The equilibrium constant

2)      The equilibrium constant, K_c

• An equilibrium constant can be defined in terms of concentrations for the following reaction:

aA + bB ¾ cC + dD         

 

where a = moles of A etc

• It can be shown that:

K_c =     [C]^c[D]^d

                                                                        [A]^a[B]            ^b

           

where [A] = concentration of A in mol dm^-3 etc

• Once equilibrium moles are known they need to be converted into concentrations before being put into the K_c expression. Concentration = moles/volume (in dm³)
• If the volume is not known, use V to represent it and the V’s will almost certainly cancel
• The units for the K_c can be determined by substituting and cancelling

Example 1 continued

Writing the K_C expression for the equilibrium gives:

                        K_C=       [CH₃COOH][C₂H₅OH]

                                    [CH₃COOC₂H₅][H₂O]

Calculating equilibrium concentrations gives

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
Equilibrium conc	6.0/V	3.0/V	2.0/V	2.0/V

                        K_C=       [2.0/V][2.0/V]

                                    [6.0/V][3.0/V]

                        =          4.0/18.0

                        =          0.22

                        Units = (mol dm^-3)(mol dm^-3)

                                    (mol dm^-3)(mol dm^-3)

                        =          no units (as they all cancel - this should be stated, not left blank)

                                   

Source: OCR Module 2816/01 June 2008 part question