ACIDS & BASES

• Many common substances in our daily lives are acids and bases. Oranges, lemons and vinegar are examples of acids. In addition, our stomachs contain acids that help digest foods. Antacid tablets taken for heartburn and ammonia cleaning solutions are examples of bases.
• General properties associated with acids include the following:
• sour taste
• change color of litmus from blue to red
• react with metals to produce gas
• react with bases to produce salt and water
• General properties associated with bases include the following:
• bitter taste
• slippery soapy feeling
• change color of litmus from red to blue
• react with acids to produce salt and water
• The most common definition of acids and bases was formulated by the Swedish chemist Svante Arrhenius in 1884.
• According to the Arrhenius definition,
• Acids are substances that produce hydronium ion ( ) in aqueous solution

• Bases are substances that produce hydroxide ion in aqueous solution

BRØNSTED-LOWRY ACIDS & BASES

• The Arrhenius definition of acids and bases is limited to aqueous solutions.
• A broader definition of acids and bases was developed by Brønsted and Lowry in the early century.
• According to Brønsted-Lowry definition, an acid is a proton donor, and a base is a proton acceptor.

• A substance that can act as a Brønsted-Lowry acid and base (such as water) is called amphiprotic.
• In Brønsted-Lowry definition, any pair of molecules or ions that can be interconverted by transfer of a proton is called conjugate acid-base pair.
  

BRØNSTED-LOWRY ACIDS & BASES

Examples:

1. Identify the conjugate acid-base pairs for each reaction shown below:

2. Write the formula for the conjugate acid for each base shown:

3. Write the formula for the conjugate base for each acid shown:

ACID & BASE STRENGTH

• According to the Arrhenius definition, the strength of acids and bases is based on the amount of their ionization in water.
• Strong acids and bases are those that ionize completely in water.
• Strong acids and bases are strong electrolytes.

• Weak acids and bases are those that ionize partially in water, and are therefore written as reversible reactions (indicated by ).
• Weak acids and bases are weak electrolytes.

ACID & BASE STRENGTH

• Listed below are the formulas and names of common acids and bases, and comparison of their characteristics.

IONIZATION OF WATER

• As noted previously, water can act both as an acid and a base.
• In pure water, one water molecule donates a proton to another water molecule to produce ions.

• In pure water, the transfer of protons between water molecules produces equal numbers of and ions. However, the number of ions produced in pure water is very small, as indicated below:

• When the concentrations of and are multiplied together, the ionproduct constant ( ) is formed.

• All aqueous solutions have and ions. An increase in the concentration of one of the ions will cause an equilibrium shift that causes a decrease in the other one.

ACIDIC & BASIC SOLUTIONS

• When [ ] and [ ] are equal in a solution, it is neutral.
• When [ ] is greater than [ ] in a solution, it is acidic.

• When [ ] is greater than [ ] in a solution, it is basic.

ACIDIC & BASIC SOLUTIONS

Examples:

1. Calculate the in a solution with . Classify the solution as acid or basic.

2. Calculate the in a solution with . Classify the solution as acid or basic.

3. Calculate the in a solution with . Classify the solution as acid or basic.
4. Calculate in solution prepared by dissolving 2.8 g KOH to make 45 mL of solution.

THE pH SCALE

• The acidity of a solution is commonly measured on a scale.

• The pH scale ranges from 0-14, where acidic solutions are less than 7 and basic solutions are greater than 7 .

Examples:

1. The [ ] of a liquid detergent is . Calculate its pH .

2. The pH of black coffee is 5.3. Calculate its .

3. The of a solution is . Calculate its pH .
4. The pH of tomato juice is 4.1. Calculate its .
5. The [ ] of a cleaning solution is . What is the pH of this solution?

6. Calculate the pH of a solution.

BUFFERS

• The pH of water and most solutions change dramatically when a small amount of acid or base are added to it. However, when a small amount of acid or base is added to a buffer, the pH does not change very much.
• A buffer solution maintains the of a solution by neutralizing the added acid or base. Proper physiological functioning in the human body requires a very tight balance between the concentrations of acids and bases in the blood. A variety of buffering systems permits blood and other bodily fluids to maintain a narrow pH range.

• A buffer solution consists of a weak acid and its conjugate base or a weak base and its conjugate acid. Both buffer systems work similarly, but we will concentrate on the former type to understand how a buffer works.
• An example of such buffer is a mixture of acetic acid (weak acid) and sodium acetate (conjugate base). In this buffer, acetic acid is represented as HX and the acetate ion is represented as . The sodium ion is the cation associated with the acetate ion and is not part of the buffer system.
• In an ideal buffer, the mixture of weak acid and conjugate base are equal in amounts, but could be slightly different
  
  
  from one another.

HOW BUFFERS WORK

• The acetic acid dissociates in water and produces a small amount of and acetate ion, as shown below. The added sodium acetate increases the concentration of the acetate ion necessary for proper buffering capability.

• When a small amount of acid is added to this buffer, the acetate ion neutralizes it to produce the weak acid and water, as shown below. As a result, the concentration of the weak acid (HX) increases slightly while the concentration of the conjugate base ( ) decreases slightly. However, the pH of the solution changes slightly.

• When a small amount of base is added to this buffer, the acetic acid neutralizes it to produce the acetate ion and water, as shown below. As a result, the concentration of the weak acid (HX) decreases slightly while the concentration of the conjugate base ( ) increases slightly. However, the pH of the solution changes slightly.

CALCULATING pH OF BUFFERS

• Earlier we discussed that weak acids dissociate partially in solution as described by the equation below:

• The degree of dissociation of a weak acid in solution can be quantified by the acid dissociation constant which can be written as:

• The greater the dissociation constant, the more the acid ionizes and the greater the [ ] it produces in solution. Listed below are acid dissociation constants for some common acids.

• Rearranging the acid dissociation constant above, we can calculate the [ ] as:

• The pH of the buffer solution can then be calculated from the .

CALCULATING pH OF BUFFERS

Examples:

1. The for acetic acid is . Calculate the pH of a buffer that is and .
2. One of the buffer systems in blood is . The for is . What is the pH of a buffer that is prepared from and .
3. One of the buffer systems used in blood is .
   a) Write an equation that shows how this buffer neutralizes added acid.
   b) Write an equation that shows how this buffer neutralizes added base.