Acids and bases

Acids and Bases

In this section, you will learn about acids and bases, their properties, and their reactions. Acids and bases are essential in chemistry and play a significant role in various chemical reactions.

What are Acids?

Acids are substances that can donate protons (H⁺ ions) in a chemical reaction. They have a sour taste and can conduct electricity when dissolved in water. Acids turn blue litmus paper red.

Common Acids

• Hydrochloric acid (HCl): Found in the stomach, it aids in digestion.
• Sulfuric acid (H₂SO₄): Used in car batteries.
• Acetic acid (CH₃COOH): Found in vinegar.

What are Bases?

Bases are substances that can accept protons or donate hydroxide ions (OH^-) in a chemical reaction. They have a bitter taste and feel slippery. Bases turn red litmus paper blue.

Common Bases

• Sodium hydroxide (NaOH): Used in soap making.
• Calcium hydroxide (Ca(OH)₂): Used in plaster and cement.
• Ammonium hydroxide (NH₄OH): Used in cleaning products.

Properties of Acids and Bases

Acids and bases have distinct properties that help identify them:

• Conductivity: Both acids and bases conduct electricity when dissolved in water.
• pH Scale: The pH scale measures the acidity or basicity of a solution. It ranges from 0 to 14. A pH less than 7 indicates an acid, a pH of 7 indicates neutrality, and a pH greater than 7 indicates a base.
• Neutralisation: When an acid reacts with a base, they neutralise each other to form water and a salt.

Acid-Base Reactions

Acid-base reactions are a type of chemical reaction where an acid reacts with a base to form water and a salt. This process is called neutralisation.

Example of an Acid-Base Reaction

Consider the reaction between hydrochloric acid (HCl) and sodium hydroxide (NaOH):

HCl + NaOH → NaCl + H₂O

In this reaction:

• HCl is the acid.
• NaOH is the base.
• NaCl is the salt formed (sodium chloride).
• H₂O is water.

The equation shows that one mole of hydrochloric acid reacts with one mole of sodium hydroxide to produce one mole of sodium chloride and one mole of water.

Identifying Acids and Bases

There are several methods to identify acids and bases, including:

• Litmus Test: Use litmus paper. Blue litmus paper turns red in an acid, and red litmus paper turns blue in a base.
• pH Indicator: Use pH indicators such as phenolphthalein or universal indicator to determine the pH of a solution.

Applications of Acids and Bases

Acids and bases are used in various applications:

• Industry: Acids are used in the production of fertilizers, plastics, and explosives.
• Medicine: Acids and bases are used in pharmaceuticals and medical treatments.
• Household: Many cleaning products are either acidic or basic, such as vinegar (acid) and baking soda (base).

Acid-Base Theories

There are several theories to explain the behaviour of acids and bases:

• Arrhenius Theory: Acids produce H⁺ ions in water, while bases produce OH^- ions.
• Brønsted-Lowry Theory: Acids are proton donors, and bases are proton acceptors.
• Lewis Theory: Acids are electron pair acceptors, and bases are electron pair donors.

Example of Acid-Base Theory Application

According to the Brønsted-Lowry theory, in the reaction of acetic acid (CH₃COOH) with water:

CH₃COOH + H₂O ⇌ CH₃COO⁻ + H₃O⁺

Here, acetic acid donates a proton to water, forming the acetate ion (CH₃COO^-) and the hydronium ion (H₃O⁺).

Summary

• Acids donate protons and turn blue litmus red.
• Bases accept protons and turn red litmus blue.
• Neutralisation occurs when an acid reacts with a base to form water and a salt.
• The pH scale measures acidity and basicity.

Check your understanding

1. What is the chemical formula for sulfuric acid?
2. Explain the difference between an acid and a base.
3. What is produced when hydrochloric acid reacts with sodium hydroxide?
4. Describe how you would use litmus paper to identify an acid.