Humboldt State University ® Department of Chemistry

Richard A. Paselk

Chem 107

Fundamentals of Chemistry

Fall 2009

Lecture Notes: 3 December

© R. Paselk 2005
 
     
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Acids and Bases

What are acids and bases? There are three major definitions. We will look at two in which the proton is a major defining component (the third, Lewis definition, is not needed for our study).

(Although I will signify protons in water as H+, you should realize that naked protons do not exist in water - they are always hydrated. At a minimum we see the hydronium ion, H3O+. But hydronium ion is in fact also generally thought to be hydrated, so you will sometimes see hydrogen ion represented as H5O2+, H7O3+, etc.)

        H2O right arrow  H+  + OH- 
         acid       conj. base
                 
     H+ +  OH-  right arrow  H2O  
        base    conj. acid    
                 
 H3O+   left arrow  H+  +  H2O  right arrow  OH- + H+ 

 

conj. acid 

     

 acid

base

  conj. base    

Strong vs. Weak Acids & Bases

These terms have nothing to do with concentration, rather they refer to the degree of dissociation of an acid or base:

Aqueous Hydrogen Ion Concentration and pH

The concentration of hydronium ion in water is extremely influential on all kinds of chemistry. The range of hydronium ion concentration in water is also vast, with extremes of about 10M to about 10-15M, and commonly ranging from 1M - 10-14M. Imagine plotting [H3O+] vs. volume of acid added to a base solution in a titration. If you had one cm on the graph paper = 10-14M, then you would need a piece of paper 109 km long (greater than the distance from the Sun to Jupiter) to plot this titration! Obviously a more convenient measure is needed. This is easily accomplished by looking instead at the logarithm of [H+] and defining a new term,

pH = -log[H+]

Turns out that the concentration of hydrogen ion in water is related to the concentration of hydroxide ion due to the equilibrium dissociation of water:

H2O equilibrium arrow H+ + OH-, so

K = [H+][OH-] / [H2O]

But the concentration of water remains essentially the same in dilute solution,

so by convention we define the dissociation constant or ion product for water:

Kw= [H+][OH-] = 1.0 x 10-14 @ 25 °C

pH

Let's look at pH a bit:

Examples:

Note that the "p" has the more general meaning of "-log[]". Thus pOH is -log [OH-], pCa = -log [Ca2+], etc.


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Last modified 3 December 2009