Chemical Equilibrium and Kc, Kp
Dynamic nature, equilibrium constant expressions and Le Chatelier's principle. (Chemistry › Equilibrium, NEET UG syllabus.)
What is Chemical Equilibrium and Kc, Kp?
A dynamic state in a reversible reaction where the rates of the forward and reverse reactions are equal, leading to constant concentrations of reactants and products.
Key formula / rule: Equilibrium Constant (Kc)
Key points
- Define chemical equilibrium and explain its dynamic nature.
- State the Law of Mass Action.
- Write the expression for Kc for any given reversible reaction.
- Write the expression for Kp for any given gaseous reversible reaction.
Common exam trap
Not balancing the chemical equation before writing the equilibrium expression.
Definitions
- Term
Chemical Equilibrium
- Meaning
A dynamic state in a reversible reaction where the rates of the forward and reverse reactions are equal, leading to constant concentrations of reactants and products.
- Term
Equilibrium Constant (Kc)
- Meaning
The ratio of the product of molar concentrations of products to that of reactants, each raised to their stoichiometric coefficients, at equilibrium.
- Term
Equilibrium Constant (Kp)
- Meaning
The ratio of the product of partial pressures of gaseous products to that of gaseous reactants, each raised to their stoichiometric coefficients, at equilibrium.
- Term
Law of Mass Action
- Meaning
At a given temperature, the ratio of the product of molar concentrations (or partial pressures) of products to that of reactants, each raised to the power of their stoichiometric coefficients, is a constant (the equilibrium constant).
Learning objectives
Define chemical equilibrium and explain its dynamic nature.
State the Law of Mass Action.
Write the expression for Kc for any given reversible reaction.
Write the expression for Kp for any given gaseous reversible reaction.
Derive and apply the relationship between Kp and Kc.
Calculate Kc or Kp given equilibrium concentrations or partial pressures.
Understand the significance of the magnitude of Kc/Kp.
Formulae
- Name
Equilibrium Constant (Kc)
- Note
For aA + bB ⇌ cC + dD; concentrations in mol/L. Pure solids and liquids are excluded.
- Expression
Kc = ([C]c [D]d) / ([A]a [B]b)
- Name
Equilibrium Constant (Kp)
- Note
For aA + bB ⇌ cC + dD (all gaseous); partial pressures. Pure solids and liquids are excluded.
- Expression
Kp = ((PC)c (PD)d) / ((PA)a (PB)b)
- Name
Relationship between Kp and Kc
- Note
R = Ideal gas constant (0.0821 L·atm/mol·K), T = Absolute temperature (K).
- Expression
Kp = Kc(RT)^Δng
- Name
Change in moles of gas (Δng)
- Note
Only gaseous species are considered for Δng.
- Expression
Δng = (sum of stoichiometric coefficients of gaseous products) - (sum of stoichiometric coefficients of gaseous reactants)
Prerequisites
Basic stoichiometry and mole concept.
Understanding of chemical reactions and reversible reactions.
Knowledge of states of matter (solid, liquid, gas).
Ideal Gas Law (PV=nRT) for understanding partial pressures.
Common mistakes
Not balancing the chemical equation before writing the equilibrium expression.
Including pure solids or liquids in the Kc or Kp expressions.
Incorrectly calculating Δng (e.g., including non-gaseous species or wrong sign).
Using incorrect units for R or T (e.g., R in J/mol·K instead of L·atm/mol·K, or T in Celsius).
Confusing Kc with Kp or using the wrong constant for a given problem type.
Forgetting to raise concentrations/pressures to their stoichiometric coefficients.
Keywords
Equilibrium
Kc
Kp
Law of Mass Action
Dynamic Equilibrium
Reversible Reaction
Partial Pressure
Concentration
Δ ng
Practice preview
For the reaction N₂(g) + 3H₂(g) ⇌ 2NH₃(g), what is the relationship between Kp and Kc?…
easy
What is the correct expression for the equilibrium constant, Kc, for the reaction 2SO₂(g) + O₂(g) ⇌ 2SO₃(g)?…
easy
At a certain temperature, 2 moles of A and 2 moles of B are mixed in a 2 L container. At equilibrium, 1 mole of C is formed according to the reaction A(g) + B(g) ⇌ C(g). What is the value of Kc?…
medium