Acids, Bases, and Salts

Welcome to your comprehensive study guide for CXC Chemistry 2024-2025 on the topic of Acids, Bases, and Salts. This lesson covers all the essential concepts, definitions, reactions, and applications you need to excel in your examinations.

1. Introduction to Acids and Bases

Acids and bases are fundamental classes of chemical compounds with distinct properties that play crucial roles in chemistry and everyday life.

1.1 Historical Development of Acid-Base Theories

• The Arrhenius Theory (1884): Defines acids as substances that produce hydrogen ions (H+) in water and bases as substances that produce hydroxide ions (OH-) in water.
• The Brønsted-Lowry Theory (1923): Defines acids as proton (H+) donors and bases as proton acceptors.
• The Lewis Theory (1923): Defines acids as electron pair acceptors and bases as electron pair donors.

1.2 Properties of Acids

• Taste sour (never taste chemicals in the laboratory!)
• Turn blue litmus paper red
• Have pH values less than 7
• React with metals to produce hydrogen gas
• React with carbonates to produce carbon dioxide
• Conduct electricity in aqueous solutions
• Neutralize bases to form salts and water

1.3 Properties of Bases

• Taste bitter (never taste chemicals in the laboratory!)
• Feel slippery or soapy
• Turn red litmus paper blue
• Have pH values greater than 7
• Conduct electricity in aqueous solutions
• Neutralize acids to form salts and water

2. Acid-Base Theories

2.1 Arrhenius Theory

According to Arrhenius:

• An acid is a substance that produces hydrogen ions (H+) in aqueous solution
• A base is a substance that produces hydroxide ions (OH-) in aqueous solution

The main limitation of the Arrhenius theory is that it only applies to aqueous solutions and cannot explain the basic properties of compounds like NH₃ which do not contain hydroxide ions.

2.2 Brønsted-Lowry Theory

According to Brønsted-Lowry:

• An acid is a proton (H+) donor
• A base is a proton (H+) acceptor

This theory introduces the concept of conjugate acid-base pairs:

• A conjugate acid is formed when a base accepts a proton
• A conjugate base is formed when an acid donates a proton

2.3 Lewis Theory

According to Lewis:

• An acid is an electron pair acceptor
• A base is an electron pair donor

This is the most general acid-base theory and explains reactions where no proton transfer occurs.

3. Strength of Acids and Bases

3.1 Strong and Weak Acids

The strength of an acid depends on its degree of ionization in aqueous solution:

• Strong acids completely ionize in water (e.g., HCl, HNO₃, H₂SO₄)
• Weak acids partially ionize in water (e.g., CH₃COOH, H₂CO₃, H₃PO₄)

3.2 Strong and Weak Bases

Similarly, the strength of a base depends on its degree of ionization:

• Strong bases completely ionize in water (e.g., NaOH, KOH, Ca(OH)₂)
• Weak bases partially ionize in water (e.g., NH₃, Na₂CO₃)

3.3 pH Scale

The pH scale measures the concentration of hydrogen ions (H⁺) in a solution:

• pH = -log[H⁺]
• pH < 7: Acidic solution
• pH = 7: Neutral solution (at 25°C)
• pH > 7: Basic/alkaline solution

Similarly, pOH measures the concentration of hydroxide ions:

• pOH = -log[OH⁻]
• pH + pOH = 14 (at 25°C)

4. Acid-Base Indicators

Acid-base indicators are substances that change color at specific pH values, helping to determine the pH of a solution.

5. Neutralization Reactions

Neutralization is a reaction between an acid and a base that produces water and a salt.

5.1 Types of Neutralization Reactions

• Strong acid + Strong base: Forms a neutral salt (pH = 7)
• Strong acid + Weak base: Forms an acidic salt (pH < 7)
• Weak acid + Strong base: Forms a basic salt (pH > 7)
• Weak acid + Weak base: Forms a salt whose pH depends on the relative strengths of the acid and base

5.2 Titration

Titration is a laboratory technique used to determine the concentration of an acid or base by neutralizing it with a standard solution of known concentration.

• The equivalence point is reached when the acid and base have neutralized each other completely.
• The endpoint is detected using an indicator that changes color.

5.3 Titration Curves

Titration curves show the change in pH during a titration. The shape of the curve depends on the strengths of the acid and base involved.

6. Salts

Salts are ionic compounds formed from the neutralization of acids and bases.

6.1 Formation of Salts

Salts can be formed through various methods:

• Direct combination of elements: 2Na + Cl₂ → 2NaCl
• Neutralization reactions: HCl + NaOH → NaCl + H₂O
• Reaction of an acid with a metal: Zn + 2HCl → ZnCl₂ + H₂
• Reaction of an acid with a carbonate: CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂

6.2 Types of Salts

Salts can be categorized based on their formation and properties:

• Normal salts: Formed when all hydrogen ions in an acid are replaced by metal or ammonium ions (e.g., NaCl, KNO₃).
• Acid salts: Formed when not all hydrogen ions in an acid are replaced (e.g., NaHSO₄, KHCO₃).
• Basic salts: Formed when a base does not fully neutralize an acid, leaving hydroxide (e.g., Mg(OH)Cl).
• Double salts: Contain two different cations in the same crystalline structure (e.g., KAl(SO₄)₂·12H₂O, commonly known as alum).
• Complex salts: Contain a central metal ion surrounded by other molecules or ions (e.g., [Fe(CN)₆]⁴⁻ in potassium ferrocyanide).

6.3 Properties of Salts

• Salts are usually crystalline solids.
• They have high melting and boiling points.
• Most salts dissolve in water, dissociating into their respective ions.
• Some salts undergo hydrolysis in water, affecting the pH of the solution.
• Salts can be hygroscopic (absorbing moisture from the air) or efflorescent (losing water to the air).

6.4 Applications of Salts

• Food industry: Table salt (NaCl) is used for seasoning and food preservation.
• Medical uses: Epsom salt (MgSO₄) is used in bath therapy and medicine.
• Agriculture: Fertilizers such as ammonium nitrate (NH₄NO₃) and potassium sulfate (K₂SO₄).
• Industrial applications: Sodium carbonate (Na₂CO₃) is used in glass and soap manufacturing.
• Water treatment: Aluminum sulfate (Al₂(SO₄)₃) is used in water purification.

7. Buffers

Buffers are solutions that resist changes in pH when small amounts of acid or base are added. They are essential in biological systems and industrial applications.

7.1 Types of Buffers

• Acidic buffers: Contain a weak acid and its salt (e.g., CH₃COOH + CH₃COONa).
• Basic buffers: Contain a weak base and its salt (e.g., NH₃ + NH₄Cl).

7.2 Importance of Buffers

• Maintain pH in biological systems (e.g., blood buffer system).
• Used in the pharmaceutical industry to stabilize drug formulations.
• Essential in biochemical reactions, where pH must remain constant.
• Used in industrial processes, such as dyeing and fermentation.

Glossary

• Acid: A substance that donates hydrogen ions (H⁺) in a solution.
• Base: A substance that accepts hydrogen ions or donates hydroxide ions (OH⁻).
• pH Scale: A scale from 0 to 14 that measures the acidity or basicity of a solution.
• Neutralization: A chemical reaction between an acid and a base to produce a salt and water.
• Buffer: A solution that maintains a stable pH when small amounts of acid or base are added.
• Salt: An ionic compound formed from the reaction of an acid and a base.

6. Salts

Salts are ionic compounds formed from the neutralization of acids and bases.

6.1 Formation of Salts

Salts can be formed through various methods:

• Direct combination of elements: 2Na + Cl₂ → 2NaCl
• Neutralization reactions: HCl + NaOH → NaCl + H₂O
• Reaction of an acid with a metal: Zn + 2HCl → ZnCl₂ + H₂
• Reaction of an acid with a carbonate: CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂

6.2 Types of Salts

Salts can be categorized based on their formation and properties:

• Normal salts: Formed when all hydrogen ions in an acid are replaced by metal or ammonium ions (e.g., NaCl, KNO₃).
• Acid salts: Formed when not all hydrogen ions in an acid are replaced (e.g., NaHSO₄, KHCO₃).
• Basic salts: Formed when a base does not fully neutralize an acid, leaving hydroxide (e.g., Mg(OH)Cl).
• Double salts: Contain two different cations in the same crystalline structure (e.g., KAl(SO₄)₂·12H₂O, commonly known as alum).
• Complex salts: Contain a central metal ion surrounded by other molecules or ions (e.g., [Fe(CN)₆]⁴⁻ in potassium ferrocyanide).

6.3 Properties of Salts

• Salts are usually crystalline solids.
• They have high melting and boiling points.
• Most salts dissolve in water, dissociating into their respective ions.
• Some salts undergo hydrolysis in water, affecting the pH of the solution.
• Salts can be hygroscopic (absorbing moisture from the air) or efflorescent (losing water to the air).

6.4 Applications of Salts

• Food industry: Table salt (NaCl) is used for seasoning and food preservation.
• Medical uses: Epsom salt (MgSO₄) is used in bath therapy and medicine.
• Agriculture: Fertilizers such as ammonium nitrate (NH₄NO₃) and potassium sulfate (K₂SO₄).
• Industrial applications: Sodium carbonate (Na₂CO₃) is used in glass and soap manufacturing.
• Water treatment: Aluminum sulfate (Al₂(SO₄)₃) is used in water purification.

7. Buffers

Buffers are solutions that resist changes in pH when small amounts of acid or base are added. They are essential in biological systems and industrial applications.

7.1 Types of Buffers

• Acidic buffers: Contain a weak acid and its salt (e.g., CH₃COOH + CH₃COONa).
• Basic buffers: Contain a weak base and its salt (e.g., NH₃ + NH₄Cl).

7.2 Importance of Buffers

• Maintain pH in biological systems (e.g., blood buffer system).
• Used in the pharmaceutical industry to stabilize drug formulations.
• Essential in biochemical reactions, where pH must remain constant.
• Used in industrial processes, such as dyeing and fermentation.

Self-Assessment Questions

Test your understanding of acids, bases, and salts with these questions. Click on the plus sign (+) to reveal the answers.

• 1. What is the definition of an acid according to the Arrhenius theory? [+] Click for solution

  An acid is a substance that increases the concentration of H⁺ ions in an aqueous solution.

• 2. What is the difference between a strong acid and a weak acid? [+] Click for solution

  A strong acid completely ionizes in water (e.g., HCl), whereas a weak acid only partially ionizes (e.g., CH₃COOH).

• 3. Write the balanced chemical equation for the reaction of hydrochloric acid with sodium hydroxide. [+] Click for solution

  HCl + NaOH → NaCl + H₂O

• 4. How can you differentiate between an acid and a base using indicators? [+] Click for solution

  Acids turn blue litmus red, while bases turn red litmus blue.

• 5. What is a buffer solution? [+] Click for solution

  A buffer is a solution that resists changes in pH when small amounts of acid or base are added.

• 6. What is the definition of an acid? [+] Click for solution

  An acid is a substance that donates protons (H⁺) in a solution.

• 7. What is the pH range of a basic solution? [+] Click for solution

  A basic solution has a pH greater than 7.

• 8. Give an example of a strong acid and a strong base. [+] Click for solution

  Strong acid: Hydrochloric acid (HCl). Strong base: Sodium hydroxide (NaOH).

• 9. What is the reaction equation for the neutralization of HCl and NaOH? [+] Click for solution

  HCl + NaOH → NaCl + H₂O

• 10. What is the difference between a normal salt and an acid salt? [+] Click for solution

  A normal salt forms when all hydrogen ions in an acid are replaced, while an acid salt forms when only some hydrogen ions are replaced.

• 11. Why are salts important in everyday life? [+] Click for solution

  Salts are used in food, medicine, agriculture, water treatment, and industry.