Brønsted-Lowry Concept of Acids and Bases

                                                                     Std  5e 


Brøsted-Lowry  is the best way to describe and define acids and bases.  It is different from what Arrhenius had to say during the late 1800’s.  The Arrhenius definition of acids & bases spoke of   H+ ions from acids and  OH- ions from bases.  If you want a complete explanation for why we are better off with the Brøsted-Lowry definition click Why we don't use the Arrhenius theory.  This explanation comes from The Chem Team (Diamond Bar High).


Brøsted-Lowry definition of acids & bases simply stated:

            acid = a proton donor (can give up a proton)

            base = a proton acceptor (can take on a proton)

            conjugate base = what is left of the acid after it loses the proton

            conjugate acid = is the base after it takes the proton


NOTE:   a proton is the same thing as a hydrogen nucleus, which is the same as a hydrogen ion.


Let’s do a few examples that should clear this up for you. 


1.  Hydrofluoric acid (WEAK ACID) in water. 


                                Acid                base                                                                conj base                               conj acid

         Eqn A     HF     +     H2O         --->                       F-         +        H3O+ 


         WEAK ACID

       (hydrofluoric acid  HF)


         HF  donates a proton to H2O .   After HF lost its proton (H nucleus) all that is left is the conjugate base F- ion.  After H2O gets the proton, it becomes the hydronium ion  H3O+ .  


         We can write this same equation this way:


                                     Acid                      base                                    conj base                 conj acid

         Eqn B        H3O+     +         F-           <---              H2O      +       HF


         WEAK ACID

       (hydrofluoric acid  HF)



Take time with each of these and look and make sure that you can see the differences.

         To really understand, you may need 10 or more minutes to look back and forth at the differences.  Print out this page to make it easier to look back and forth.


1.      Big and small arrows go different ways in Eqn A and B

2.      In Eqn A    the HF   is the acid,  in Eqn B   HF    is the conjugate acid

3.      In Eqn A   the H2O  is the base, in Eqn B   H2O  is the conjugate base

4.      In Eqn B   the H3O+ is the acid,  in Eqn A H3O+  is the conjugate acid

5.      In Eqn B   the   F-     is the base, in Eqn A   F-      is the conjugate base


In all equations the length of the arrows indicates which way the equation “wants” to go (products or reactants). 

STRONG ACID OR BASE  big arrow will go towards the side with the ions.

WEAK ACID OR BASE       big arrow will go towards the side without the ions.

As we saw in the demonstration with the lightbulb in class, more ions gives you a brighter light – brighter light means more electricity is conducted through the solution.


2.      HCl  (STRONG ACID)  in water 

         This time you decide which way the big and small arrows should go.  You label the acid, base, conjugate acid and conjugate base, see what the differences are as shown above for the weak acid.  Also, try the reverse reactioin as I did above for the weak acid. 



                HCl   +    H2O             ----                     H3O+        +      Cl-   





After you have everything labeled, come to me and I’ll let you know if you have done it right.  This will be worth an extra credit stamp!