Brønsted–Lowry Acids and Bases
What Is the Brønsted–Lowry Theory?
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.
Brønsted–Lowry acid
A Brønsted–Lowry acid is a proton (H⁺) donor, which gives up a hydrogen ion in a reaction.
Brønsted–Lowry base
A Brønsted–Lowry base is a proton (H⁺) acceptor, which gains a hydrogen ion in a reaction.
Consider the reaction between hydrogen chloride (HCl) and ammonia (NH₃):
$$
\text{HCl} + \text{NH}_3 \rightarrow \text{NH}_4^+ + \text{Cl}^-
$$
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.
- 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.
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, $ \mathrm{H_3O^+} $:
$$
\mathrm{H^+} + \mathrm{H_2O} \rightarrow \mathrm{H_3O^+}
$$
Thus, when you see $ \mathrm{H^+} $, it is shorthand for the hydronium ion, $ \mathrm{H_3O^+} $.
- Hydrochloric acid (HCl) dissociates in water:
$$
\text{HCl} \rightarrow \text{H⁺(aq)} + \text{Cl⁻(aq)}
$$ - This is more accurately represented as:
$$
\text{HCl} + \text{H₂O} \rightarrow \text{H₃O⁺(aq)} + \text{Cl⁻(aq)}
$$ - Ammonia (NH₃) reacts with water to form hydroxide ions:
$$
\text{NH₃} + \text{H₂O} \leftrightharpoons \text{NH₄⁺(aq)} + \text{OH⁻(aq)}
$$
- In IB Chemistry, both $ \text{H⁺(aq)} $ and $ \text{H₃O⁺(aq)} $ are acceptable representations of the proton in aqueous solutions.
- However, keep in mind that $ \text{H₃O⁺} $ is the actual species present in water.
Reaction Between HCl and Water
$$
\text{HCl} + \text{H₂O} \rightarrow \text{H₃O⁺} + \text{Cl⁻}
$$
- HCl donates a proton (H⁺), so it is the acid.
- H₂O accepts the proton, so it is the base.
Reaction Between NH₃ and HCl
$$
\text{NH₃} + \text{HCl} \rightarrow \text{NH₄⁺} + \text{Cl⁻}
$$
- NH₃ accepts a proton, so it is the base.
- HCl donates a proton, so it is the acid.
Amphiprotic Nature of Water
Water can act as both an acid and a base:
- As an acid:
$$
\text{H₂O} \rightarrow \text{H⁺} + \text{OH⁻}
$$ - As a base:
$$
\text{H₂O} + \text{H⁺} \rightarrow \text{H₃O⁺}
$$
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.
- An alkali is a type of base that dissolves in water to produce hydroxide ions (OH⁻).
- Ammonia (NH₃) is a base but not an alkali because it is only partially soluble in water.
- Sodium hydroxide (NaOH) is both a base and an alkali because it dissolves in water to form $ \text{OH⁻(aq)} $:
$$
\text{NaOH} \rightarrow \text{Na⁺(aq)} + \text{OH⁻(aq)}
$$
Students often assume that all bases are alkalis. Remember: while all alkalis are bases, not all bases are alkalis.
In the reaction $ \text{HCO₃⁻} + \text{H₂O} \leftrightharpoons \text{H₂CO₃} + \text{OH⁻} $, can you identify the acid and the base?
