Acids and Bases: Brønsted-Lowry Theory

Objectives

• Review the concept of acidity and basicity.
• Define the Brønsted-Lowry Theory of conjugate acid-base pairs.
• Illustrate conjugate acid-base pairs.

Acids and Bases: General Concepts

• Acidity and basicity are extremes that describe chemicals, similar to how hot and cold describe temperature.
• Mixing acids and bases can neutralize their effects, analogous to mixing hot and cold water to even out the temperature.

The pH Scale

• The pH scale measures how acidic or basic a substance is.
• The scale ranges from 0 to 14.
  • pH of 7: Neutral.
  • pH less than 7: Acidic.
  • pH greater than 7: Basic.
• Pure water has a pH of 7 (neutral).
• When chemicals are mixed with water, the solution can become acidic or basic.

The Brønsted-Lowry Theory of Acids and Bases

• Generally regarded as the classical theory of acids and bases.
• Acid: A substance that is a proton (H+) donor.
  • Examples: Hydrochloric acid (HCl), sulfuric acid ($H<em>2SO</em>4$), and nitric acid ($HNO_3$).
• Base: A substance that is a proton (H+) acceptor.
  • Examples: Sodium hydroxide (NaOH) and sodium hydrogencarbonate ($NaHCO_3$ or baking soda).

Water as a Solvent

• Water is composed of $H_2O$ molecules and can dissolve many substances.
• Acidic substances, when dissolved in water, ionize to give hydrogen ions ($H^+$) or, more specifically, hydroxonium ions ($H_3O^+$).
• Example:
  $HCl + H<em>2O \rightleftharpoons H</em>3O^+ + Cl^-$
• Basic substances, when dissolved in water, ionize to give hydroxide ions ($OH^-$).
• Example:
  $NH<em>3 + H</em>2O \rightleftharpoons NH_4^+ + OH^-$

Acid-Base Conjugate Pairs

• Consider the reaction:
  $HCl + H<em>2O \rightleftharpoons H</em>3O^+ + Cl^-$
• The removal of a proton ($H^+$) from an acid produces its conjugate base.
• The reception of a proton by a base produces its conjugate acid.
• In the example above:
  • The removal of $H^+$ from HCl produces the chloride ion ($Cl^−$), which is the conjugate base of the acid HCl.