Bronsted-Lowry Acids and Bases: Definitions, Conjugate Pairs, and Applications

Bronsted-Lowry Theory of Acids and Bases

Introduction to Bronsted-Lowry Definitions

The Bronsted-Lowry theory, developed in 1923 by Johannes Bronsted and Thomas Lowry, expanded the concept of acids and bases beyond the limitations of the Arrhenius definition. This theory is fundamental in GOB Chemistry for understanding acid-base reactions in aqueous and non-aqueous solutions.

• Bronsted-Lowry Acid: A substance that donates a proton (H+) to another substance.

• Bronsted-Lowry Base: A substance that accepts a proton (H+) from another substance.

• Unlike Arrhenius acids and bases, Bronsted-Lowry acids and bases are not limited to aqueous solutions.

• Every Arrhenius acid is a Bronsted-Lowry acid, but not all Bronsted-Lowry acids are Arrhenius acids.

Conjugate Acid-Base Pairs: Bronsted-Lowry acids and bases always occur in pairs called conjugate acid/base pairs. When an acid donates a proton, it forms its conjugate base; when a base accepts a proton, it forms its conjugate acid.

• Example pairs: OH- / H2O, H3O+ / H2O

Conjugate Acids and Bases

Writing Formulas for Conjugate Bases

To determine the conjugate base of a given acid, remove one proton (H+) from the acid. The resulting species is the conjugate base.

• Key Point: The conjugate base has one less H and a charge decreased by +1.

• Example: For HSO4-, the conjugate base is SO42- (remove an H+, charge becomes more negative).

• Example: For H2SO5, the conjugate base is HSO5- (remove an H+, charge becomes more negative).

Writing Formulas for Conjugate Acids

To determine the conjugate acid of a given base, add one proton (H+) to the base. The resulting species is the conjugate acid.

• Key Point: The conjugate acid has one more H and a charge increased by +1.

• Example: For VO2-, the conjugate acid is HVO2 (add an H+, charge becomes more positive).

• Example: For NH2NH2, the conjugate acid is NH2NH3+ (add an H+, charge becomes more positive).

Identifying Acids, Bases, and Conjugate Pairs in Reactions

Example Reactions

In chemical reactions, identify the acid, base, conjugate acid, and conjugate base by tracking proton transfer.

• Example 1: $\mathrm{HF\ (aq) + H_2O\ (l) \rightarrow F^-\ (aq) + H_3O^+\ (aq)}$ HF: acid H2O: base F-: conjugate base H3O+: conjugate acid

• Example 2: $\mathrm{CN^-\ (aq) + H_2O\ (l) \rightarrow HCN\ (aq) + OH^-\ (aq)}$ CN-: base H2O: acid HCN: conjugate acid OH-: conjugate base

Practice: Identifying Bronsted-Lowry Acids and Bases

Given a list of compounds, determine which act as Bronsted-Lowry acids or bases.

• CH4: base

• HCN: acid

• NH3: base

• Br2: conjugate base

Practice Problems and Applications

Writing Chemical Equations for Acid-Base Reactions

Determine the products of acid-base reactions by identifying the transfer of protons.

• Example: Carbonate ion reacting with water: $\mathrm{CO_3^{2-}\ (aq) + H_2O\ (l) \rightarrow HCO_3^-\ (aq) + OH^-\ (aq)}$ CO32-: base H2O: acid HCO3-: conjugate acid OH-: conjugate base

Summary Table: Bronsted-Lowry Acid-Base Pairs

The following table summarizes the relationship between acids, bases, and their conjugate pairs:

Species	Role	Conjugate Pair	Change
HSO4-	Acid	SO42- (Conjugate Base)	Remove H+, charge -1
VO2-	Base	HVO2 (Conjugate Acid)	Add H+, charge +1
NH2NH2	Base	NH2NH3+ (Conjugate Acid)	Add H+, charge +1

Key Equations

• General acid-base reaction: $\mathrm{HA + B \rightarrow A^- + HB^+}$

• Conjugate base formation: $\mathrm{Acid - H^+ \rightarrow Conjugate\ Base}$

• Conjugate acid formation: $\mathrm{Base + H^+ \rightarrow Conjugate\ Acid}$

Additional info: The notes also reference the importance of charge changes when forming conjugate acids and bases, and the identification of acids and bases in reactions is a key skill for GOB Chemistry students.