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Acids and Bases - Theories of acids and bases

Grade 11IBChemistry

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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The Brønsted-Lowry theory defines an acid as a proton (H+H^+) donor and a base as a proton (H+H^+) acceptor.

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A conjugate acid-base pair consists of two species that differ by a single H+H^+ ion. When an acid HAHA loses a proton, it forms its conjugate base A−A^-.

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Amphiprotic species are substances that can act as both a Brønsted-Lowry acid and a Brønsted-Lowry base (e.g., H2OH_2O, HCO3−HCO_3^-, H2PO4−H_2PO_4^-).

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The Lewis theory defines an acid as an electron-pair acceptor and a base as an electron-pair donor.

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All Brønsted-Lowry acids are Lewis acids (as the H+H^+ ion accepts an electron pair), but not all Lewis acids are Brønsted-Lowry acids (e.g., BF3BF_3 or AlCl3AlCl_3).

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When a Lewis base reacts with a Lewis acid, a coordinate covalent bond (dative bond) is formed.

📐Formulae

Acid⇌Base+H+Acid \rightleftharpoons Base + H^+

HA(aq)+B(aq)⇌A−(aq)+HB+(aq)HA(aq) + B(aq) \rightleftharpoons A^-(aq) + HB^+(aq)

Kw=[H+][OH−]=1.0×10−14 at 298 KK_w = [H^+][OH^-] = 1.0 \times 10^{-14} \text{ at } 298\text{ K}

BF3+:NH3→F3B←NH3BF_3 + :NH_3 \rightarrow F_3B\leftarrow NH_3

💡Examples

Problem 1:

Identify the conjugate acid-base pairs in the following reaction: NH3(aq)+H2O(l)⇌NH4+(aq)+OH−(aq)NH_3(aq) + H_2O(l) \rightleftharpoons NH_4^+(aq) + OH^-(aq).

Solution:

Pair 1: NH3NH_3 (base) and NH4+NH_4^+ (conjugate acid). Pair 2: H2OH_2O (acid) and OH−OH^- (conjugate base).

Explanation:

NH3NH_3 accepts a proton to become NH4+NH_4^+, making it the Brønsted-Lowry base. H2OH_2O donates a proton to become OH−OH^-, making it the Brønsted-Lowry acid.

Problem 2:

Explain why BF3BF_3 acts as an acid in the reaction BF3+NH3→BF3NH3BF_3 + NH_3 \rightarrow BF_3NH_3 according to the Lewis theory.

Solution:

BF3BF_3 is the Lewis acid because it accepts a lone pair of electrons from the nitrogen atom in NH3NH_3.

Explanation:

Boron in BF3BF_3 has an incomplete octet (only 6 valence electrons) and can accommodate an incoming electron pair from NH3NH_3 to form a coordinate bond.

Problem 3:

Show the amphiprotic nature of the hydrogen carbonate ion (HCO3−HCO_3^-) using equations.

Solution:

As an acid: HCO3−(aq)+H2O(l)⇌CO32−(aq)+H3O+(aq)HCO_3^-(aq) + H_2O(l) \rightleftharpoons CO_3^{2-}(aq) + H_3O^+(aq). As a base: HCO3−(aq)+H3O+(aq)⇌H2CO3(aq)+H2O(l)HCO_3^-(aq) + H_3O^+(aq) \rightleftharpoons H_2CO_3(aq) + H_2O(l).

Explanation:

In the first reaction, HCO3−HCO_3^- donates a proton (H+H^+). In the second, it accepts a proton, demonstrating its ability to act as both a Brønsted-Lowry acid and base.

Theories of acids and bases - Revision Notes & Key Formulas | IB Grade 11 Chemistry