AUTOIONIZATION OF WATER

Before we study acids and bases, let's first look at the autoionization of water.

In this reaction, two water molecules react forming the hydronium ion and the hydroxide ion. The equilibrium expression for this reaction is:

In pure water the hydronium ion concentration and the hydroxide ion concentration are equal to 1.00 E-7 and the water is said to be neutral.

ACIDS AND EQUILIBRIUM

An acid is any compound which can donate a proton. It reacts with water in the equation below. If the acid is a strong acid (ionizes 100%), the equilibrium lies entirely to the right. However, if the acid is a weak acid (ionizes < 100%), the equilibrium is dependent on the acid equilibrium constant, Ka. An acid equilibrium expression can only be written for weak acids.

BASES AND EQUILIBRIUM

A base is any compound which can accept a proton. It reacts with water in the equation below. If the base is a strong base (ionizes 100%), the equilibrium lies entirely to the right. However, if the base is a weak base (ionizes <100%), the equilibrium is dependent on the base equilibrium constant, Kb. A base equilibrium expression can only be written for weak bases.

CONJUGATE ACID-BASE PAIRS

A pair of compounds or ions that differ by the presence of one proton (H+) is called a conjugate acid-base pair. For instance in the base hydrolysis above, BH+ and B are a conjugate acid-base pair. In addition, the water and the hydroxide ion are also a conjugate acid-base pair. Therefore, in every reaction that involves and acid or a base, there is a conjugate acid-base pair.

The acid equilibrium constant and the base equilibrium constant for a conjugate acid and a conjugate base can be related.

Therefore, if you know the kA for a weak acid (HA), you can calculate the Kb for its conjugate base (A-).

THE pH SCALE

The pH scale allows scientists to represent acid and base concentrations in small numbers. pH represents the hydronium ion concentration and pOH represents the hydroxide ion concentration. They are related by the equations below.

BUFFER SOLUTIONS

In a buffer solution, the pH is resistance to change. This means that the addition of an acid or a base does not affect the hydronium ion concentration. Human blood is an example of a buffer solution. A buffer is usually prepared from roughly equal quantities of a conjugate acid-base pair.

1. a weak acid and its conjugate base
2. a weak base and its conjugate acid

The pH of a buffer solution can be calculated using the Henderson-Hasselbalch equation.

This is equation will be useful during a titration when trying to find the pH.

TITRATIONS AND ACID/BASE EQUILIBRIUM

A titration is an example of an acid/base equilibrium. In an titration, an acid and a base react to form a salt and water. Here are some important definitions.

• equivalence point - when the moles of acid or base added equals the moles of acid or base which are present
• midpoint - when the moles of acid or base added is 1/2 the moles of acid or base which are present
• indicator - a dye used to indicate when the reaction is complete. It changes color at the equivalence point.

There are five types of acid/base titrations which can occur:

1. Strong Acid with Strong Base
2. Strong Base with Strong Acid
3. Weak Acid with Strong Base
4. Weak Base with Strong Acid
5. Weak Acid with Weak Base

Now, for our purposes, we will only study the first four titrations. For a thorough study of all types of titrations, please visit your lecture text.

In a each titration, there are 5 important regions

1. At time = 0, before the titration begins
2. Before the equivalence point
3. At the midpoint
4. At the equivalence point
5. After the equivalence point

The table below outlines the four types of titrations and the five important regions of each titration. The table tells you what type of equation to use at which point in a titration to calculate the pH of the titration.

IMPORTANT NOTES:

1. E.P - Equivalence Point
2. SA, SB, WA, WB - Strong Acid, Strong Base, Weak Acid, Weak Base
3. ICE Table - Initial, Change and Equilibrium Table - Please refer to General Equilibrium
4. SRFC Table - Starting Moles, Reacting Moles, Final Moles, Concentration Table. This table is similar to an ICE table. To learn how to do an SRFC table, follow the titration example below.
5. This table is only a study guide. It may not be used for any exams.

Titration Example Problem: Calculate the pH at t = 0, at 5.00 mL, at 10.00 mL, at the equivalence point and at 40.00 ml in a titration of 25.0 mL of 0.120 M formic acid with 0.105 M NaOH.(kA of formic acid = 1.8 E-4)

1. First, identify the reaction: WA with a SB

2. At t = 0, must do an ICE table with the WA

3. At 5.00 ml: this is before the EP, so do SRFC, then use Henderson-Hasselbalch

4. At 10.00 ml, also before EP, do SRFC, then use Henderson-Hasselbalch

5. At the equivalence point, do SRFC, then do ICE table with the conjugate base of the weak acid

6. At 40.00 ml: this is after the EP so do SRFC and use the excess strong base to calculate the pH