What is the difference between Kc and Kp

Kc is defined as $K_c = \frac{[products]^{\nu_{p}}}{[reactants]^{\nu_{r}}}$ using molarity. Kp replaces each concentration with the corresponding partial pressure: $K_p = \frac{(P_{products})^{\nu_{p}}}{(P_{reactants})^{\nu_{r}}}$. Use Kc when concentrations are measured, and Kp when only gas pressures are known.

Relation between Kc and Kp

Convert Kc to Kp for a gas‑phase reaction

1. 1Write the balanced equation and count gas moles on each side.
2. 2Calculate $\Delta n = \sum \nu_{gas,products} - \sum \nu_{gas,reactants}$.
3. 3Apply $K_p = K_c(RT)^{\Delta n}$ using R = 0.08206 L·atm·mol⁻¹·K⁻¹ and the temperature in kelvin.

Key points to remember

• Kc uses molarity, Kp uses pressure.
• $\Delta n$ can be zero; then $K_p = K_c$.
• Units cancel in the ratio, so Kc and Kp are dimensionless in practice.

Example calculation for 2NH₃(g) ⇌ N₂(g)+3H₂(g) at 400 K