Chemistry Notes: Acids and Bases

Acids and Bases

Introduction

• ‘ Acids and bases are fundamental concepts in chemistry with specific characteristics and behaviors.

Characteristics of Acids and Bases

• Acids:

  • Taste: Sour

  • React with certain metals, leading to dissolution.

  • Cause color changes in plant dyes.

• Bases:

  • Taste: Bitter

  • Physical properties: Slippery to the touch.

  • Corrosive nature.

Theories of Acids and Bases

• Two primary theories that facilitate understanding of the chemistry of acids and bases:

  1. Arrhenius Theory

  2. Brønsted-Lowry Theory

Arrhenius Theory of Acids and Bases

Definition

• Acid: A substance that, when dissolved in water, dissociates to produce hydrogen ions (H⁺) also referred to as protons.

  • Example: Hydrochloric acid (HCl)

  • Dissociation:
    $HCl(aq) <br>ightleftharpoons H^+(aq) + Cl^-(aq)$

• Base: A substance that, when dissolved in water, dissociates to produce hydroxide ions (OH⁻).

  • Example: Sodium hydroxide (NaOH)

  • Dissociation:
    $NaOH(aq) <br>ightleftharpoons Na^+(aq) + OH^-(aq)$

Arrhenius Theory Problem

• Case Study on Ammonia (NH₃):

  • When ammonia dissolves in water, it exhibits basic properties; however, it does not produce hydroxide (OH⁻) ions directly. This limitation indicates the necessity for a broader acid-base theory.

Brønsted-Lowry Theory of Acids and Bases

Definition

• Acid: Defined as a proton (H⁺) donor, a substance that gives away a hydrogen ion in a chemical reaction.

• Base: Defined as a proton (H⁺) acceptor, a substance that accepts a hydrogen ion in a chemical reaction.

Key Difference from Arrhenius Theory

• The Brønsted-Lowry definition does not require the presence of water to classify the acid or base, allowing for a wider variety of substances to fit within the definitions. Both the donation of protons by acids and acceptance of protons by bases are evident in chemical reactions.