Learning Outcomes
i. Define the Law of Mass Action and its significance.
ii. Explain the relationship between the concentration of reactants and products at equilibrium.
iii. Write the equilibrium constant expression.
iv. Apply the equilibrium constant expression to predict the direction of a reaction.
Introduction
In the previous lessons, we explored the concept of chemical equilibrium and the dynamic nature of forward and reverse reactions. This lesson delves into the quantitative aspect of equilibrium, introducing the Law of Mass Action, a fundamental principle that governs the behavior of reversible reactions.
i. Law of Mass Action: A Guiding Principle
The Law of Mass Action, formulated by Guldberg and Waage in the late 19th century, states that the rate of a chemical reaction is proportional to the product of the active masses of the reactants. Active mass refers to the concentration of a substance in a reaction mixture.
ii. Mathematical Representation of Equilibrium
At equilibrium, the forward and reverse reactions occur at equal rates, and the net change in concentrations is zero. The Law of Mass Action can be mathematically expressed using the equilibrium constant expression:
Kc = [Products]a/[Reactants]b
where:
- Kc is the equilibrium constant, a specific value for a particular reaction at a given temperature.
- [Products]a represents the concentrations of the products raised to their respective stoichiometric coefficients.
- [Reactants]b represents the concentrations of the reactants raised to their respective stoichiometric coefficients.
iii. Equilibrium Constant: A Quantitative Measure: The equilibrium constant (Kc) is a quantitative measure of the tendency of a reaction to reach equilibrium. A large Kc value indicates that the equilibrium favors the products, while a small Kc value indicates that the equilibrium favors the reactants.
iv. Predicting Reaction Direction
The equilibrium constant can also be used to predict the direction of a reaction when not at equilibrium. If the reaction quotient (Q), calculated using the current concentrations of reactants and products, is less than Kc, the reaction will proceed in the forward direction to increase the concentrations of products. Conversely, if Q is greater than Kc, the reaction will proceed in the reverse direction to increase the concentrations of reactants.
The Law of Mass Action provides a quantitative framework for understanding chemical equilibrium, enabling us to predict the behavior of reactions and assess their tendency to favor the formation of products or reactants. This law plays a crucial role in various fields, including chemical engineering, industrial chemistry, and environmental science.
