CHEMISTRY FORM FIVE/SIX

BUFFER SOLUTION

Is a solution which has ability to maintain its PH value constant on either addition of acid or base provided that its buffering capacity is not exceeded.

         

                      TYPES OF BUFFER SOLUTION

1 Acidic buffer solution PH>7

                                                  2 Basic buffer solutions PH<7

                                                         PREPARATION OF BUFFER SOLUTION

                            Acidic buffer solution

Ø  Is prepared from reaction btn weak acid and  strong salts of the same acid,for example ,The mixture containing acetic acid(CH3COOH)  and sodium acetate(CH3COONa),is an example of acidic buffer solution

                                                                     Basic buffer solution

Ø  Is prepared from the reaction btn weak base and strong salt of the same base,forexample A mixture containing ammonium chloride(NH4Cl),is a basic buffer solution.

MECHANISM OF HOW BUFFER SOLUTIONS WORK TO MAINTAIN THEIR PH VALUE

Ø  At first Acetic acid in its aqueous solution  dissociates partially to give small amount of H ions.

CH3COOH    --------    CH3COO ions and H Ions

Ø  When the sodium acetate is added into Acetic acid solution 

                                                         CH3COONa   -------   CH3COO ions and Na ions

                                 Ionization of CH3COONa increases the acetate ions (CH3COOions) in the solution.

                Therefore the increase in CH3COO ions by CH3COOH will cause the equilibrium of Acetic acid to shift more backward hence lead to formation of CH3COOH, Because CH3COO ions will react with Ions of H, being formed from dissociation of Acetic acid originally.

Ø  The new equilibrium will be re established which H ions that remain after establishment of new equilibrium will be the one which determines, the PH of the resulting buffer solution and must remain constant since the PH value of the buffer solution has to remain un changed.

  

              Worked examples on buffer solution

1.       What will happen to the PH of buffer solution if acidic solution like HCL,is added into acidic buffer solution.

                                                                            solution

                                               CH3COOH   ---------   CH3COOions and Hions

                                             CH3COONa    ----------    CH3COOions and Na ions

                                                HCL   ------------                  H ions and CL ions

Ø  The H ions being released by Hcl will disturb the dynamic equilibrium position of CH3COOH .In this case  the equilibrium has to move backward to form more acid,since H ions become combined with CH3COO ions of salts to form acetic acid.

     The PH value of acidic solution will remain fairly constant due to the new re established equilibrium.  

2.       What happens when NaoH becomes added into Acetic buffer solution?

solution

                                                  CH3COOH      --------           CH3COO ions and H ions

                                                     CH3COONa   ---------            CH3COO ions and Na ions

                                                  NaOH      ------------                       Na ions and OH ions

Ø  OH ions released by NaOH,combine with H ions from original CH3COOH to form water,hense causing more acid to dissociate therefore make the equilibrium to move this will result  into decrease of concentration of acetic acid.

                                              Attempt this question below

3.       What is the role of weak acid like CH3COOH in Acidic buffer solution or weak base(NH4OH) In basic buffer solution?

                                                     PH     EQUATION FOR BUFFER SOLUTION

           ACIDIC BUFFER SOLUTION

Ø  Consider dissociation of acetic acid

                                                                        

CH3COOH------------CH3COO ions and H ion

                          NOTE we shall continue with this part after you understand the first part.