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Acids and Bases - Theories of Acids and Bases

Grade 9IB

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

🔑Concepts

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Arrhenius Theory: An acid is a substance that dissociates in water to produce hydrogen ions (H+H^+), while a base dissociates to produce hydroxide ions (OH−OH^-).

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Brønsted-Lowry Theory: An acid is defined as a proton (H+H^+) donor, and a base is defined as a proton (H+H^+) acceptor.

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Conjugate Acid-Base Pairs: These consist of two species that differ by only one proton (H+H^+). When an acid loses a proton, it forms its conjugate base; when a base gains a proton, it forms its conjugate acid.

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Amphiprotic Substances: Species that can act as both a Brønsted-Lowry acid and a base, such as water (H2OH_2O) or hydrogen carbonate (HCO3−HCO_3^-).

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Hydronium Ion: In aqueous solutions, H+H^+ ions do not exist in isolation but bond with water to form the hydronium ion, represented as H3O+H_3O^+.

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Strong vs. Weak: Strong acids/bases dissociate completely in aqueous solution, whereas weak acids/bases only partially dissociate.

📐Formulae

HA(aq)→H(aq)++A(aq)−HA_{(aq)} \rightarrow H^+_{(aq)} + A^-_{(aq)}

BOH(aq)→B(aq)++OH(aq)−BOH_{(aq)} \rightarrow B^+_{(aq)} + OH^-_{(aq)}

HA+B⇌A−+HB+HA + B \rightleftharpoons A^- + HB^+

H2O+H2O⇌H3O++OH−H_2O + H_2O \rightleftharpoons H_3O^+ + OH^-

pH=−log⁡10[H+]pH = -\log_{10}[H^+]

💡Examples

Problem 1:

Identify the Brønsted-Lowry acid, base, conjugate acid, and conjugate base 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:

NH3NH_3 is the base, H2OH_2O is the acid, NH4+NH_4^+ is the conjugate acid, and OH−OH^- is the conjugate base.

Explanation:

NH3NH_3 accepts a proton to become NH4+NH_4^+, making it the base. H2OH_2O donates a proton to become OH−OH^-, making it the acid. The resulting species on the product side are their respective conjugates.

Problem 2:

What is the conjugate base of the acid H2SO4H_2SO_4?

Solution:

HSO4−HSO_4^-

Explanation:

A conjugate base is formed by removing one proton (H+H^+) from the acid. Therefore: H2SO4−H+=HSO4−H_2SO_4 - H^+ = HSO_4^-.

Problem 3:

Write the dissociation equation for the Arrhenius acid HClHCl in water.

Solution:

HCl(aq)→H+(aq)+Cl−(aq)HCl(aq) \rightarrow H^+(aq) + Cl^-(aq) or HCl(aq)+H2O(l)→H3O+(aq)+Cl−(aq)HCl(aq) + H_2O(l) \rightarrow H_3O^+(aq) + Cl^-(aq)

Explanation:

According to Arrhenius, HClHCl is an acid because it increases the concentration of H+H^+ ions when dissolved in water.