Equilibrium Constant (Kc vs Kp) - NEET Notes, Formula & Common Mistakes
Equilibrium·2 min read·NEET 2026
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.
Frequently Asked Questions
What is the main difference between Kc and Kp?▾
Kc is the equilibrium constant expressed using the molar concentrations of reactants and products. Kp is the equilibrium constant expressed using the partial pressures of gaseous reactants and products. Both constants describe the state of equilibrium for a reversible reaction but use different units of measurement for the species involved.
When is Kp equal to Kc?▾
Kp is equal to Kc when the change in the number of moles of gaseous species (Δng) in the balanced chemical equation is zero. In such cases, the term (RT)^Δng becomes (RT)^0 = 1, simplifying the relation Kp = Kc(RT)^Δng to Kp = Kc.
How do I calculate Δng for a reaction?▾
Δng is calculated as the sum of the stoichiometric coefficients of all gaseous products minus the sum of the stoichiometric coefficients of all gaseous reactants. It's essential to only consider species that are in the gaseous phase; solids, liquids, and aqueous solutions are excluded from this calculation.
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