Acids and Bases

Learning Objectives

• Acids and Bases
• Common Acids and Bases
• Arrhenius and Bronsted Lowry theories of acids and bases
• Neutralisation reactions
• Conjugate acids and bases
• Apply knowledge to exam-style questions

Introduction to acids and bases

Acids are substances that:

• React with metals producing hydrogen gas
• Turn blue litmus red

Strong acids:

• Hydrochloric acid (HCl)
• Sulfuric acid (H₂SO₄)
• Nitric acid (HNO₃)

Weak acids (everyday examples):

• Ethanoic acid (CH₃COOH) in vinegar
• Hydrochloric acid (HCl) in the stomach (digestion, killing bacteria)
• Lactic acid in sour milk

Hydrochloric acid (HCL), sulfuric acid ($H\_2SO^4$) and nitric acid ($HNO\_3$) are examples of strong acids.

Common everyday acids

Vinegar containing the weak acid ethanoic acid ($CH\_3COOH$).

Hydrochloric acid in the stomach to kill bacteria in food.

Sour milk containing the weak acid lactic acid.

Bases are substances that:

• Turn red litmus blue
• React with acids to form salts.

Strong bases (alkalis):

• Sodium hydroxide (NaOH)
• Potassium hydroxide (KOH)

Weaker bases (everyday examples):

• Magnesium hydroxide (Mg(OH)₂) in milk of magnesia
• Sodium carbonate (Na₂CO₃) in washing soda
• Calcium carbonate (CaCO₃) in limestone

Alkalis are bases that dissolve in water (e.g. NaOH). Bases like calcium carbonate are not soluble, so they are bases but not alkalis.

Neutralisation reactions

Example of a neutralisation reaction: hydrochloric acid reacts with a base to form a salt and water:

HCl + NaOH → NaCl + H₂O

Everyday example of neutralisation

If you have stomach acid indigestion it is probably due to excess acid (HCL). To neutralise the acid, milk of magnesia can be taken which contains the base magnesium hydroxide($Mg(OH)\_2$**)**which neutralises the acid to form the salt magnesium chloride according to the following reaction:

2HCl + Mg(OH)₂ → MgCl₂ + 2H₂O

Theories of acids and bases

The Arrhenius theory (1903): the Swedish chemist Arrhenius put forward the following definitions of acids and bases.

Two major limitations of the Arrhenius theory are:

1. Arrhenius restricted his definition to aqueous (water) solution. However other solvents (liquids) like methylbenzene or liquid ammonia could apply.
2. The hydrogen ion does not exist on its own but reacts with water to form hydronium ion as follows: H⁺ + H₂O → H₃O⁺

Bronsted Lowry theory

Other points of the Bronsted-lowry theory are:

• The stronger the acid the more readily it donates a proton
• The stronger the base the more readily it accepts a proton
• An acid-base reaction involves the transfer of a proton from an acid to a base

Conjugate acids and bases

What is the conjugate acid of water?

H₂O + H⁺ → H₃O⁺

A conjugate base is formed when a proton ($H^+$) is removed from an acid.

What is the conjugate base of sulfuric acid?

H₂SO₄ → HSO₄⁻ + H⁺

It is important to remember that the stronger the acid the weaker the conjugate base that is formed.