Kp and Kc
For gas reactions, K can be written with partial pressures (Kp) or concentrations (Kc). They are related by Kp = Kc(RT)^Δn, where Δn is moles of gas on the product side minus the reactant side.
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Analogy
The same measurement in two units, like miles and kilometres, with a conversion factor.
Example
Δn = 2 − (1 + 3) = −2Kp = Kc (RT)⁻²If Δn = 0, Kp = Kc
In practice
Industrial gas reactions are often described with Kp because pressures are easy to measure.
Practice
Flashcards
- Relation between Kp and Kc?Kp = Kc(RT)^Δn.
Questions people ask
When is Kp equal to Kc?
When the number of gas moles is the same on both sides, Δn = 0, and (RT) raised to the power zero is 1. Examples are H₂ + I₂ ⇌ 2HI and N₂ + O₂ ⇌ 2NO. In other reactions the two constants differ.
Which value of R do I use for Kp?
Use R = 0.08206 L·atm/(mol·K) when Kc uses mol/L and Kp uses atm, with T in kelvin. Keep the units consistent with your data. The conversion Kp = Kc(RT)^Δn only works if Kc is in mol/L and pressure is in atmospheres.