Kp relation Calculator
Relationship between Kp and Kc.
Formula first
Overview
The Kp relation defines the mathematical link between equilibrium constants derived from partial pressures and those derived from molar concentrations. This relationship accounts for the work done by or on a gas system during a reaction where the total number of gaseous moles changes, assuming ideal gas behavior.
Symbols
Variables
K_c = Equilibrium Kc, R = Gas Constant, T = Temperature, n = Delta n, K_p = Equilibrium Kp
Apply it well
When To Use
When to use: Apply this equation when converting between pressure-based (Kp) and concentration-based (Kc) constants for reactions involving gases. It requires the reaction temperature in Kelvin and the change in moles of gas, calculated as product gas moles minus reactant gas moles.
Why it matters: This allows scientists to predict equilibrium positions across different measurement units, which is essential for industrial processes like the Haber-Bosch ammonia synthesis. It clarifies why pressure changes only affect the equilibrium position when there is a net change in gaseous moles (dn ≠ 0).
Avoid these traps
Common Mistakes
- Forgetting the sign of Δ n.
- Using wrong R value.
One free problem
Practice Problem
For the synthesis of ammonia, N₂(g) + 3H₂(g) ⇌ 2NH₃(g), the value of Kc is 0.045 at 500 K. Using R = 0.0821 L·atm/(mol·K), calculate the value of Kp.
Solve for:
Hint: Calculate the change in moles (dn) by subtracting reactant gas moles (1+3) from product gas moles (2).
The full worked solution stays in the interactive walkthrough.
References
Sources
- Atkins' Physical Chemistry
- McQuarrie, Simon, Physical Chemistry: A Molecular Approach
- Wikipedia: Equilibrium constant (specifically the section 'Relationship between Kp and Kc')
- NIST CODATA
- IUPAC Gold Book
- McQuarrie & Simon, Physical Chemistry: A Molecular Approach
- Brown, LeMay, Bursten, Chemistry: The Central Science
- Edexcel A-Level Chemistry — Equilibria