Getting Started
Acid-base reactions are a fundamental class of chemical processes that govern everything from biological systems to industrial manufacturing. At the molecular level, these reactions are defined by the transfer of a single, simple particle: a proton. Understanding how and why this proton transfer occurs allows us to predict the products of reactions and explain the characteristic properties of acidic and basic solutions.
What You Should Be able to Do
After completing this section, you should be able to perform the following tasks:
Define a chemical species as a Brønsted-Lowry acid or base based on its behavior in a reaction.
Analyze a proton-transfer reaction to identify the acid, the base, the conjugate acid, and the conjugate base.
Explain why water can act as either an acid or a base depending on the chemical environment.
Identify the two conjugate acid-base pairs within a given acid-base reaction.
Describe the inverse relationship between the strength of an acid and the strength of its conjugate base.
Key Concepts & Analysis
The Brønsted-Lowry theory provides a powerful framework for understanding acid-base chemistry as a process of proton transfer. This model focuses on the roles that reactants play—as either proton donors or proton acceptors—and the resulting products that are formed.
Inputs & Preconditions
For a Brønsted-Lowry acid-base reaction to occur, two key components are required:
An acid: A molecule or ion that can donate a proton (H⁺). This species must contain a hydrogen atom bonded in such a way that it can be released as an H⁺ ion. This is often a hydrogen bonded to a highly electronegative atom.
A base: A molecule or ion that can accept a proton. This species must have a region of high electron density, typically a lone pair of electrons, to form a new bond with the incoming H⁺.
Most of these reactions are studied in an aqueous solution, where water itself can participate as either an acid or a base.
Key Steps / The Mechanism
The core mechanism of any Brønsted-Lowry acid-base reaction is the transfer of one proton (H⁺) from the acid to the base.
Consider the general reaction:
HA + B ⇌ A⁻ + HB⁺
Proton Donation: The acid, HA, donates its proton (H⁺). The electrons from the H-A bond remain with A, forming the species A⁻.
Proton Acceptance: The base, B, uses its lone pair of electrons to form a new covalent bond with the incoming proton (H⁺), forming the species HB⁺.
Example 1: A Strong Acid in Water
When hydrogen chloride gas dissolves in water, it reacts as follows:
HCl(g) + H₂O(l) → Cl⁻(aq) + H₃O⁺(aq)
Step 1: HCl donates a proton.
Step 2: H₂O accepts the proton.
Example 2: A Weak Base in Water
When ammonia dissolves in water, an equilibrium is established:
NH₃(aq) + H₂O(l) ⇌ NH₄⁺(aq) + OH⁻(aq)
Step 1: H₂O donates a proton.
Step 2: NH₃ accepts the proton.
Notice that in the first example, water acted as a base, while in the second, it acted as an acid. A substance that can act as either an acid or a base is called amphoteric or amphiprotic.
Outputs & Effects
The products of a proton-transfer reaction are themselves an acid and a base. They are referred to as conjugates of the original reactants.
Conjugate Base: The species that remains after an acid has donated its proton (e.g., A⁻ is the conjugate base of the acid HA).
Conjugate Acid: The species that is formed when a base accepts a proton (e.g., HB⁺ is the conjugate acid of the base B).
An acid and its conjugate base (or a base and its conjugate acid) are collectively known as a conjugate acid-base pair. These pairs always differ by a single H⁺.
| Reaction | Reactant Acid | Reactant Base | Product Conjugate Base | Product Conjugate Acid |
|---|---|---|---|---|
| HCl + H₂O → Cl⁻ + H₃O⁺ | HCl | H₂O | Cl⁻ | H₃O⁺ |
| NH₃ + H₂O ⇌ NH₄⁺ + OH⁻ | H₂O | NH₃ | OH⁻ | NH₄⁺ |
| HNO₂ + H₂O ⇌ NO₂⁻ + H₃O⁺ | HNO₂ | H₂O | NO₂⁻ | H₃O⁺ |
The two conjugate pairs in the first reaction are HCl/Cl⁻ and H₂O/H₃O⁺.
Controls & Limiting Factors
The extent to which an acid-base reaction proceeds is controlled by the relative strengths of the acids and bases involved.
A strong acid is one that readily donates its proton, meaning the reaction proceeds almost completely to products. Its conjugate base is consequently very weak and has a negligible tendency to accept a proton back.
A weak acid donates its proton less readily, establishing an equilibrium where a significant amount of the un-ionized acid remains. Its conjugate base is a relatively weak base.
This leads to a critical inverse relationship:
The stronger the acid, the weaker its conjugate base.
The stronger the base, the weaker its conjugate acid.
For example, HCl is a very strong acid, so its conjugate base, Cl⁻, is an extremely weak base. Acetic acid (CH₃COOH) is a weak acid, so its conjugate base, the acetate ion (CH₃COO⁻), is a weak base.
Key Models & Representations
Identifying the components of an acid-base reaction is a critical skill. The following matrix organizes the roles of each species in several key reactions. A conjugate pair is formed by linking a reactant with its corresponding product.
| Reaction Equation | Acid (Proton Donor) | Base (Proton Acceptor) | Conjugate Base (of the Acid) | Conjugate Acid (of the Base) |
|---|---|---|---|---|
| HF(aq) + H₂O(l) ⇌ F⁻(aq) + H₃O⁺(aq) | HF | H₂O | F⁻ | H₃O⁺ |
| H₂SO₄(aq) + H₂O(l) → HSO₄⁻(aq) + H₃O⁺(aq) | H₂SO₄ | H₂O | HSO₄⁻ | H₃O⁺ |
| HCO₃⁻(aq) + OH⁻(aq) ⇌ CO₃²⁻(aq) + H₂O(l) | HCO₃⁻ | OH⁻ | CO₃²⁻ | H₂O |
| H₃O⁺(aq) + NH₃(aq) ⇌ H₂O(l) + NH₄⁺(aq) | H₃O⁺ | NH₃ | H₂O | NH₄⁺ |
Conjugate Pairs for the first reaction: HF/F⁻ and H₂O/H₃O⁺.
Key Terms, Quantities, & Concepts
Brønsted-Lowry Acid: A chemical species that acts as a proton (H⁺) donor in a reaction.
Brønsted-Lowry Base: A chemical species that acts as a proton (H⁺) acceptor in a reaction; it must have a lone pair of electrons.
Proton Transfer: The fundamental process in a Brønsted-Lowry acid-base reaction where an H⁺ ion moves from the acid to the base.
Conjugate Acid-Base Pair: Two species in a reaction that differ from each other by the presence or absence of a single proton (e.g., NH₄⁺ and NH₃).
Conjugate Acid: The species formed when a base gains a proton.
Conjugate Base: The species that remains after an acid has lost a proton.
Amphoteric (or Amphiprotic): A substance, such as water, that has the ability to act as either an acid or a base depending on the species it is reacting with.
Hydronium Ion (H₃O⁺): The ion formed when a water molecule accepts a proton from an acid. It is the form in which protons exist in aqueous solution.
Skill Snapshots
Causation
A molecule containing a polar H-X bond (where X is electronegative) causes it to have the potential to act as a Brønsted-Lowry acid by donating H⁺.
The presence of a lone pair of electrons on an atom causes a molecule to have the potential to act as a Brønsted-Lowry base by accepting H⁺.
The complete ionization of a strong acid in water causes the formation of a conjugate base that is exceptionally weak.
Comparison
A Brønsted-Lowry acid is a proton donor, whereas a Brønsted-Lowry base is a proton acceptor.
A conjugate acid has one more proton and a charge that is more positive by one unit than its conjugate base.
In the reaction with ammonia (NH₃), water acts as an acid, whereas in the reaction with hydrogen chloride (HCl), water acts as a base.
Change and Continuity Over Time (CCOT)
Baseline: Before the reaction, an acid molecule (e.g., HF) and a base molecule (e.g., H₂O) exist independently in solution.
Change 1: During the reaction, the covalent bond between H and F breaks, and a new covalent bond forms between H and the oxygen atom of H₂O.
Change 2: The products, a fluoride ion (F⁻) and a hydronium ion (H₃O⁺), are formed, resulting in a change in the chemical composition of the solution.
Continuity: The fluorine, oxygen, and hydrogen atoms are all conserved throughout the proton-transfer process; they are merely rearranged into new species.
Common Misconceptions & Clarifications
Misconception: The terms "acid" and "conjugate acid" are interchangeable.
Clarification: The term "acid" typically refers to a reactant, while "conjugate acid" refers to a product. The conjugate acid is what the reactant base becomes after it accepts a proton.
Misconception: A proton (H⁺) exists as a free, isolated particle in water.
Clarification: An H⁺ ion is a bare proton and is extremely reactive. In an aqueous solution, it is immediately captured by a water molecule to form the more stable hydronium ion, H₃O⁺. So, H⁺(aq) is a shorthand for H₃O⁺(aq).
Misconception: The Arrhenius definition (acids produce H⁺, bases produce OH⁻) is the only definition.
Clarification: The Brønsted-Lowry definition is broader and more useful. It correctly identifies species like ammonia (NH₃) as bases because they accept protons, even though they do not contain a hydroxide group in their formula.
Misconception: Any two species on opposite sides of the reaction arrow can be a conjugate pair.
Clarification: A conjugate acid-base pair must differ by exactly one proton. In the reaction H₂SO₄ + H₂O → HSO₄⁻ + H₃O⁺, the pairs are H₂SO₄/HSO₄⁻ and H₂O/H₃O⁺. The species H₂SO₄ and H₃O⁺ are both acids, but they are not a conjugate pair.
One-Paragraph Summary
The Brønsted-Lowry theory defines acid-base chemistry as the process of proton transfer. In any such reaction, an acid donates a single proton (H⁺) to a base, which accepts it using a lone pair of electrons. This transfer results in the formation of a conjugate base (the deprotonated acid) and a conjugate acid (the protonated base). These reactant-product sets are known as conjugate acid-base pairs, and their relative strengths are inversely related: a strong acid yields a very weak conjugate base. Water is a key amphoteric substance in aqueous chemistry, capable of acting as either an acid or a base, which allows it to facilitate a vast range of proton-transfer reactions.