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Calculate The Equilibrium Constant for The Following Reaction at 25

Reviewed by Calculator Editorial Team

This calculator helps you determine the equilibrium constant (Keq) for a chemical reaction at 25°C (298.15 K) using the van't Hoff equation. The equilibrium constant is a measure of the ratio of product concentrations to reactant concentrations at equilibrium.

How to Calculate the Equilibrium Constant

The equilibrium constant for a reaction is calculated using the concentrations of reactants and products at equilibrium. The standard formula is:

Keq = [Products]/[Reactants]

For a general reaction:

aA + bB ⇌ cC + dD

The equilibrium constant would be:

Keq = ([C]c[D]d) / ([A]a[B]b)

Steps to Calculate

  1. Write the balanced chemical equation for your reaction
  2. Determine the stoichiometric coefficients (a, b, c, d)
  3. Measure the equilibrium concentrations of all species
  4. Plug the values into the equilibrium constant formula
  5. Calculate the numerical value of Keq

Note: The equilibrium constant is temperature-dependent. This calculator assumes standard conditions (25°C) unless you specify otherwise.

The Equilibrium Constant Formula

The equilibrium constant formula is derived from the law of mass action. For a reaction:

aA + bB ⇌ cC + dD

The equilibrium constant is given by:

Keq = ([C]c[D]d) / ([A]a[B]b)

Where:

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

The equilibrium constant can also be expressed in terms of partial pressures for gaseous reactions:

Kp = (PCcPDd) / (PAaPBb)

Worked Example

Let's calculate the equilibrium constant for the reaction:

N2 + 3H2 ⇌ 2NH3

At equilibrium, the concentrations are:

  • [N2] = 0.10 M
  • [H2] = 0.15 M
  • [NH3] = 0.30 M

The equilibrium constant is calculated as:

Keq = [NH3]2 / ([N2][H2]3)

Keq = (0.30)2 / (0.10 × (0.15)3)

Keq = 0.09 / (0.10 × 0.003375)

Keq = 0.09 / 0.0003375 ≈ 266.7

The equilibrium constant for this reaction is approximately 266.7 at 25°C.

Interpreting the Equilibrium Constant

The value of the equilibrium constant tells you about the position of equilibrium:

  • Keq > 1: Products favored at equilibrium
  • Keq ≈ 1: Equal amounts of reactants and products
  • Keq < 1: Reactants favored at equilibrium

For the example above (Keq ≈ 266.7), we can conclude that:

  • The reaction strongly favors the formation of ammonia
  • At equilibrium, most of the reactants have been converted to products

Remember that the equilibrium constant is temperature-dependent. This calculator assumes standard conditions (25°C) unless you specify otherwise.

FAQ

What is the difference between Keq and Kp?
Keq is expressed in terms of concentrations (molarity), while Kp is expressed in terms of partial pressures. They are related by the equation Kp = Keq × (RT)Δn, where Δn is the change in the number of moles of gas.
How does temperature affect the equilibrium constant?
The equilibrium constant is temperature-dependent. The van't Hoff equation relates the change in Keq to temperature changes. This calculator assumes standard conditions (25°C) unless you specify otherwise.
What if my reaction is reversible?
The equilibrium constant applies to both reversible and irreversible reactions. For irreversible reactions, the equilibrium constant is effectively infinite, indicating complete conversion to products.
Can I use this calculator for gas-phase reactions?
Yes, this calculator can be used for gas-phase reactions. Just remember to use partial pressures (Kp) instead of concentrations (Keq) if needed.