Page 61 - Pharmaceutical_Analytical_Chemistry_1_Theoretical_Notes_Level_1

Mansoura National University
Pharm D-Clinical Pharmacy Program Level 1 Pharm. Anal. Chem. 1 (PC 101)

2. Brønsted-Lowry Theory

➢ The Brønsted-Lowry theory defines an acid as a proton donor and a base as

a proton acceptor in a proton-transfer-reaction.

So, is water an acid or a base?

• In one example, we said that water was acting as a base, and in another

example we said that it was acting as an acid.
• Sometimes a molecule can donate a proton (act as an acid) and sometimes it

can accept a proton (act as a base).

• Molecules that have this ability to act as both an acid and a base are called

amphoteric or amphiprotic.

• Water is the most common example of an amphoteric substance.

▪ A conjugate acid is the part formed when a base accepts a proton.

▪ A conjugate base is the part that remains when an acid donates a proton.

▪ In any acid-base reaction, there are two conjugate acid-base pairs.

▪ The stronger the acid, the weaker is its conjugate base & vice versa.

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+
e.g.: HCl (very strong acid) → H + Cl (very weak conjugate-base)

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