5.5.1 Brønsted–Lowry Acid & Bases

What is the Brønsted–Lowry Theory of Acids and Bases?

The Brønsted–Lowry theory defines acids and bases based on their ability to donate or accept protons ( $\text{H}^+$ ):

Brønsted–Lowry Acid: A proton donor. In solution, acids release $\text{H}^+$ ions.
Brønsted–Lowry Base: A proton acceptor. Bases accept $\text{H}^+$ ions to form their conjugate acids. This definition particularly helpful when examining acid-base reactions as proton transfer reactions.

Strong Acids and Strong Bases

Strong Acids

Strong acids completely dissociate in solution, releasing all their $\text{H}^+$ ions:

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Example: Hydrochloric acid ( $HCl$ ) $HCl$ dissociates as follows:

\text{HCl (aq)} \rightarrow \text{H}^+ \text{(aq)} + \text{Cl}^- \text{(aq)}

HCl is a monoprotic acid as each molecule releases one $\text{H}^+$ ion.

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Example: Sulfuric acid ( $H₂SO₄$ ) Sulfuric acid ( $H₂SO₄$ ) is diprotic, as each molecule releases two $\text{H}^+$ ions:

\text{H}_2\text{SO}_4 \rightarrow 2\text{H}^+ \text{(aq)} + \text{SO}_4^{2-} \text{(aq)}

Strong Bases

Strong bases dissociate fully in water, producing hydroxide ions ( $\text{OH}^-$ ):

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Example: Sodium hydroxide ( $NaOH$ )

$NaOH$ dissociates as follows:

\text{NaOH (aq)} \rightarrow \text{Na}^+ \text{(aq)} + \text{OH}^- \text{(aq)}

The $\text{OH}^-$ ion readily accepts a proton (forming water, $\text{H}_2\text{O}$ ), making it a strong proton acceptor and a base.

Weak Acids and Weak Bases

Weak Acids

Weak acids only partially dissociate in solution, creating an equilibrium that lies far to the left:

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Example: Ethanoic acid ( $CH₃COOH$ ), a common organic acid.

The dissociation equilibrium is:

\text{CH}_3\text{COOH (aq)} \rightleftharpoons \text{CH}_3\text{COO}^- \text{(aq)} + \text{H}^+ \text{(aq)}

Ethanoic acid is a monoprotic acid because it can release only one $\text{H}^+$ ion per molecule.

Weak Bases

Weak bases only partially ionize in solution, creating an equilibrium that also lies far to the left:

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Example: Ammonia ( $NH₃$ ), which reacts with water to form hydroxide ions:

\text{NH}_3 + \text{H}_2\text{O} \rightleftharpoons \text{NH}_4^+ + \text{OH}^-

Ammonia produces fewer $\text{OH}^-$ ions compared to strong bases, indicating it's a weak base.

Acid-Base Equilibria

In acid-base reactions, equilibrium often exists between conjugate acid-base pairs. For example, in the dissociation of ethanoic acid, $\text{CH}_3\text{COOH}$ and $\text{CH}_3\text{COO}^-$ form a conjugate acid-base pair.

Conjugate Pairs:

An acid-base reaction involves a transfer of protons between conjugate pairs.
The acid donates a proton to the base, establishing an equilibrium where both conjugate pairs coexist in solution.

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Summary

Acids are proton donors; bases are proton acceptors.
Strong acids and bases completely dissociate in solution, while weak acids and bases only partially dissociate, resulting in an equilibrium.
Acid-base reactions involve proton transfer, often forming conjugate acid-base pairs that help maintain equilibrium in the system.

Brønsted–Lowry Acid & Bases Simplified Revision Notes for A-Level AQA Chemistry

5.5.1 Brønsted–Lowry Acid & Bases

What is the Brønsted–Lowry Theory of Acids and Bases?

Strong Acids and Strong Bases

Strong Acids

Strong Bases

Example: Sodium hydroxide ( $NaOH$ )

Weak Acids and Weak Bases

Weak Acids

Example: Ethanoic acid ( $CH₃COOH$ ), a common organic acid.

Weak Bases

Example: Ammonia ( $NH₃$ ), which reacts with water to form hydroxide ions:

Acid-Base Equilibria

Summary

500K+ Students Use These Powerful Tools to Master Brønsted–Lowry Acid & Bases For their A-Level Exams.

Other Revision Notes related to Brønsted–Lowry Acid & Bases you should explore

Brønsted–Lowry Acid & Bases Simplified Revision Notes for A-Level AQA Chemistry

5.5.1 Brønsted–Lowry Acid & Bases

What is the Brønsted–Lowry Theory of Acids and Bases?

Strong Acids and Strong Bases

Strong Acids

Strong Bases

Example: Sodium hydroxide (NaOHNaOHNaOH)

Weak Acids and Weak Bases

Weak Acids

Example: Ethanoic acid (CH3COOHCH₃COOHCH3​COOH), a common organic acid.

Weak Bases

Example: Ammonia (NH3NH₃NH3​), which reacts with water to form hydroxide ions:

Acid-Base Equilibria

Summary

500K+ Students Use These Powerful Tools to Master Brønsted–Lowry Acid & Bases For their A-Level Exams.

Other Revision Notes related to Brønsted–Lowry Acid & Bases you should explore

Example: Sodium hydroxide ( $NaOH$ )

Example: Ethanoic acid ( $CH₃COOH$ ), a common organic acid.

Example: Ammonia ( $NH₃$ ), which reacts with water to form hydroxide ions: