Equilibrium Constant for Heterogeneous Reactions
Understanding and calculating the equilibrium constant for reactions involving reactants and products in different phases, excluding pure solids and liquids from the expression.
What is Equilibrium Constant for Heterogeneous Reactions?
A state of equilibrium in a system where reactants and products exist in two or more different physical states (phases).
Key formula / rule: Kc for Heterogeneous Reactions
Key points
- Identify heterogeneous reactions.
- Write correct equilibrium constant expressions for heterogeneous reactions.
- Explain why pure solids and liquids are excluded.
- Calculate equilibrium constants using the simplified expressions.
Common exam trap
Including the concentrations or partial pressures of pure solids and liquids in the Kc or Kp expression.
Definitions
- Term
Heterogeneous Equilibrium
- Meaning
A state of equilibrium in a system where reactants and products exist in two or more different physical states (phases).
- Term
Activity
- Meaning
A thermodynamic concept representing the effective concentration or partial pressure of a substance, normalized relative to a standard state. For pure solids and liquids, activity is taken as unity.
Learning objectives
Identify heterogeneous reactions.
Write correct equilibrium constant expressions for heterogeneous reactions.
Explain why pure solids and liquids are excluded.
Calculate equilibrium constants using the simplified expressions.
Formulae
- Name
Kc for Heterogeneous Reactions
- Note
Concentrations of pure solids (s) and pure liquids (l) are omitted.
- Expression
Kc = \frac{[Productsg, aq]coeff}{[Reactantsg, aq]coeff}
- Name
Kp for Heterogeneous Reactions
- Note
Partial pressures of gaseous species. Pure solids (s) and pure liquids (l) are omitted. Aqueous species are not included in Kp.
- Expression
Kp = \frac{(P_{Productsg})coeff}{(P_{Reactantsg})coeff}
Prerequisites
Understanding of chemical equilibrium.
Concept of concentration (molarity) and partial pressure.
Writing balanced chemical equations.
Stoichiometry.
Common mistakes
Including the concentrations or partial pressures of pure solids and liquids in the Kc or Kp expression.
Confusing pure liquids (like water as a solvent) with solutions.
Incorrectly applying the rule to substances that are not pure solids or liquids (e.g., aqueous solutions).
Forgetting to raise the concentrations/pressures to their stoichiometric coefficients.
Keywords
Heterogeneous equilibrium
Equilibrium constant
Kc
Kp
Pure solids
Pure liquids
Gaseous species
Aqueous species
Activity
Phase
Practice preview
Consider the reaction: N₂(g) + 3H₂(g) ⇌ 2NH₃(g). If the equilibrium constant K<0xE1><0xB5><0x9C> is expressed in terms of molar concentrations, what is the expression for K<0xE1><0xB5><0x9C>?…
easy
For the reaction: CaCO₃(s) ⇌ CaO(s) + CO₂(g), what is the correct expression for the equilibrium constant K<0xE1><0xB5><0x9C>?…
easy
Which of the following is TRUE regarding the equilibrium constant expression for a heterogeneous reaction?…
easy