How does the Brønsted–Lowry definition of an acid differ from its definition of a base?

An acid is defined as a proton ($H^+$) donor, whereas a base is defined as a proton ($H^+$) acceptor. The relationship is functional, meaning a substance acts as an acid only when it transfers a proton to a base.

What is the difference between a Brønsted acid and its conjugate base?

The conjugate base is the species that remains after the acid has donated one proton. Structurally, the conjugate base has one fewer hydrogen atom and a charge that is one unit more negative than the original acid.

In an equilibrium reaction, how do the 'reactants' compare to the 'conjugates' in terms of their roles?

The reactants (on the left) act as the primary acid and base for the forward reaction. The conjugates (on the right) act as the acid and base for the reverse reaction, attempting to reform the original reactants.

Why is it an error to identify $H_2O$ and $OH^-$ as a conjugate pair in a reaction where $H_2O$ becomes $H_3O^+$?

In that specific reaction, $H_2O$ is acting as a base by accepting a proton to become $H_3O^+$. Therefore, $H_3O^+$ is its conjugate acid; $OH^-$ would only be the conjugate base if $H_2O$ were donating a proton.

What common mistake is made regarding the charge of a conjugate acid?

Students often forget to increase the charge by +1 when adding a proton. For example, the conjugate acid of $SO_4^{2-}$ is $HSO_4^-$, not $HSO_4^{2-}$.

What is the 'two-proton trap' in identifying conjugate pairs?

A conjugate pair must differ by exactly one proton. Identifying $H_2CO_3$ and $CO_3^{2-}$ as a pair is an error because they differ by two protons; the correct pairs are $(H_2CO_3 / HCO_3^-)$ and $(HCO_3^- / CO_3^{2-})$.

Define a 'Brønsted Acid' in terms of subatomic particles.

A Brønsted acid is a species that donates a proton. Since a hydrogen-1 atom consists of one proton and one electron, the $H^+$ ion is simply a bare proton.

What is a 'Conjugate Acid-Base Pair'?

It is a pair of chemical species that transform into each other by the gain or loss of a single proton ($H^+$). They are always found on opposite sides of a chemical equation.

What does the term 'Amphiprotic' mean in Brønsted–Lowry theory?

It describes a substance that can act as either a Brønsted acid (proton donor) or a Brønsted base (proton acceptor) depending on the reaction conditions. Water is a classic example.

What is a 'Brønsted Base'?

A Brønsted base is any species capable of accepting a proton ($H^+$). This usually involves the base using a lone pair of electrons to form a new bond with the proton.

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Advanced Physical Chemistry

Brønsted–Lowry Acid & Bases

Summary

The Brønsted–Lowry theory defines acids and bases through the exchange of protons ( $H^+$ ions). This framework shifts the focus from the presence of specific ions in solution to the functional behavior of chemical species during a reaction, where acids act as donors and bases act as acceptors, creating a reciprocal relationship known as conjugate pairs.

1. Definition & Core Concepts

Brønsted Acid: A chemical species that has the ability to donate a proton ( $H^+$ ) to another species. In this context, a 'proton' refers to a hydrogen atom that has lost its electron, leaving only the nucleus.
Brønsted Base: A chemical species that can accept a proton ( $H^+$ ) from another species. This typically requires the base to possess a lone pair of electrons that can form a coordinate bond with the incoming proton.
Proton Transfer: The fundamental event in a Brønsted–Lowry reaction is the movement of a hydrogen ion from the acid to the base. Unlike other definitions, this theory does not require the reaction to occur in an aqueous environment, though it is most commonly studied there.

Diagram showing a proton (H+) moving from a Brønsted acid (HA) to a Brønsted base (B).

2. Conjugate Acid-Base Pairs

3. Methods & Techniques

4. Key Distinctions

5. Exam Strategy & Tips

6. Common Pitfalls & Misconceptions