뒤로Chemical Equilibrium and the Equilibrium Constant (K)
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Chemical Equilibrium
Introduction to Equilibrium
Chemical equilibrium occurs when the rates of the forward and reverse reactions in a chemical system are equal, resulting in constant concentrations of reactants and products over time. This concept is fundamental in understanding how chemical reactions behave under different conditions.
Dynamic equilibrium: Both forward and reverse reactions continue to occur, but there is no net change in the concentrations of reactants and products.
Equilibrium position: The relative concentrations of reactants and products at equilibrium.
The Equilibrium Constant (K)
Definition and Interpretation
The equilibrium constant, K, is a numerical value that expresses the ratio of the concentrations of products to reactants at equilibrium, each raised to the power of their respective coefficients in the balanced chemical equation.
K > 1: Products are favored; the equilibrium lies to the right.
K < 1: Reactants are favored; the equilibrium lies to the left.
K = 1: Neither reactants nor products are favored; significant amounts of both are present at equilibrium.
The general form of the equilibrium constant expression for a reaction:
is:
where [X] denotes the concentration of species X at equilibrium.
States of Matter in K Expressions
Only gases and aqueous species are included in the equilibrium expression.
Solids and liquids are omitted because their concentrations do not change.
Writing Equilibrium Expressions
Examples
Example 1:
Example 2: (since both are solids, the equilibrium constant is not dependent on their concentrations)
Example 3:
Practice Problems
Comparing Reactant and Product Amounts Using K
Given the value of K, determine whether reactants or products are present in greater amounts at equilibrium:
Reaction | K Value | Greater Amount at Equilibrium |
|---|---|---|
N2(g) + O2(g) ↔ 2 NO(g) | 1.0 × 10-5 | Reactants |
2 CO(g) ↔ CO2(g) + C(s) | 2.2 × 102 | Products |
2 BrCl(g) ↔ Br2(g) + Cl2(g) | 1 | Neither (similar amounts) |
Writing Equilibrium Expressions for Reverse Reactions
For the reverse reaction, the equilibrium constant is the reciprocal of the forward reaction's K value.
Example: If , then
Application: Calculating Equilibrium Concentrations
Given K and equilibrium concentrations of all but one species, solve for the unknown concentration using the equilibrium expression.
Example: For , at 500 K. If M, M, solve for :
Rearrange to solve for :
Summary Table: Interpreting K Values
K Value | Interpretation |
|---|---|
K > 1 | Products favored at equilibrium |
K < 1 | Reactants favored at equilibrium |
K = 1 | Neither favored; significant amounts of both |
Key Terms
Equilibrium constant (K): A value that expresses the ratio of product to reactant concentrations at equilibrium.
Dynamic equilibrium: The state in which the forward and reverse reactions occur at the same rate.
Homogeneous equilibrium: All reactants and products are in the same phase.
Heterogeneous equilibrium: Reactants and products are in different phases.
