Brønsted–Lowry Acids and Bases
Quick Notes
- A Brønsted–Lowry acid is a proton (H⁺) donor.
- A Brønsted–Lowry base is a proton (H⁺) acceptor.
- In aqueous solution, a proton may be shown as:
- H⁺(aq)
- H₃O⁺(aq) (hydronium ion)
- Acid–base reactions involve proton transfer.
- Acid–base conjugate pairs differ by one H⁺.
- A base accepts a proton; an alkali is a base that dissolves in water to form OH⁻(aq).
Full Notes
The Brønsted–Lowry Theory
There are several different ways to describe acids and bases in chemistry. At this level, we use the Brønsted–Lowry theory:
- Brønsted–Lowry acid: A substance that donates a proton (H⁺).
- Brønsted–Lowry base: A substance that accepts a proton (H⁺).
This theory applies to all states, not just aqueous solutions.
ExampleHCl + H₂O → H₃O⁺ + Cl⁻
- HCl is the acid (donates H⁺).
- H₂O is the base (accepts H⁺ to form H₃O⁺).
Example NH₃ + H₂O ⇌ NH₄⁺ + OH⁻
- NH₃ is the base (accepts H⁺).
- H₂O is the acid (donates H⁺).
H⁺(aq) and H₃O⁺(aq)
In aqueous solutions, H⁺ ions are never found “alone” – they bond with water molecules to form hydronium ions, H₃O⁺.
When an acid molecule dissociates in water, a water molecule accepts a H⁺ ion from the acid, becoming H₃O⁺. The H₃O⁺ ion is what actually reacts with any added base.
H⁺(aq) and H₃O⁺(aq) are interchangeable in chemical equations, but H₃O⁺ gives a more accurate picture of what happens in solution.
Base or Alkali?
A base accepts a H⁺ ion, regardless of whether it dissolves in water.
An alkali is a soluble base that dissolves in water and releases OH⁻ ions into solution.
Example Comparing bases and alkalis
- NaOH is an alkali. It dissolves in water and dissociates, releasing Na⁺(aq) and OH⁻(aq).
- NH₃ is an alkali as it dissolves in water and accepts a H⁺ ion from a water molecule, forming NH₄⁺(aq) and OH⁻: NH₃ + H₂O ⇌ NH₄⁺ + OH⁻
- CuO is a base (reacts with acids and accepts H⁺) but not an alkali as it is insoluble.
Summary
- A Brønsted–Lowry acid donates a proton, while a base accepts one.
- Acid–base reactions involve conjugate pairs differing by one H⁺.
- H⁺(aq) can also be represented as H₃O⁺(aq) in aqueous solutions.
- An alkali is a soluble base that produces OH⁻ ions in water.
- Definitions of acids and bases have evolved over time with scientific understanding.
Linked Course Question
Why has the definition of acid evolved over time?
The definition of acids has changed as our understanding of chemistry has developed.
- They were first defined through simple observations, such as sour taste and reactions with metals.
- Arrhenius described acids as substances that increase H⁺ concentration in water.
- The Brønsted–Lowry definition expanded this, defining acids as H⁺ ion donors – a more universal model that also applies to non-aqueous reactions.
- The Lewis definition goes further, describing acids as electron pair acceptors, and includes many reactions where no H⁺ ions are involved.
Each model is not about being right or wrong – they reflect how scientific ideas evolve to better explain experimental evidence and broaden our understanding of chemical behaviour.