CHAPTER 15 NOTES

I. ACID- BASE REACTIONS

A. ACID- BASE REACTIONS ARE USUALLY DOUBLE-DISPLACEMENT REACTIONS.

B. ACID- BASE REACTIONS ARE CLASSIFIED BY THE STRENGTH OF THE ACID AND BASE.

1. STRONG ACID-STRONG BASE

2. WEAK ACID- STRONG BASE

3. WEAK BASE- STRONG ACID

C. REPRESENTING ACID-BASE REACTIONS BY IONIC AND NET IONIC EQUATIONS SHOWS WHAT IS HAPPENING SUBMICROSCOPICALLY.

D. ACIDS AND BASES CAN BE IDENTIFIED USING A HYDROGEN- ION TRANSFER DEFINITION.

1. AN ACID IS AN H⁺ DONOR

2. A BASE IS AN H⁺ ACCEPTOR.

E. WHEN ACIDS AND BASES REACT, THE pH OF THE FINAL SOLUTION IS DEPENDENT UPON THE NATURE OF THE REACTANTS

II. APPLICATIONS OF ACID-BASE REACTIONS

A. TITRATIONS

Introduction to Acid-Base Titrations
The beaker on the left contains a solution of hydrochloric acid and a few drops of phenolphthalein indicator which is colorless in acid. The other beakers contains a solution of sodium hydroxide.
When you mix the acid with the base they neutralize each other and form a salt
The phenolphthalein turned red because we added a slight excess of base.
HCl reacts with NaOH to form NaCl and water.

If you know how much base is needed to exactly neutralize the acid then you can calculate how much acid was present if you also know the concentration of the base.

We need a way to determine when the base has neutralized the acid and a way to measure the volume of the base.

	Buret A buret is a long glass tube with markings so you can read the volume of liquid in it. Zero is at the top to make it easier to measure the volume drained out. The stopcock at the bottom allows you to control the flow of liquid from the buret.
Let's add the base, NaOH, from the buret into the acid, HCl, in this beaker. A pH electrode in the beaker is connected to a pH meter so we can monitor the pH of the solution. A few drops of phenolphthalein have been added to the HCl so you can see the color change as base is added.
Titration of a strong acid with a strong base.
As a solution of NaOH is added to the HCl a plot of pH vs. volume of added base is made.
About 30 mL of base have been added. Notice the rapid change in pH.
The phenolphthalein indicator has changed from colorless to red indicating that the solution is basic.
Notice that the pH is not changing rapidly at this point in the titration.

Here is our plot of pH vs. mL of added NaOH solution.

Titration is the general process of determining the molarity of an acid or a

base through the use of an acid-base reaction. It is an experimental procedure in which a standard solution is used to determine the molarity of an unknown

medium. A standard solution is one of known molarity. The titration involves the gradual addition of one solution to another until the solute in the first solution has completely reacted with the solute in the second solution. This point is called the equivalence point. The equivalence point is detected using an indicator. The point at which the indicator changes color is called the endpoint of the titration.

The most common titrations involve the reaction of an acid solution with a

basic solution. The reaction of the acid and base is termed a neutralization. The products of this reaction are a salt and water.

acid + base à salt + water

The reaction between a strong acid (e.g., HCl or HNO₃) and a strong base

(e.g., NaOH) gives salts (e.g., NaCl or NaNO₃). Since these salts are products of strong acids and strong bases, the resulting solution is neutral.

For a strong acid/strong base titration, the pH at the equivalence point is 7;

but only a small amount of reagent causes a major pH change. The titration curve for the neutralization reaction is shown in Figure 15-1a.

Curves can be produced using a pH meter connected to a chart recorder. The indicator selected should change color in the pH range from about 4 to 10. Phenolphthalein is usually used since it is easy to detect visually a slight pink color from a colorless liquid.

Reaction between a strong acid (HCl, HNO₃, or H₂SO₄) and a weak base

(NH₃) also produces salts (NH₄Cl, NH₄NO₃, or (NH₄)₂SO₄). These salts hydrolyze to form slightly acidic solutions. The titration curve for this reaction is shown in Figure 15-1b.

Methyl orange can be used as an indicator because of the low pH region in which it changes color.

The reaction between a weak acid (CH₃COOH) and a strong base (NaOH)

gives a salt (NaCH₃COO). Such salts hydrolyze to give a slightly basic solution.

The titration curve for this reaction is shown in Figure 15-1c.

Any indicator changing color in the higher pH ranges could be used, but phenolphthalein is most frequently used.

The concentration of the acid and basic solutions will change the position of

the curves only slightly (especially at the start and completion of the titration) in relation to the pH.

B. A BUFFER IS A SOLUTION THAT MAINTAINS A RELATIVELY CONSTANT pH WHEN H⁺ IONS OR OH^- IONS ARE ADDED.

1.) THE pH OF BLOOD IS CONTROLLED IN PART BY A BUFFER COMPOSED OF CARBONIC ACID, H₂CO₃, AND THE HYDROGEN CARBONATE ION, HCO₃^-

C. ANTACIDS ARE BASES THAT REACT WITH STOMACH ACID

D. Other applications