What is the Kc to Kp Converter?
Quick Answer: A tool that converts the equilibrium constant expressed in concentration (Kc) to the one expressed in pressure (Kp) using the formula Kp = Kc (RT)Δn. Input temperature, change in moles of gas (Δn), and Kc to get Kp. A visual indicates the gaseous equilibrium shift.
Theory of Kc and Kp Relationship
For a gaseous reaction aA + bB ⇌ cC + dD, the equilibrium constants in terms of concentration (Kc) and pressure (Kp) are related by:
Kp = Kc (RT)Δn
where Δn = (c + d) − (a + b) (change in moles of gaseous species), R is the ideal gas constant, and T is the absolute temperature in Kelvin. When Δn = 0, Kp = Kc. The value of R depends on the pressure units: 0.0821 L·atm/(mol·K) for atmospheres, or 8.314 J/(mol·K) for pressure in Pascal (though Kp in Pa would use R = 8.314 m3·Pa/(mol·K) with appropriate volume conversion). The tool supports both common R values.
Step-by-Step Examples
Example 1: Reaction with Δn = 1
- Set Kc = 1.0, T = 298 K, Δn = 1, R = 0.0821.
- Kp = 1.0 × (0.0821 × 298)1 = 24.46.
- The display shows Kp in atm units.
Example 2: No change in moles (Δn = 0)
- Δn = 0, Kc = 2.5. Kp = 2.5 (identical).
Example 3: Reverse conversion
- Click "Swap" to convert Kp back to Kc using Kc = Kp / (RT)Δn.
Frequently Asked Questions
What does Δn represent?
Δn is the difference between the total moles of gaseous products and total moles of gaseous reactants. Solids and liquids are ignored.
Which R value should I use?
Use 0.0821 L·atm/(mol·K) if Kp is in atmospheres. Use 8.314 J/(mol·K) if pressure is in Pascal (or N/m2), noting that Kp in Pa uses R in m3·Pa/(mol·K) which is equivalent to 8.314.
Can I convert Kp back to Kc?
Yes, the formula is Kc = Kp / (RT)Δn. The Swap button performs this calculation.
Why is Kp independent of pressure when Δn = 0?
Because (RT)0 = 1, so the two constants are numerically equal, regardless of pressure.
Is the converter accurate for non‑ideal gases?
No, this relation is exact for ideal gases. For real gases, fugacity corrections are needed.