7.5 Magnitude of the Equilibrium Constant

K tells you where equilibrium lies

The equilibrium constant K compares product concentrations to reactant concentrations at equilibrium. When K >> 1, products are strongly favored — the reaction proceeds nearly to completion. When K << 1, reactants are favored and very little product forms. When K is close to 1, significant amounts of both reactants and products are present at equilibrium.

Large K = reaction goes far right

A large K (e.g., K = 10^10) means the forward reaction is essentially complete — at equilibrium, the mixture is mostly products. This doesn't tell you about speed (kinetics), only about the extent of reaction. A reaction can have a huge K but still be very slow if activation energy is high.

Small K = reaction barely proceeds

A small K (e.g., K = 10^-10) means the equilibrium mixture is mostly reactants, with only a tiny amount of product formed. This is common for reactions with very stable reactants or weak tendency to form products, such as dissociation of a very weak acid.

Key terms

K >> 1

Equilibrium favors products; reaction proceeds nearly to completion.

K << 1

Equilibrium favors reactants; very little product forms.

K ≈ 1

Significant amounts of both reactants and products present at equilibrium.

Does K tell you reaction speed?

No — K describes extent (position) of equilibrium, not kinetics/rate.

Example: very small Ka

Indicates a weak acid that dissociates only slightly in water, leaving mostly undissociated acid.

Try an AP-style question

A reaction has K = 1 × 10⁵. What is true at equilibrium?

1. Products are strongly favored
2. The reaction is fast
3. Equal amounts of reactant and product
4. Reactants are strongly favored