Bronsted-Lowry Concept

Bronsted-Lowry Theory of Acid-Base Behavior

The Arrhenius theory was the only theory used to explain the behaviour of acids and bases for about forty years. In 1923, J. N. Bronsted in Denmark and T. M. Lowry in England independently, and almost simultaneously, proposed the modern “protonic” or Bronsted-Lowry theory of acid-base behaviour. According to the Bronsted-Lowry concept, an acid is any compound or ion which can give up a proton, while a base is any compound or ion which can accept a proton.

Any species which can either accept or give up a proton is said to be amphiprotic. Thus the water molecule is amphiprotic, since it can give up a proton, H2O –> H+ + OH, to form the hydroxyl ion OH. Alternatively, water can accept a proton to form the hydronium ion H3O+, according to the equation H+ + H2O –> H3O+. The above two equations can be combined to give the dissociation equation for water: 2H2O –> H3O+ + OH.

 

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)

 

DIFFERENCES BETWEEN TWO EQUATIONS

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   H2is 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!

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