What NEET Asks
- Direct questions on the units of Kc or Kp for a given reaction are frequent.
- Numerical problems involving the relation between Kp and Kc often require correct unit handling.
- Conceptual questions on when Kp and Kc are unitless are common.
Key Points
- Kc is the equilibrium constant expressed in terms of molar concentrations (mol/L or M). Its unit depends on Δn.
- Kp is the equilibrium constant expressed in terms of partial pressures (atm or bar). Its unit also depends on Δn.
- Δn is the change in the number of moles of gaseous products minus gaseous reactants (Δn = n_products(gas) - n_reactants(gas)).
- If Δn = 0, both Kc and Kp are dimensionless (unitless).
- Pure solids and pure liquids are omitted from Kc and Kp expressions as their concentrations/activities are constant.
- The standard unit for concentration is mol/L, and for pressure is atm or bar.
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: Gas constant (0.0821 L atm mol⁻¹ K⁻¹ if pressure in atm, 8.314 J mol⁻¹ K⁻¹ if pressure in Pa).
- T: Absolute temperature in Kelvin.
- Δn: (Sum of stoichiometric coefficients of gaseous products) - (Sum of stoichiometric coefficients of gaseous reactants).
Common Mistakes
- Students often include stoichiometric coefficients of solids or liquids when calculating Δn. Remember, only gaseous species count!
- Don't confuse the units of R (Gas Constant) when applying Kp = Kc(RT)^Δn. Use R = 0.0821 L atm mol⁻¹ K⁻¹ for pressure in atm.
- Incorrectly assigning units to Kc or Kp when Δn is negative or zero.
Rapid Revision
Units of Kc = (mol/L)^Δn. Units of Kp = (atm)^Δn. For Δn=0, both are unitless. Always check only gaseous species for Δn. Remember Kp = Kc(RT)^Δn for interconversion.