Relationship between Kc and Kp
Deriving and applying the mathematical relationship between Kc and Kp for gaseous reactions.
What is Relationship between Kc and Kp?
The equilibrium constant expressed in terms of molar concentrations of reactants and products.
Key formula / rule: Relationship between Kp and Kc
Key points
- Understand the difference between Kc and Kp.
- Derive the relationship between Kc and Kp.
- Apply the relationship Kp = Kc(RT)^(Δn) to solve problems.
- Identify when Kp = Kc.
Common exam trap
Forgetting to consider only gaseous species when calculating Δn.
Definitions
- Term
Kc
- Meaning
The equilibrium constant expressed in terms of molar concentrations of reactants and products.
- Term
Kp
- Meaning
The equilibrium constant expressed in terms of partial pressures of gaseous reactants and products.
- Term
Δn
- Meaning
The difference between the total number of moles of gaseous products and the total number of moles of gaseous reactants in a balanced chemical equation.
Learning objectives
Understand the difference between Kc and Kp.
Derive the relationship between Kc and Kp.
Apply the relationship Kp = Kc(RT)^(Δn) to solve problems.
Identify when Kp = Kc.
Calculate Δn correctly for various gaseous reactions.
Formulae
- Name
Relationship between Kp and Kc
- Note
Where R is the universal gas constant, T is the absolute temperature in Kelvin, and Δn is the change in the number of moles of gaseous components.
- Expression
Kp = Kc(RT)^(Δn)
- Name
Change in moles of gaseous components (Δn)
- Note
Only gaseous species are considered for Δn.
- Expression
Δn = (Sum of stoichiometric coefficients of gaseous products) - (Sum of stoichiometric coefficients of gaseous reactants)
Prerequisites
Concept of chemical equilibrium.
Definition and calculation of Kc.
Definition and calculation of Kp.
Ideal Gas Law (PV=nRT).
Partial pressures.
Stoichiometry of gaseous reactions.
Common mistakes
Forgetting to consider only gaseous species when calculating Δn.
Using incorrect units for R or temperature T.
Applying the formula to reactions involving solids or liquids.
Confusing Kc and Kp expressions.
Incorrectly calculating Δn (e.g., summing coefficients instead of subtracting).
Keywords
Equilibrium constant
Kc
Kp
Partial pressure
Molar concentration
Gaseous reaction
Δn
Ideal gas law
Chemical equilibrium
Practice preview
What is the correct mathematical relationship between the equilibrium constant Kp and Kc for a reversible gaseous reaction?…
easy
For the reversible reaction N2(g) + 3H2(g) ⇌ 2NH3(g), what is the value of Δn_g, which is used in the relationship between Kp and Kc?…
easy
For the dissociation of phosphorus pentachloride, PCl5(g) ⇌ PCl3(g) + Cl2(g), if Kc = 0.04 M at 500 K, what is the approximate value of Kp? (Given R = 0.0821 L atm mol^-1 K^-1)…
medium