How to Calculate Chemical Equilibrium

Method 1: Calculating from Equilibrium Concentrations

• 1

  Convert all figures into the correct units: moles per liter in solution, or partial pressures (usually Atm) in gaseous equilibrium. Leave out pure solids and liquids. Since the reaction takes place in a solution or a mixture of gases, only dissolved substances or gases participate.

  Make sure you are using concentrations rather than amounts: for example, do not enter in "3.00 grams of NaCl" but instead use the weight of NaCl from periodic table values to calculate the number of moles: 3.00 grams / 58.44 grams per mole gives 0.0513 moles. Then divide the number of moles by the volume of the total solution in liters to arrive at moles per liter or molarity, a correct unit for concentration.

• 2

  Apply the stoichiometric coefficients from the balanced chemical equation as exponents for the concentrations in the equilibrium expression, as follows:

  [Products]^x/ [Reactants]^y

  In this form the square brackets indicate concentration, and the "x" and "y" denote the stoichiometric coefficients from the balanced chemical equation. For each product or reactant concentration, make sure to use the amount of that chemical present when the reaction has reached equilibrium (the end of the reaction) not the amount present at the beginning of the reaction or some other point.

• 3

  Multiply the concentrations of products or reactants (taken to their correct powers) together on each side of the dividing line if the problem gives multiple reactants or multiple products that participate in the equilibrium. For example, express this reaction:

  H2O + HCl -> H3O+ + Cl-

  as

  ([H3O+] * [Cl-]) / [HCl].

  Water, a pure liquid, does not participate in the equilibrium.

• 4

  Divide the result on the products side of the dividing line by the result on the reactants side of the dividing line. This method calculates K, the equilibrium constant for the reaction.

Method 2: Calculating from Gibbs Free Energy

• 5

  Write out the expression relating Gibbs free energy to equilibrium constant, as follows:

  delta G0 = -RT ln K

  The standard Gibbs free energy of a reaction, delta G0, describes whether conditions favor the reaction. Several ways exist to calculate the Gibbs free energy of a reaction. The sign convention dictates that a favorable reaction has a negative delta G.

• 6

  Convert the temperature to Kelvin if given in other units. If given in Celsius, add 273.15. Then divide delta G by the quantity of (-R * T). This gives a result equal to (ln K), or the natural logarithm of the equilibrium constant.

  R is a universal constant, approximately equal to 8.314 J / mol * K. Remember to track whether the number is given in joules or kilojoules: the value of R usually given requires a number in joules, not kilojoules, so convert accordingly.

• 7

  Raise e (a constant approximately equal to 2.7183) to the power of (ln K) to extract from the natural log and reach an answer equal to K. You will probably require a scientific calculator to complete this operation.