Brønsted–Lowry Acid & Bases (AQA A-Level Chemistry): Revision Notes
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 ():
- Brønsted–Lowry Acid: A proton donor. In solution, acids release ions.
- Brønsted–Lowry Base: A proton acceptor. Bases accept 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 ions:
Example: Hydrochloric acid () dissociates as follows:
HCl is a monoprotic acid as each molecule releases one ion.
Example: Sulfuric acid () Sulfuric acid () is diprotic, as each molecule releases two ions:
Strong Bases
Strong bases dissociate fully in water, producing hydroxide ions ():
Example: Sodium hydroxide ()
dissociates as follows:
The ion readily accepts a proton (forming water, ), 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:
Example: Ethanoic acid (), a common organic acid.
The dissociation equilibrium is:
Ethanoic acid is a monoprotic acid because it can release only one ion per molecule.
Weak Bases
Weak bases only partially ionise in solution, creating an equilibrium that also lies far to the left:
Example: Ammonia (), which reacts with water to form hydroxide ions:
Ammonia produces fewer 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, and 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.
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.