Equilibrium quantities

Revision Notes

1)      Equilibrium quantities

• A reversible reaction reaches a position of dynamic equilibrium where both the forward and backward reactions are taking place at the same rate and concentrations of chemicals are constant
• At equilibrium there will always be a mixture of reactants and products
• The moles present at equilibrium can be worked out using the equation and the moles present at the start

Example 1

The hydrolysis of ethyl ethanoate is a reversible reaction. The equation for the equilibrium is shown below.

            CH₃COOC₂H₅ + H₂O ¾ CH₃COOH + C₂H₅OH

A student mixed together 8.0 mol ethyl ethanoate and 5.0 mol water. He also added a small amount of hydrochloric acid to catalyse the reaction.

The student left the mixture until it had reached equilibrium at constant temperature. He found that 2.0 mol of ethanoic acid had formed.

The information in the question is summarised in the table below.

Component	CH3COOC2H5	H2O	CH3COOH	C2H5OH
Initial mol	8.0	5.0	0.0	0.0
Equilibrium mol			2.0

The equation says that for every mole of CH₃COOH made, one mole of C₂H₅OH is made so the equilibrium moles of ethanol is also 2.0

The equation also says that for every mole of CH₃COOH made, one mole of CH₃COOC₂H₅ is used up so the equilibrium moles of ethyl ethanoate is initial moles - 2.0 = 6.0 mol

The equation also says that for every mole of CH₃COOH made, one mole of H₂O is used up so the equilibrium moles of water is initial moles - 2.0 = 3.0 mol

The completed table is as follows.

Component	CH3COOC2H5	H2O	CH3COOH	C2H5OH
Initial mol	8.0	5.0	0.0	0.0
Equilibrium mol	6.0	3.0	2.0	2.0

Source: OCR Module 2816/01 June 2008 part question