Weak Acids and Bases

Acids and Bases: Basic Concepts Weak Acids Acetic acid, HC2H3O2, is a good example of a weak acid. The molecular structure of a weak acid determines the extent to which the acid ionizes in water. A solution of weak acid contains a mixture of un-ionized acid molecules, hydronium ions, and the corresponding conjugate base.

Acids and Bases: Basic Concepts Weak Acids The concentration of the un-ionized acid is always the greatest of the three concentrations.

Acids and Bases: Basic Concepts Weak Bases Ammonia is a weak base because most of its molecules don’t react with water to form ions. Other examples of bases that produce so few OH– ions that they are considered to be weak bases are Al(OH)3, and Fe(OH)3.

Strength Is Not Concentration Acids and Bases: Basic Concepts Strength Is Not Concentration Although the terms weak and strong are used to compare the strengths of acids and bases, dilute and concentrated are terms used to describe the concentration of solutions. The combination of strength and concentration ultimately determines the behavior of the solution.

pH of weak acids and bases Finding pH of strong acids and bases is simple…multiply concentration of acid or base by the number of acidic hydrogens or hydroxide ions, then take the negative log. Weak acids and bases are a little more complicated. They do not dissociate completely, so one must decide what amount is dissociated. This is done using the Ka value. To do this, one must consider the net ionic reaction: HC2H3O2 + H2O(l)  C2H3O2- + H3O+

In this reaction, acetic acid reacts with water to form the acetate ion (Conjugate base) and the hydronium ion. If you recall from last chapter, you can write an equilibrium expression by dividing products by reactants. So Ka=[C2H3O2-][H3O+] _______________ [HC2H3O2]

So all you do is substitute in and solve for the hydronium ion, which allows you to find pH Bases are the same, except you use Kb. Kb=[OH-][Conjugate acid] ______________________ [base]

Practice question 1 Find the pH of 0.05 M acetic acid-HC2H3O2 (Ka= 1.8 x 10-5 )

Find pOH of 0.5M aniline C6H5NH2 (kb=3.8 x 10-10)

Find pH of 2.00 M nitrous acid (HNO2). Ka=4.0 x 10 -4

Acids and Bases: Additional Concepts Acid-Base Titrations The general process of determining the molarity of an acid or a base through the use of an acid-base reaction is called an acid-base titration. Click box to view movie clip.

Acids and Bases: Additional Concepts Acid-Base Titrations The known reactant molarity is used to find the unknown molarity of the other solution. Solutions of known molarity that are used in this fashion are called standard solutions. In a titration, the molarity of one of the reactants, acid or base, is known, but the other is unknown.

You know that NaOH and HCl react completely. Acids and Bases: Additional Concepts Acid-Base Titrations You know that NaOH and HCl react completely. You know the concentration of the NaOH solution, so it is your standard solution.

Acids and Bases: Additional Concepts Acid-Base Titrations You can use the reaction, the volumes of acid and base used, plus the molarity of the base to determine the molarity of the unlabeled HCl.

To do this, simply use the following formula: [H+]Vacid=[OH-]Vbase