Law of mass action

Definition

The law of mass action states that the rate of a chemical reaction is directly proportional to the product of the concentrations of the reactants, each raised to a power equal to its coefficient in the balanced chemical equation. It is fundamental in deriving equilibrium constants.

5 Must Know Facts For Your Next Test

1. The law of mass action can be used to write the expression for an equilibrium constant (K).
2. For a reaction $aA + bB \rightleftharpoons cC + dD$, the equilibrium constant expression is $K = \frac{[C]^c[D]^d}{[A]^a[B]^b}$.
3. Equilibrium constants are temperature-dependent and will change if the temperature changes.
4. The values of equilibrium constants provide insight into whether products or reactants are favored at equilibrium.
5. In heterogeneous equilibria involving gases and solids, only gaseous components and aqueous solutions appear in the equilibrium constant expression.

Review Questions

• How does the law of mass action relate to the equilibrium constant expression?
• What is the general form of an equilibrium constant expression for a given chemical reaction?
• Why do only certain phases (gaseous and aqueous) appear in equilibrium constant expressions?

Related terms

Equilibrium Constant (K): A numerical value that expresses the ratio of concentrations of products to reactants at equilibrium for a given reaction.

Reaction Quotient (Q): A ratio similar to the equilibrium constant but calculated using initial concentrations; it predicts which direction a reaction will proceed to reach equilibrium.

Le Chatelier's Principle: States that if an external change is applied to a system at equilibrium, the system adjusts itself to partially counteract that change and re-establishes equilibrium.