistry curricula for Classes 11 and 12. The relationship between Kp and Kc using Kp = Kc(RT)^Δn is a key concept in physical chemistry for competitive exams like JEE (Advanced and Main) and NEET. Students should use R = 0.0821 L·atm/mol·K and temperature in Kelvin.
United States (US): The equilibrium constant Kp is taught in AP Chemistry and undergraduate General Chemistry courses. The American Chemical Society (ACS) includes gas-phase equilibrium computations in standardized exams. Partial pressures are typically expressed in atmospheres (atm) in textbook problems, and the standard state for gases is 1 atm.
United Kingdom (UK): Kp calculations are part of A-level Chemistry curricula across OCR, AQA, and Edexcel exam boards. Students learn to write Kp expressions for homogeneous gas equilibria and to calculate Kp from equilibrium partial pressures or mole fractions with total pressure. The ideal gas equation PV = nRT is used to relate partial pressures to concentrations.
Frequently Asked Questions
What is Kp in chemistry?
Kp is the equilibrium constant expressed in terms of partial pressures of gaseous reactants and products. For a reaction aA(g) + bB(g) ⇌ cC(g) + dD(g), Kp = (P_C^c × P_D^d) / (P_A^a × P_B^b), where each partial pressure is raised to the power of its stoichiometric coefficient.
How do you calculate Kp from partial pressures?
Enter the stoichiometric coefficients and equilibrium partial pressures (in atm) of each gaseous reactant (A, B) and product (C, D) in the reaction aA + bB ⇌ cC + dD. The calculator computes Kp = (P_C^c × P_D^d) / (P_A^a × P_B^b). A Kp value greater than 1 means products dominate at equilibrium, while less than 1 means reactants dominate.
What is the relationship between Kp and Kc?
Kp and Kc are related by the equation Kp = Kc(RT)^Δn, where R is the ideal gas constant (0.082057 L·atm/mol·K), T is the temperature in Kelvin, and Δn = (c + d) - (a + b) is the change in the number of moles of gas. When Δn = 0, Kp = Kc. Our calculator can convert between Kp and Kc when you optionally provide Kc and temperature values.
What does Δn represent in Kp calculations?
Δn is the change in the number of moles of gas during the reaction, calculated as (sum of product coefficients) - (sum of reactant coefficients). If Δn > 0, more gas molecules are produced than consumed. If Δn < 0, gas molecules are consumed. If Δn = 0, Kp = Kc and pressure changes do not affect equilibrium position.
When should I use Kp instead of Kc?
Use Kp when dealing with gas-phase reactions where partial pressures are measured or known, rather than molar concentrations. Kp is typically used when pressures are expressed in atmospheres (atm) or bar. For reactions involving only gases, Kp is often more convenient since partial pressures are directly measurable from manometers or pressure gauges.
How does temperature affect Kp?
Temperature changes affect Kp according to the van't Hoff equation. For exothermic reactions, increasing temperature decreases Kp. For endothermic reactions, increasing temperature increases Kp. The relationship is ln(K₂/K₁) = -ΔH°/R × (1/T₂ - 1/T₁), where ΔH° is the standard enthalpy change of the reaction.
Can Kp be used for reactions with solids or liquids?
In equilibrium expressions for heterogeneous reactions, pure solids and liquids have constant activity (considered as 1) and do not appear in the Kp expression. Only gaseous species with measurable partial pressures are included in the Kp calculation. For example, in CaCO₃(s) ⇌ CaO(s) + CO₂(g), Kp = P_CO₂.
What units does Kp have?
Kp can have units of pressure raised to the power of Δn (change in gas moles). When Δn = 0, Kp is dimensionless. When Δn ≠ 0, Kp has units of (atm)^Δn. However, since these are ratios relative to a standard state of 1 atm, the numerical value is often reported without units in equilibrium calculations.