Law of Chemical Equilibrium and Equilibrium Constant
Defining the law of mass action and deriving the general expression for the equilibrium constant (K).
What is Law of Chemical Equilibrium and Equilibrium Constant?
At a given temperature, the ratio of the product of concentrations of products to the product of concentrations of reactants, each raised to the power of their stoichiometric coefficient, is constant for a reversible reaction at equilibrium.
Key formula / rule: Equilibrium Constant (Kc)
Key points
- Define the law of chemical equilibrium.
- Derive the expression for the equilibrium constant (Kc).
- Understand the concept of Kp and its relation to Kc.
- Interpret the significance of the magnitude of the equilibrium constant.
Common exam trap
Forgetting to raise concentrations/pressures to the power of stoichiometric coefficients.
Definitions
- Term
Law of Chemical Equilibrium
- Meaning
At a given temperature, the ratio of the product of concentrations of products to the product of concentrations of reactants, each raised to the power of their stoichiometric coefficient, is constant for a reversible reaction at equilibrium.
- Term
Equilibrium Constant (Kc)
- Meaning
The numerical value of the ratio of the product of molar concentrations of products to the product of molar concentrations of reactants, each raised to the power of their stoichiometric coefficient, for a reversible reaction at equilibrium at a specific temperature.
- Term
Partial Pressure Equilibrium Constant (Kp)
- Meaning
The numerical value of the ratio of the product of partial pressures of gaseous products to the product of partial pressures of gaseous reactants, each raised to the power of their stoichiometric coefficient, for a reversible gaseous reaction at equilibrium at a specific temperature.
Learning objectives
Define the law of chemical equilibrium.
Derive the expression for the equilibrium constant (Kc).
Understand the concept of Kp and its relation to Kc.
Interpret the significance of the magnitude of the equilibrium constant.
Formulae
- Name
Equilibrium Constant (Kc)
- Note
For aA + bB ≤> cC + dD, where concentrations are in mol/L.
- Expression
Kc = ([C]c [D]d) / ([A]a [B]b)
- Name
Partial Pressure Equilibrium Constant (Kp)
- Note
For gaseous reactions, where P is partial pressure.
- Expression
Kp = (PC^c PD^d) / (PA^a PB^b)
- Name
Relationship between Kp and Kc
- Note
R = 0.08314 L bar/mol K or 8.314 J/mol K, T in Kelvin, Δn = (moles of gaseous products) - (moles of gaseous reactants).
- Expression
Kp = Kc(RT)^Δn
Prerequisites
Understanding of chemical reactions (reversible and irreversible).
Concept of concentration (molarity).
Basic algebra and exponents.
Common mistakes
Forgetting to raise concentrations/pressures to the power of stoichiometric coefficients.
Including pure solids or liquids in the Kc expression.
Confusing Kc and Kp or incorrectly calculating Δn.
Assuming Kc changes with concentration or pressure (it only changes with temperature).
Keywords
Chemical Equilibrium
Law of Mass Action
Equilibrium Constant
Kc
Kp
Reversible Reaction
Partial Pressure
Concentration
Stoichiometric Coefficient
Temperature Dependence
Practice preview
For a reversible reaction, the rate of the forward reaction is proportional to:…
easy
For the reaction A(g) + B(g) <=> C(g) + D(g), if the initial concentrations of A and B are both 0.5 M, and at equilibrium, the concentration of C is 0.3 M, what is the value of Kc?…
medium
The equilibrium constant (Kc) for the reaction N2(g) + 3H2(g) <=> 2NH3(g) is expressed as:…
easy