What NEET Asks
- Numerical problems directly applying the Kp and Kc relation are frequent.
- Conceptual questions on conditions where Kp = Kc or Δng = 0 are common.
- Often combined with Le Chatelier's principle or reaction stoichiometry. Expect 1-2 questions from Equilibrium annually.
Key Points
- Kc is the equilibrium constant expressed in terms of molar concentrations of species.
- Kp is the equilibrium constant expressed in terms of partial pressures for purely gaseous reactions.
- Kp and Kc are related by the equation Kp = Kc(RT)^Δng.
- Δng = (sum of stoichiometric coefficients of gaseous products) - (sum of stoichiometric coefficients of gaseous reactants).
- Solids and pure liquids are not included in Δng calculation or the equilibrium constant expressions.
- The units of Kc and Kp vary depending on the value of Δng.
Must-Know Formula / Reaction
Kp = Kc (RT)^Δng
- Kp: Equilibrium constant in terms of partial pressures.
- Kc: Equilibrium constant in terms of molar concentrations.
- R: Universal gas constant (0.0821 L atm mol⁻¹ K⁻¹ or 8.314 J mol⁻¹ K⁻¹). Use 0.0821 if pressure is in atm and volume in L.
- T: Absolute temperature in Kelvin.
- Δng: Change in moles of gaseous products minus gaseous reactants.
Common Mistakes
- Students often forget to convert temperature from Celsius to Kelvin.
- Don't include stoichiometric coefficients of solid or liquid phases in Δng calculation.
- Incorrectly calculating Δng by summing all product/reactant coefficients instead of only gaseous ones.
Rapid Revision
Remember Kp = Kc(RT)^Δng. Δng is crucial: (gaseous product moles) - (gaseous reactant moles). If Δng = 0, then Kp = Kc. Ensure R's units match pressure/volume. Always convert T to Kelvin.