Calculate The Equilibrium Constant for The Following Two Reactions
'/%3E%3Ccircle cx='48' cy='39' r='19' fill='%23f2c9a5'/%3E%3Cpath d='M27 35c3-16 39-17 42 2-8-8-27-9-42-2z' fill='%23374151'/%3E%3Cpath d='M21 86c5-24 49-28 56 0z' fill='%232563eb'/%3E%3Ccircle cx='41' cy='41' r='2' fill='%23111827'/%3E%3Ccircle cx='55' cy='41' r='2' fill='%23111827'/%3E%3Cpath d='M42 52c4 3 9 3 13 0' stroke='%2392400e' stroke-width='3' fill='none' stroke-linecap='round'/%3E%3C/svg%3E)
Reviewed by Calculator Editorial Team
Calculating the equilibrium constant (Keq) for chemical reactions is essential in chemistry for predicting reaction behavior. This guide explains how to determine Keq using the equilibrium constant expression and provides practical examples.
Introduction
The equilibrium constant (Keq) is a quantitative measure of the position of a chemical equilibrium. It indicates the ratio of product concentrations to reactant concentrations at equilibrium. For a general reaction:
aA + bB ⇌ cC + dD
The equilibrium constant expression is:
Keq = [C]c[D]d / [A]a[B]b
Where [ ] represents the molar concentration of each species. The value of Keq helps predict whether a reaction will proceed forward or reverse to reach equilibrium.
Equilibrium Constant Formula
The equilibrium constant formula varies depending on the stoichiometry of the reaction. For a general reaction:
aA + bB ⇌ cC + dD
The equilibrium constant expression is:
Keq = [C]c[D]d / [A]a[B]b
Key points about the equilibrium constant:
- Keq is temperature-dependent
- For gas-phase reactions, concentrations can be replaced with partial pressures
- Pure solids and liquids are not included in the expression
- The value of Keq indicates reaction favorability:
- Keq > 1: Products favored
- Keq = 1: Equal amounts of reactants and products
- Keq < 1: Reactants favored
Calculating Keq
To calculate Keq, you need to know the equilibrium concentrations of all species involved in the reaction. Here's the step-by-step process:
- Write the balanced chemical equation
- Determine the equilibrium constant expression
- Measure or calculate the equilibrium concentrations of all species
- Substitute the concentrations into the equilibrium constant expression
- Calculate the numerical value of Keq
Note: For reactions involving gases, you can use partial pressures instead of concentrations if the ideal gas law is applicable.
Example Calculation
Let's calculate Keq for the following reaction:
N2(g) + 3H2(g) ⇌ 2NH3(g)
At equilibrium, the concentrations are:
- [N2] = 0.10 M
- [H2] = 0.15 M
- [NH3] = 0.30 M
The equilibrium constant expression is:
Keq = [NH3]2 / ([N2][H2]3)
Substituting the equilibrium concentrations:
Keq = (0.30)2 / (0.10 × (0.15)3)
Keq = 0.09 / (0.10 × 0.00375)
Keq = 0.09 / 0.00375 = 24
The equilibrium constant for this reaction is 24, indicating that the reaction strongly favors the formation of ammonia.
FAQ
What does a Keq value greater than 1 mean?
A Keq value greater than 1 indicates that the reaction favors the formation of products at equilibrium. The larger the value, the more products are formed relative to reactants.
How does temperature affect the equilibrium constant?
The equilibrium constant is temperature-dependent. For exothermic reactions, increasing temperature shifts the equilibrium toward reactants (lower Keq), while for endothermic reactions, increasing temperature shifts the equilibrium toward products (higher Keq).
Can Keq be negative?
No, the equilibrium constant cannot be negative. It is always a positive value because concentrations and pressures are always positive quantities.