Buffers – Ch 19.4                          Std 5

        

Buffers are created by mixing a weak acid and the salt of its conjugate base.

                       

HC2H3O2  <------------------------  H+   +   C2H3O2-  

(weak acid)              -->

 

         Notice the equilibrium favors the acid (left) very strongly.  That is why it is a weak acid.  Not many ions get formed because it doesn’t ionize much.

 

NaC2H3O2  -------------------------->  Na+  +  C2H3O2-

(salt of conj. base)

         Notice the salt ( NaC2H3O2 ) ionization is 100% , which means that all of the salt is ionized and none of the original NaC2H3O2 is left.  This puts a lot of the Na+ and C2H3O2-  ions into the water.

 

         After we put     HC2H3O2    and      NaC2H3O2     into the water, the ions and substances that can affect the pH are:

                                    HC2H3O2     and     C2H3O2-      (IN THE WATER)

 

         We now call this solution a buffer.  This buffer will have a pH = 4 (±2) depending on the concentrations of the ions. 

 

         Let’s see what happens when we add acids or bases to the buffer:

        

a.  ADD ACID 

HNO3   ----->   H+  +  NO3-       NEXT       H+  +  C2H3O2-  ----------->  HC2H3O2

(strong acid)                                                                                         <--

                                          IT MIXES & REACTS                   

At first   H+  ions from the HNO3 make the solution more acidic (lower pH), but they are quickly   “grabbed”  by the C2H3O2- ions which act as “assassin” ions and and hold on to the H+ ions.  This will  form HC2H3O2, which returns the pH close to 4.   The pH is again close to the original pH = 4 because the H+ ions taken by the C2H3O2- ions are no longer in the water.

 

         b.  ADD BASE

NaOH  ---->  Na+  +  OH-    NEXT  OH-  +  HC2H3O2  ---------->  H2O  +  C2H3O2 1-

(strong base)                                                                                  <--

                                      IT MIXES & REACTS

The OH-  ions act like “assassins” and  “grab”  the  H+ ions away from the HC2H3O2  to form water and C2H3O21- ions.  That  removes OH- ions from the solution and makes more C2H3O21- ions.   Adding the OH-  ions only makes the solution more basic for a short time.  When this second reaction takes place the pH come back close to pH = 4  because the OH- ions are used up forming water.

 

Here are additional explanations of Buffers given in class

 

         Buffer example questions

 

1.  Hydrofluoric acid (HF) a weak acid,  and potassium fluoride (KF) the salt of its conjugate base

HF                               -->                      H+      +         F- 

Weak acid    <--------------------------                      conjugate base

 

     =============================================

 

         KF       --------------------------->                    K+      +        F-

Salt of conj base

 

         If we add OH-  (NaOH)  -  What combines with the OH- ions so the pH doesn’t change much?  

 

            If we add H+ (HCl) – What combines with the H+ ions so the pH doesn’t change much?

                                                                                Answers

 

2.  Acetic acid (HC2H3O2 ), and sodium acetate (NaC2H3O2 ), the salt of the conjugate base.

HC2H3O2                          -->           H+     +    C2H3O2-

Weak acid   <----------------------                  conjugate base

 

NaC2H3O2    ---------------------->    Na+    +   C2H3O2-

salt of conjugate base

 

         If we add OH-  (NaOH)  -  What combines with the OH- ions so the pH doesn’t change much?  

 

            If we add H+ (HCl) – What combines with the H+ ions so the pH doesn’t change much?

                                                                Answers

 

3.   Can salts of strong acids act as buffers by themselves?

 

            Examples:     NaCl,  KCl, CaCl2 , MgCl2                    

                                                    Answers

 

Here is an important summary of buffer information:

     The a  and  b below explain how a buffer works to keep the pH relatively constant when acids or bases are added to their solutions.

 

            a.  Buffers resist change in pH even if H+  or  OH-  ions are added from strong acids or bases.

 

b.  H+ ions react with conjugate bases of weak acids.  OH- ions combine with the H+ ions from weak acids, taking the hydrogen away from them.