How to Calculate Equilibrium

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Chemical equilibrium refers to a state in which the products of a reaction are being converted back to reactants at the same rate that the reactants are being converted to products. You can calculate the equilibrium constant, K, using a formula. You can then use this constant to calculate the concentration of a particular reactant at equilibrium.

Things You'll Need

  • Balanced chemical equation
  • Concentration of reactants or equilibrium constant
  • Calculator

Finding the Equilibrium Constant

  • Write the balanced chemical equation on your paper. Circle the coefficients. For example, in the equation:

    N2 + 3H2 <-> 2NH3, circle the 3 and the 2.

  • Write the formula for finding the equilibrium constant using the circled coefficients. The general formula is:

    K = [product 1]^coefficient [product 2]^coefficient / [reactant 1]^coefficient [reactant 2]^coefficient.

    [] is shorthand for "concentration of."

    For N2 + 3H2 <-> 2NH3, given concentrations of 0.875 M, 0.598 M, and 0.105 M respectively, the formula for the equilibrium coefficient is:

    [0.105 M]^2/[0.875 M][0.598 M]^3

  • Calculate the equilibrium constant.

Finding Equilibrium Concentrations

  • Write the equation for the balanced chemical reaction. Write all known equilibrium concentrations for the reaction underneath the relevant products. For example, use the equation N2 + 3H2 <-> 2NH3 with known concentrations at equilibrium for N2 of 0.231 M and H2 of 0.629 M. Write the chemical equation:

    N2 + 3H2 <-> 2NH3.

    Underneath N2, write 0.231 M. Underneath H2 write 0.629 M.

  • Write the equilibrium constant equation and rearrange it to solve for the missing concentration. You need the value of the equilibrium constant. For N2 + 3H2 <-> 2NH3, K = 0.0589.

    0.0589 = [NH3]^2/[0.231 M] * [0.629 M]^3.

    Rearrange to solve for [NH3]:

    0.0589 0.231 M (0.629M)^3 = [NH3]^2.

  • Take the square root of both sides to find [NH3].

References

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