Magnitude of the Equilibrium Constant
Learning Objective 7.5.A
Explain how very large or very small values of K relate to the relative concentrations of species at equilibrium.
Quick Notes
- K >> 1: products favored (equilibrium lies to the right).
- K << 1: reactants favored (equilibrium lies to the left)
- K ≈ 1: comparable amounts of reactants and products at equilibrium.
What Does the Magnitude of K Mean?
The equilibrium constant (K) is a value that indicates the ratio of product to reactant concentrations at equilibrium.
- If K is very large (e.g., K > 10⁴), the numerator of the expression (products) must be much greater than the denominator (reactants).
Reaction goes nearly to completion. - If K is very small (e.g., K < 10⁻⁴), the numerator (products) is much smaller than the denominator (reactants).
Only a small amount of product forms - the reaction doesn't noticeably occur - If K is near 1, the concentrations of products and reactants are roughly comparable
Example — Very Large K
K = 1.8 × 105
Reaction: HCl(aq) → H+(aq) + Cl−(aq)
Interpretation: Nearly complete dissociation: almost all HCl exists as ions in solution.
Example — Very Small K
K = 1.3 × 10−13
Reaction: CH3COOH(aq) ⇌ H+(aq) + CH3COO−(aq)
Interpretation: Hardly dissociates — most acetic acid remains as molecules.
Summary
- The size of K indicates how far a reaction proceeds before equilibrium.
- Use K to predict whether reactants or products are favored.
- Large K → product-favored; Small K → reactant-favored.