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Calculate Keq for The Following Reaction

Reviewed by Calculator Editorial Team

The equilibrium constant (K_eq) is a fundamental concept in chemical equilibrium that quantifies the ratio of product concentrations to reactant concentrations at equilibrium. This calculator helps you determine K_eq for any given reaction by analyzing the stoichiometry and initial concentrations.

What is K_eq?

The equilibrium constant (K_eq) is a numerical value that describes the position of a chemical equilibrium. It's calculated from the concentrations of the products and reactants at equilibrium, each raised to the power of their respective stoichiometric coefficients.

K_eq values provide important information about the extent of a reaction and the direction in which it proceeds. A large K_eq indicates that the reaction strongly favors products, while a small K_eq suggests that reactants are favored.

How to Calculate K_eq

Calculating K_eq involves several steps:

  1. Write the balanced chemical equation for the reaction
  2. Determine the stoichiometric coefficients for each species
  3. Measure or calculate the equilibrium concentrations of all species
  4. Apply the K_eq formula using these values

The exact calculation depends on whether the reaction is homogeneous (all components in the same phase) or heterogeneous (multiple phases).

K_eq Formula

For a general reaction: aA + bB ⇌ cC + dD

The equilibrium constant expression is:

K_eq = [C]c[D]d / [A]a[B]b

Where:

  • [A], [B], [C], [D] = equilibrium concentrations of species
  • a, b, c, d = stoichiometric coefficients

For gas-phase reactions, partial pressures can be used instead of concentrations if the reaction is at constant pressure.

Example Calculation

Consider the reaction: 2NO(g) + O₂(g) ⇌ 2NO₂(g)

At equilibrium, the concentrations are:

  • [NO] = 0.10 M
  • [O₂] = 0.20 M
  • [NO₂] = 0.30 M

The K_eq calculation would be:

K_eq = [NO₂]2 / ([NO]2[O₂])

K_eq = (0.30)2 / ((0.10)2(0.20))

K_eq = 0.09 / (0.002)

K_eq = 45

This indicates the reaction strongly favors the formation of NO₂.

Interpreting K_eq

The magnitude and sign of K_eq provide important information:

  • Large K_eq (>100): Reaction strongly favors products
  • Small K_eq (<0.01): Reaction strongly favors reactants
  • K_eq ≈ 1: Reaction is at equilibrium with roughly equal concentrations
  • K_eq is unitless because it's a ratio of concentrations

Temperature affects K_eq - it changes with temperature according to the van't Hoff equation.

FAQ

What does a K_eq of 1 mean?
A K_eq of 1 means the reaction is at equilibrium with equal concentrations of products and reactants.
How does temperature affect K_eq?
K_eq is temperature-dependent. The van't Hoff equation relates changes in K_eq to changes in temperature.
Can K_eq be negative?
No, K_eq is always positive because concentrations are squared in the expression.
What if a reactant or product is a solid or liquid?
Solids and liquids are not included in the K_eq expression because their concentrations are constant.
How precise should my measurements be for K_eq?
Concentrations should be measured to at least 3 significant figures for reliable K_eq values.