Chapter_16_Notes_-_CHEM_1133_Broering

Chemistry: An Atoms-Focused Approach

Date: 2/19/2025Class: CHEM 1133Edition: 2ndCopyright: 2017, W.W. Norton & Company

Chapter 16: Acid Base Theory

Definitions of Acids and Bases

• Arrhenius Acid: A compound that produces positive hydrogen ions (H⁺) or hydronium ions (H₃O⁺) when dissolved in aqueous solutions, thereby increasing the concentration of H⁺ in the solution. Example: Hydrochloric acid (HCl) in water.

• Arrhenius Base: A compound that produces hydroxide ions (OH⁻) in aqueous solutions, increasing the concentration of OH⁻ in the solution, which can neutralize acids. Example: Sodium hydroxide (NaOH).

• Ionization: The process in which electrolytes dissociate into their respective ions upon dissolving in water, resulting in the formation of H⁺ and OH⁻ ions which are crucial for acid-base reactions.

Brønsted-Lowry Theory

• Brønsted-Lowry Acid: Defined as a substance that donates H⁺ ions (protons) to other substances, thereby participating in acid-base reactions without needing to be in an aqueous solution. Example: Acetic acid (CH₃COOH).

• Brønsted-Lowry Base: A substance that accepts H⁺ ions, acting as a proton acceptor in chemical reactions. Example: Ammonia (NH₃), which can gain a proton to form ammonium (NH₄⁺).

Hydrogen Ions and Protons

• Hydrogen Ion (H⁺): Composed solely of a proton without any associated electrons, which means it cannot exist freely in aqueous conditions; it forms the hydronium ion (H₃O⁺) upon encountering water molecules.

Reaction:[ ext{H}^+(aq) + ext{H}_2 ext{O}(l) ightarrow ext{H}_3 ext{O}^+(aq) ]

Acid Ionization and Strength

• Strong Acid: A type of acid that completely ionizes in water, leading to a high concentration of H⁺ ions in solution. Example: [ ext{HCl}(aq) ightarrow ext{H}^+(aq) + ext{Cl}^-(aq) ]

• Weak Acid: An acid that does not fully ionize and results in an equilibrium established between the undissociated species and its ions in solution. Example:[ ext{HF}(aq) ightleftharpoons ext{H}^+(aq) + ext{F}^-(aq) ]

Strong Acids

• List of Strong Acids: Hydrobromic acid (HBr), Hydroiodic acid (HI), Nitric acid (HNO₃), Sulfuric acid (H₂SO₄), Perchloric acid (HClO₄), Chloric acid (HClO₃).

These acids are characterized by their ability to completely dissociate in aqueous solution, leading to significantly lower pH values.

Weak Acids and Their Strengths

• Ka Values: Weak acids are often ranked by their acid dissociation constant (Ka), which is a quantitative measure of their acid strength in solution. Acids with Ka values significantly smaller than 1 favor the reactants over the products. Typical ranking of some weak acids by decreasing strength:

• Ranking of Weak Acids by Ka:

  • HF > HNO₂ > HCOOH > CH₃COOH > HOCl

Base Ionization and Strength

• Strong Bases: Completely dissociate in water, resulting in a high concentration of OH⁻ ions. Example: Sodium hydroxide (NaOH) completely dissociates to give Na⁺ and OH⁻ in solution.

• Weak Bases: Do not fully dissociate in solution and have a lower concentration of hydroxide ions compared to strong bases. Example: Ammonia (NH₃) reacts with water to form hydroxide ions:[ ext{NH}_3(aq) + ext{H}_2 ext{O}(l) ightleftharpoons ext{NH}_4^+(aq) + ext{OH}^-(aq) ] Kb is the equilibrium constant for this reaction.

Brønsted–Lowry Acids and Bases

• Conjugate Acid-Base Pair: A pair comprising an acid and its conjugate base that differ by the presence of one proton (H⁺). Example:

  • Acid: HCl

  • Conjugate Base: Cl⁻

• Amphoteric Substance: A substance that possesses the ability to act as either an acid or a base, depending on the surrounding chemical environment. Water (H₂O) is a common example, as it can accept a proton to become H₃O⁺ or donate a proton to become OH⁻.

Relative Strengths of Conjugate Acids and Bases

• Relationship: Strong acids yield weak conjugate bases due to the complete ionization of the acid. Example: Hydrochloric acid (HCl) dissociates to form Cl⁻ which is a very weak base and does not tend to re-associate with protons.

pH and the Autoionization of Water

• Autoionization of Water: Water can self-ionize at a very low concentration to produce H₃O⁺ and OH⁻ ions:[ K_w = [ ext{H}_3 ext{O}^+][ ext{OH}^-] = 1.0 imes 10^{-14} ext{ at } 25°C ]

• Relationship: The concentration of H₃O⁺ inversely affects the concentration of OH⁻, maintaining dynamic equilibrium in pure water.

Acidic, Basic, and Neutral Solutions

• Acidic Solutions: Defined as having [H₃O⁺] > [OH⁻], resulting in pH < 7.

• Basic Solutions: Characterized by [H₃O⁺] < [OH⁻], leading to pH > 7.

• Neutral Solutions: Occur when [H₃O⁺] = [OH⁻], giving a pH of exactly 7.

pH, pKa, and pKb Calculations

• pH Calculation: Defined as -log[H₃O⁺].

• pOH Calculation: Defined as -log[OH⁻].

• Relation Between Ka, Kb, pKa, and pKb: There is an inverse relationship between Ka and Kb values, where the equation pKa + pKb = 14 holds true at 25°C. This shows the interdependence of the strengths of acids and bases.

Weak Acid Example: Acetic Acid

• Acetic acid is a weak acid that partially ionizes in a solution, thus making it a suitable candidate for ICE (Initial, Change, Equilibrium) tables for equilibrium calculations.

Polyprotic Acids

• Definitions: These are acids that can donate more than one H⁺ ion per molecule. Common examples include sulfuric acid (H₂SO₄), which can donate two protons, and phosphoric acid (H₃PO₄), which can donate three.

• Acid Ionization Constants: The strength of these acids decreases with each successive ionization, where the first ionization constant is often significantly greater than the following constants.

Acid-Base Characteristics of Salts

• Generalizations of Salts: Salts can form solutions that are neutral, acidic, or basic depending on the nature of their constituent ions.

• Examples:

  • Neutral Salts: Salts formed from strong acids and strong bases like sodium chloride (NaCl).

  • Acidic Solutions: Salts containing cations that are conjugate acids of weak bases (e.g., ammonium chloride, NH₄Cl).

  • Basic Solutions: Salts with anions that are conjugate bases of weak acids (e.g., sodium fluoride, NaF).

Practice Problems and Concept Tests

• The notes include numerous practice examples that involve calculating pH and recognizing the strengths of different acids and bases. Additionally, concept tests are provided throughout to reinforce understanding and application of acid-base principles.