Review- Acid/Base theory

 

Brønsted-Lowry Acid- Substance that increases [H+]

Brønsted-Lowry Base- Substance that decreases [H+]

 

          pH = -log[H+] ;      higher [H+] = lower pH

 

Equilibrium Constant       Keq =  [products]

                                                [reactants]

 

          for self-ionization of water                  H2O   D   H+    +    OH-

 

                   Keq     =    [ H+ ] [OH-]              for [H2O] =  1000g/L   =  55.5 M

                              [H2O]                                      18 g/mol

 

          given that [ H+ ]  = [OH-]  =  1.0 x 10-7 M

 

          then             Keq = (1.0 x 10-7)( 1.0 x 10-7)     =    1.8 x 10-16

                                                55.5

 

therefore      [ H+ ] [OH-] =(Keq )(55.5) = (1.8 x 10-16)(55.5)  = 1.0 x 10-14  = Kw

 

Kw à Ion Product of water.  This describes that the concentration of hydrogen ions

and hydroxide ion are inversely proportional.

 

Can we calculate pH when  [ H+ ]  = [OH-] 

 

          pH =  - log [ H+ ]   =   - log (1.0 x 10-7)    =   7

 

So, when  [ H+ ]  = [OH-] , the solution is neutral.

 

Ionization Constant (Ka)- describes the ability of an acid to form ions in solution.

 

          For strong electrolytes: ( like strong acids)

 

                   HCl    +   H20   à H3O+     +    Cl-

 

                   So Keq         =  [ H3O+ ] [Cl- ]   >> 1 (much larger than one)

                                           [ HCl ][ H20 ]       

 

          So… this says that in solution, the prevalent forms are  H3O+  and Cl-

 

 

          Weak Electrolytes.  Do they strongly dissociate?

 

          Acetic Acid  (CH3COOH)

 

                   CH3COOH  +  H2O   à CH3COO-    +  H3O+

 

                    Keq =      [CH3COO- ][H3O+]     , but with weak electrolytes, [H2O] remains

                                   [CH3COOH][H2O]       very large and almost unchanged. IGNORE IT.

 

                               Ka  =      [CH3COO- ][H3O+]     =    1.74 x 10-5

                                                  [CH3COOH]

 

          So what does this mean?

 

 

Relating pH and Ka values. The Henderson-Hasselbalch Equation

 

          Ka describes the equilibrium concentrations of the ionic forms and acid forms.

 

          What does pH depend on?

                  

          The amount of H+ in solution is dependent upon the amount of acid that has been

dissociated.

In other words [H+] = [A-], given the equation:  HA  +  H2O à H3O+  +  A-

 

Deriving Henderson-Hasselbalch,

 

                   Ka  =      [A- ][H3O+]      

                                      [HA]

 

                   [H3O+]   =   Ka  [HA]

                                        [A- ]         

 

          Take (-log) of both sides           pH  =  pKa  - log [HA]

                                                                                      [A-]

 

                                                          pH = pKa  +  log [A-]

                                                                                    [HA]

 

 

 

         

          Can we find the pH when a weak acid is ½ dissociated if we know the Ka ?

 

                   pH  = -log Ka   when [A-]= [HA],            because if [A-] = [HA], then log 1 = 0

 

                  

pH of HAc (@ [Ac-] = [HAc])  = - log (1.74 x 10-5)  = 4.76

 

 

          Does the initial concentration of HCl affect the pH?

          Does the initial concentration of HAc affect the pH?

 

 

Changes in pH as a function in changes in [A-] or [HA]

 

What happens if the pH of an amino acid solution is decreased? Increased?

 

Example.  If the pH is 2 units below the pKa

 

                   pH =  pKa + log [A-]   à  pH =  pKa +(-2)

     [HA]

                  

                   therefore:     log [A-]      =   -2

                                            [HA]

 

                   (take 10^ of both sides)             [A-]   = 0.01

                                                                   [HA]

 

                   What does this mean?

 

 

 

          The pKa of Formic Acid is 3.75.  Calculate using the Henderson-Hasselbalch equation the pH of a solution containing 200 mL of 0.1 M Formic acid and 150 mL of 0.1 NaOH with enough water bring the entire solution volume to 1 L.

 

 

Titrations of Amino Acids

         

                                   

            

 

 

What do  pKa1  & pKa2 mean?

          Where does the Isoelectric point exist?  What does it mean?

 

Solving for Isoelectric point

 

pI  =  pKa1  +  pKa2   

                             2

 

          For alanine, the pI equals

 

                   pI =  2.3 + 9.7  = 6

                                2

                  

          So, what does this mean?

 

 

 

 

         

 

          Calculate the pI for Glutamic Acid.