L1 - Theories of Acids & Bases

objectives

• Explore the changes in definitions and models of an acid and a base over time to explain the limitations of each model, including but not limited to:

– Arrhenius’ theory

– Brønsted–Lowry theory

● write ionic equations to represent the dissociation of acids and bases in water, conjugate acid/base pairs

in solution and amphiprotic nature of some salts, for example:

– sodium hydrogen carbonate

– potassium dihydrogen phosphate

Arrhenius Acid-Base Theory

Arrhenius said that acids are hydrogen-containing compounds that ionize to yield hydrogen ions (H⁺) in aqueous solution. He also said that bases are compounds that ionize to yield hydroxide ions (OH^–) in aqueous solution.

Arrhenius Acids

•Acids that contain one ionizable hydrogen, such as nitric acid (HNO₃), are called monoprotic acids.

•Acids that contain two ionizable hydrogens, such as sulfuric acid (H₂SO₄), are called diprotic acids.

•Acids that contain three ionizable hydrogens, such as phosphoric acid (H₃PO₄) are called triprotic acids.

Arrhenius Bases

Hydroxide ions are one of the products of the dissolution of an alkali metal in water.

Arrhenius Definitions do not work for Ammonia

Limitations of Arrhenius

• Arrhenius only applies to reactions in aqueous solutions
• It does not explain the different strengths of acids and bases
• All substances MUST produce H+ and OH-

Brønsted-Lowry Acids and Bases

ACID = proton donor

HCl ——> H⁺(aq) + Cl¯(aq)

BASE = proton acceptor

NH₃ (aq) + H⁺(aq) ——> NH₄⁺(aq)

Conjugate Systems

Acids are related to bases

ACID ⇌ PROTON + CONJUGATE BASE

Bases are related to acids

BASE + PROTON ⇌ CONJUGATE ACID

For an acid to behave as an acid, it must have a base present to accept a proton...

example CH₃COO¯ + H₂O ⇌ CH₃COOH + OH¯

Base 1 acid 1 acid 2 base 2

HA + B ⇌ BH⁺ + A¯

Limitations of Bronsted-Lowry

• To be classed as an acid - a substance MUST have a proton it can lose
• The theory does not explain the reaction between acidic oxides and basic oxides

Strong Acids and Bases

STRONG ACIDS - completely dissociate (split up) into ions in aqueous solution

e.g. HCl ——> H⁺(aq) + Cl¯(aq) MONOPROTIC 1 replaceable H

HNO₃ ——> H⁺(aq) + NO₃¯(aq)

H₂SO₄ ——> 2H⁺(aq) + SO₄^2-(aq) DIPROTIC 2 replaceable H’s

STRONG BASES - completely dissociate into ions in aqueous solution

e.g. NaOH ——> Na⁺(aq) + OH¯(aq)

Amphiprotic Species

Amphiprotic Species can behave as both Bronsted-Lowry Acids and Bases meaning they can both accept and donate protons