Ionic Equilibria I: Acids and Bases Unit 8 Ionic Equilibria I: Acids and Bases

Chapter Goals Identify strong electrolytes and calculate concentrations of their ions Understand the autoionization of water Understand the pH and pOH scales Perform calculations involving pH, pOH, kw.

A Review of Strong Electrolytes RECALL:- Weak acids and bases ionize or dissociate partially, much less than 100%. In this chapter we will see that it is often less than 10%! Strong electrolytes ionize or dissociate completely. Strong electrolytes approach 100% dissociation in aqueous solutions.

A Review of Strong Electrolytes There are three classes of strong electrolytes. Strong Water Soluble Acids Remember the list of strong acids from Chapter 4.

A Review of Strong Electrolytes

A Review of Strong Electrolytes Strong Water Soluble Bases The entire list of these bases was also introduced in Chapter 4.

A Review of Strong Electrolytes Most Water Soluble Salts The solubility guidelines from Chapter 4 will help you remember these salts.

A Review of Strong Electrolytes The calculation of ion concentrations in solutions of strong electrolytes is easy. Example 18-1: Calculate the concentrations of ions in 0.050 M nitric acid, HNO3.

A Review of Strong Electrolytes Example 18-2: Calculate the concentrations of ions in 0.020 M strontium hydroxide, Sr(OH)2, solution. You do it!

The Autoionization of Water Pure water ionizes very slightly. The concentration of the ionized water is less than one-millionth molar at room temperature.

The Autoionization of Water We can write the autoionization of water as a dissociation reaction similar to those previously done in this chapter. Because the activity of pure water is 1, the equilibrium constant for this reaction is:

The Autoionization of Water Experimental measurements have determined that the concentration of each ion is 1.0 x 10-7 M at 25oC. Note that this is at 25oC, not every temperature! We can determine the value of Kc from this information.

The Autoionization of Water This particular equilibrium constant is called the ion-product for water and given the symbol Kw. Kw is one of the recurring expressions for the remainder of this chapter and Chapters 19 and 20.

The Autoionization of Water Example 18-3: Calculate the concentrations of H3O+ and OH- in 0.050 M HCl.

The pH and pOH scales A convenient way to express the acidity and basicity of a solution is the pH and pOH scales. The pH of an aqueous solution is defined as:

The pH and pOH scales In general, a lower case p before a symbol is read as the ‘negative logarithm of’ the symbol. Thus we can write the following notations.

The pH and pOH scales If either the [H3O+] or [OH-] is known, the pH and pOH can be calculated. Example 18-4: Calculate the pH of a solution in which the [H3O+] =0.030 M.

The pH and pOH scales Example 18-5: The pH of a solution is 4.597. What is the concentration of H3O+? You do it!

The pH and pOH scales A convenient relationship between pH and pOH may be derived for all dilute aqueous solutions at 250C. Taking the logarithm of both sides of this equation gives:

The pH and pOH scales Multiplying both sides of this equation by -1 gives: Which can be rearranged to this form:

The pH and pOH scales Remember these two expressions!! They are key to the next three chapters!

The pH and pOH scales The usual range for the pH scale is 0 to 14. And for pOH the scale is also 0 to 14 but inverted from pH. pH = 0 has a pOH = 14 and pH = 14 has a pOH = 0.

The pH and pOH scales Example 18-6: Calculate the [H3O+], pH, [OH-], and pOH for a 0.020 M HNO3 solution. Is HNO3 a weak or strong acid? What is the [H3O+] ?

The pH and pOH scales Example 18-6: Calculate the [H3O+], pH, [OH-], and pOH for a 0.020 M HNO3 solution.

The pH and pOH scales To help develop familiarity with the pH and pOH scale we can look at a series of solutions in which [H3O+] varies between 1.0 M and 1.0 x 10-14 M. [H3O+] [OH-] pH pOH 1.0 M 1.0 x 10-14 M 0.00 14.00 1.0 x 10-3 M 1.0 x 10-11 M 3.00 11.00 1.0 x 10-7 M 7.00 2.0 x 10-12 M 5.0 x 10-3 M 11.70 2.30