Calculations using equilibrium constant equations

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  • Created by: chunks-42
  • Created on: 06-10-15 12:36

A reactions that has reached equilibrium at a given temperature will be a mixture of reactants and products. We can use the equilibrium expression to calculate the composition of this mixture.

Calculating the composition of a reaction mixture

The reaction of ethanol and ethanoic acid is:

C2H5OH + CH3CO2H <--> CH3CO2C2H5 + H2O

We know that at equilibrium:

Kc = [CH3CO2C2H5][H2O]

       [C2H5OH][CH3CO2H]

Suppose that Kc = 4.0 at the temperature of our experiment and we want to know how much we could produce by mixing 1 mol of ethanol and 1 mol of ethanoic acid. Set out the information as shown below:

Equation : C2H5OH  + CH3CO2H  <-->  CH3CO2C2H5  +  H2O

                Ethanol      Ethanoic acid      Ethyl Ethanoate   Water

At Start :   1 mol         1 mol                 0 mol                  0 mol

At Equil:   (1-x)mol      (1-x) mol             x mol                  x mol

We do not know how many moles of ethyl ethanoate will be produced, so we call this x. The equilibrium of water will also be produced and, in doing so, x mol of both ethanol and ethanoic acid will be used up. So the equilibrium of each of these remaininn is (1-x) mol.

These figures are in moles, but we need concentrations in mol dm-3 to substitute…

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