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Acids and Bases - Theories of acids and bases

Grade 12IBChemistry

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

🔑Concepts

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A Brønsted-Lowry acid is defined as a proton (H+H^+) donor, while a Brønsted-Lowry base is a proton acceptor.

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Conjugate acid-base pairs are species that differ by a single proton. For a general reaction HA+BightleftharpoonsA−+HB+HA + B ightleftharpoons A^- + HB^+, HA/A−HA/A^- and HB+/BHB^+/B are conjugate pairs.

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Amphiprotic species can act as both Brønsted-Lowry acids and bases. Examples include H2OH_2O, HCO3−HCO_3^-, and H2PO4−H_2PO_4^-.

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A Lewis acid is defined as an electron pair acceptor, while a Lewis base is an electron pair donor.

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Lewis theory is more inclusive than Brønsted-Lowry; all Brønsted-Lowry acids are Lewis acids, but species like BF3BF_3 or AlCl3AlCl_3 are Lewis acids without being Brønsted-Lowry acids.

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When a Lewis base reacts with a Lewis acid, a coordinate covalent bond (dative bond) is formed as the base donates a lone pair to the acid's vacant orbital.

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Nucleophiles are Lewis bases (electron-rich species attracted to positive centers), and electrophiles are Lewis acids (electron-deficient species attracted to lone pairs).

📐Formulae

pH=−log⁡10[H+(aq)]pH = -\log_{10}[H^+(aq)]

[H+]=10−pH[H^+] = 10^{-pH}

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

pOH=−log⁡10[OH−(aq)]pOH = -\log_{10}[OH^-(aq)]

pH+pOH=pKw=14.00 at 298KpH + pOH = pK_w = 14.00 \text{ at } 298\text{K}

💡Examples

Problem 1:

Identify the conjugate acid-base pairs in the following equilibrium: NH3(aq)+H2O(l)ightleftharpoonsNH4+(aq)+OH−(aq)NH_3(aq) + H_2O(l) ightleftharpoons 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^+, while H2OH_2O donates a proton to become OH−OH^-. Each pair differs by exactly one H+H^+ ion.

Problem 2:

Determine which species acts as the Lewis acid and which as the Lewis base in the reaction: BF3+NH3ightarrowBF3NH3BF_3 + NH_3 ightarrow BF_3NH_3.

Solution:

Lewis Acid: BF3BF_3; Lewis Base: NH3NH_3.

Explanation:

NH3NH_3 (ammonia) has a lone pair of electrons on the nitrogen atom. BF3BF_3 (boron trifluoride) has an incomplete octet on the boron atom. NH3NH_3 donates its lone pair to BF3BF_3 to form a coordinate bond.

Problem 3:

Explain why HCO3−HCO_3^- is considered amphiprotic by providing two chemical equations.

Solution:

  1. As an acid: HCO3−(aq)+OH−(aq)ightarrowCO32−(aq)+H2O(l)HCO_3^-(aq) + OH^-(aq) ightarrow CO_3^{2-}(aq) + H_2O(l). 2. As a base: HCO3−(aq)+H3O+(aq)ightarrowH2CO3(aq)+H2O(l)HCO_3^-(aq) + H_3O^+(aq) ightarrow H_2CO_3(aq) + H_2O(l).

Explanation:

In the first reaction, HCO3−HCO_3^- donates a proton (H+H^+) to the hydroxide ion. In the second reaction, it accepts a proton from the hydronium ion.

Theories of acids and bases - Revision Notes & Key Formulas | IB Grade 12 Chemistry