Kp = Kc(RT)^Δn Derivation - NEET Notes, Formula & Common Mistakes
Equilibrium·2 min read·NEET 2026
What NEET Asks
Expect direct application of Kp = Kc(RT)^Δn in numerical problems.
Questions on calculating Δn for various reactions are frequent.
Conceptual questions about conditions where Kp = Kc or the role of non-gaseous species.
Key Points
Kp is the equilibrium constant expressed in terms of partial pressures of gaseous species.
Kc is the equilibrium constant expressed in terms of molar concentrations of species.
The relationship Kp = Kc(RT)^Δn connects these two constants for gaseous reactions.
Δn = (sum of stoichiometric coefficients of gaseous products) - (sum of stoichiometric coefficients of gaseous reactants).
Only gaseous components are considered when calculating Δn. Solids and liquids are excluded.
If Δn = 0, then Kp = Kc.
Must-Know Formula / Reaction
Kp = Kc(RT)^Δn
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 for pressure in atm; 8.314 J mol⁻¹ K⁻¹ or kPa m³ mol⁻¹ K⁻¹ for pressure in Pa/kPa).
T: Absolute temperature in Kelvin.
Δn: Change in the number of moles of gaseous species (moles of gaseous products - moles of gaseous reactants).
Common Mistakes
Students often include stoichiometric coefficients of solids or liquids when calculating Δn. Remember, only gaseous species contribute.
Using the wrong value of R or incorrect units for temperature (e.g., Celsius instead of Kelvin).
Confusing the order of products and reactants when calculating Δn (it's always products - reactants).
Rapid Revision
The relation Kp = Kc(RT)^Δn is fundamental for gaseous equilibria. Remember Δn considers only gaseous moles (products minus reactants), and T must be in Kelvin. Kp equals Kc when Δn is zero.
Frequently Asked Questions
What does Δn represent in the Kp = Kc(RT)^Δn relationship?▾
Δn represents the change in the number of moles of gaseous species during a reaction. It is calculated as the sum of the stoichiometric coefficients of gaseous products minus the sum of the stoichiometric coefficients of gaseous reactants.
Under what conditions does Kp become equal to Kc?▾
Kp becomes equal to Kc when Δn (the change in the number of moles of gaseous species) is zero. This occurs when the total number of moles of gaseous products is equal to the total number of moles of gaseous reactants.
Which value of R should be used in the Kp = Kc(RT)^Δn formula for NEET?▾
For calculations where pressure is typically in atmospheres, R = 0.0821 L atm mol⁻¹ K⁻¹ is commonly used. If pressures are given in Pascal or kPa, then R = 8.314 J mol⁻¹ K⁻¹ (or kPa m³ mol⁻¹ K⁻¹) is appropriate. Always ensure units are consistent.
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