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.