R3.1.1 Brønsted–Lowry acid-base theory

Brønsted–Lowry Acids and Bases

Why Do Acids and Bases Behave the Way They Do?

Picture this: You’re in the kitchen preparing lemonade, and a drop of lemon juice lands on your hand.
It feels tangy and sharp.
Later, while washing up, you notice the slippery feel of the soap.

These sensations—sourness and slipperiness—are caused by the chemical properties of acids and bases.

What Is the Brønsted–Lowry Theory?

Definition

Brønsted–Lowry theory

The Brønsted–Lowry theory, introduced in 1923, defines acids and bases based on their ability to transfer protons (H⁺ ions).

This framework expanded earlier definitions and applies to a wide variety of chemical reactions, even beyond water-based systems.

Definition

Brønsted–Lowry acid

A Brønsted–Lowry acid is a proton (H⁺) donor—it gives up a hydrogen ion in a reaction.

Definition

Brønsted–Lowry base

A Brønsted–Lowry base is a proton (H⁺) acceptor—it gains a hydrogen ion in a reaction.

Example

Consider the reaction between hydrogen chloride (HCl) and ammonia (NH₃):

$HCl + {NH}_{3} \to {NH}_{4}^{+} + {Cl}^{-}$

Here:

HCl acts as a Brønsted–Lowry acid because it donates an H⁺ ion.
NH₃ acts as a Brønsted–Lowry base because it accepts the H⁺ ion.

Tip

Unlike earlier theories, the Brønsted–Lowry theory is not restricted to aqueous solutions. It applies to reactions in gases, liquids, and even non-aqueous solvents, making it a more flexible model.

Brønsted–Lowry behavior in the reaction between ethanoic acid and water.

Representing Acids and Bases in Aqueous Solutions

In aqueous (water-based) systems, acids and bases interact with water.
A key concept here is the hydronium ion:
- A free proton (H⁺) does not exist independently in water. Instead, it bonds with a water molecule to form the hydronium ion, $H_{3} O^{+}$ :
  $H^{+} + H_{2} O \to H_{3} O^{+}$

Thus, when you see $H^{+}$ , it is shorthand for the hydronium ion, $H_{3} O^{+}$ .

Example

Hydrochloric acid (HCl) dissociates in water:
$HCl \to H⁺(aq) + Cl⁻(aq)$
This is more accurately represented as:
$HCl + H₂O \to H₃O⁺(aq) + Cl⁻(aq)$
Ammonia (NH₃) reacts with water to form hydroxide ions:
$NH₃ + H₂O ⇋ NH₄⁺(aq) + OH⁻(aq)$

Note

In IB Chemistry, both $H⁺(aq)$ and $H₃O⁺(aq)$ are acceptable representations of the proton in aqueous solutions. However, keep in mind that $H₃O⁺$ is the actual species present in water.

Bases vs. Alkalis: What’s the Difference?

The terms "base" and "alkali" are often confused but have distinct meanings:

A base is any substance that can accept a proton (H⁺). This includes both soluble and insoluble substances.

Example

Ammonia (NH₃) is a base but not an alkali because it is only partially soluble in water.

An alkali is a type of base that dissolves in water to produce hydroxide ions (OH⁻).

Example

Sodium hydroxide (NaOH) is both a base and an alkali because it dissolves in water to form $OH⁻(aq)$ :
$NaOH \to Na⁺(aq) + OH⁻(aq)$

Common Mistake

Students often assume that all bases are alkalis. Remember: while all alkalis are bases, not all bases are alkalis.

Examples of Brønsted–Lowry Acids and Bases

Example

Reaction Between HCl and Water

$HCl + H₂O \to H₃O⁺ + Cl⁻$

HCl donates a proton (H⁺), so it is the acid.
H₂O accepts the proton, so it is the base.

Example

Reaction Between NH₃ and HCl

$NH₃ + HCl \to NH₄⁺ + Cl⁻$

NH₃ accepts a proton, so it is the base.
HCl donates a proton, so it is the acid.

Example

Amphiprotic Nature of Water

Water can act as both an acid and a base:

As an acid:
$H₂O \to H⁺ + OH⁻$
As a base:
$H₂O + H⁺ \to H₃O⁺$

Reflection

Self review

In the reaction $HCO₃⁻ + H₂O ⇋ H₂CO₃ + OH⁻$ , can you identify the acid and the base?

Theory of Knowledge

How might advances in technology, such as the invention of the pH meter, have shaped our understanding and application of acid–base chemistry?

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Introduction to Brønsted–Lowry Acid-Base Theory

The Brønsted–Lowry theory is a fundamental concept in chemistry that defines acids and bases based on their ability to donate or accept protons (H⁺ ions).
This theory extends beyond the traditional definitions of acids and bases, allowing us to understand their behavior in a wide range of chemical reactions.

Analogy

Think of a Brønsted–Lowry acid as a person giving away a basketball (proton), while a Brønsted–Lowry base is someone receiving that basketball. The exchange of the basketball represents the transfer of a proton.

Example

When you taste lemon juice (an acid) or feel the slipperiness of soap (a base), you're experiencing the effects of substances that either donate or accept protons.

Definition

Brønsted–Lowry Acid-Base Theory

A theory that defines acids as proton donors and bases as proton acceptors.

Definition

Proton (H⁺)

A hydrogen ion that has lost its electron, essentially a hydrogen nucleus.

Common Mistake

Students often confuse the Brønsted–Lowry definition with other acid-base theories. Remember that this theory focuses on proton transfer.

Note

This theory was independently proposed by Johannes Brønsted and Thomas Lowry in 1923.