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Updates. Assignment 05 is is due Monday, Mar. 05 (in class) Midterm 2 is Thurs., March 15 Huggins 10, 7-8pm For conflicts: ELL 221, 6-7pm (must arrange at least one week in advance). Acids and Bases. Chapter 16. Binary acids (HX, H 2 X, H 3 X, H 4 X).
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Updates • Assignment 05 is is due Monday, Mar. 05 (in class) • Midterm 2 is Thurs., March 15 • Huggins 10, 7-8pm • For conflicts: ELL 221, 6-7pm (must arrange at least one week in advance)
Acids and Bases Chapter 16
Binary acids (HX, H2X, H3X, H4X) • Bond strength determines acidity within the same group (column), size • Bond polarity determines acidity within the same period (row), electronegativity
Rationalizing acidity • Two considerations • Stability of reactants • Stability of products • Stable • “Happy” • Low energy • Low number of charges • Low charge density on atom(s) • Charge distributed over multiple atoms • Strong bonds • Unstable • “Unhappy” • High energy • High number of charges • High charge density on atom(s) • Weak bonds • A highly favored reaction: unstable reactants forming stable products • Strong acid: unstable acid leads to stable conjugate base • Similarly, strong base: unstable base leads to stable conjugate acid • Weak acids and bases: intermediate in stability
Rationalizing acidity • Two considerations • Stability of reactants • Stability of products • Stable • “Happy” • Low energy • Low number of charges • Low charge density on atom(s) • Charge distributed over multiple atoms • Strong bonds • Unstable • “Unhappy” • High energy • High number of charges • High charge density on atom(s) • Weak bonds • A highly favored reaction: unstable reactants forming stable products • Strong acid: unstable acid leads to stable conjugate base • Similarly, strong base: unstable base leads to stable conjugate acid • Weak acids and bases: intermediate in stability
Oxyacids Central atoms derived from same group (same oxidation state) • More electronegative central atom polarizes the OH bond more, facilitating ionization • More electronegative central atom better able to stablize resulting negative charge following ionization, making a happier (more stable) conjugate base
Oxyacids Identical central atoms with different oxidation states • Acidity increases as oxidation number of central atom increases • Said another way: for oxyacids with the same central atom, acidity increases as the number of oxygens attached increases • Additional electronegative oxygen atoms pull electron density from the O-H bond, further increasing its polarity, facilitating ionization • Increasing the number of oxygens also helps stabilize the conjugate base by increasing its ability to “spread out” its negative charge (a happier conjugate base)
Carboxylic acids • Portion in blue known as carboxyl group and is often written as –COOH • Acids that contain a carboxyl group are called carboxylic acids, and they form the largest category of organic acids (organic referring to carbon-containing compounds, based on C-C bonds)
Carboxylic acids • Acetic acid is a weak acid (Ka = 1.8 x 10-5), whereas methanol is not an acid in water • Additional oxygen increases polarity of O-H bond and helps to stabilize the conjugate base • The conjugate base (called a carboxylate anion) can exhibit resonance (p. 284), which contributes further to the stability of the anion by spreading the negative charge over several atoms • When the three hydrogens are replaced with fluorines, the acidity increases (Ka = 5.0 x 10-1)
Acid-base properties of salt solutions • Salt solutions can be neutral, acidic, or basic, owing to the reaction of a cation or anion (or both) with water • These three solutions contain the acid-base indicator bromthymol blue. (a) The NaCl solution is neutral (pH = 7.0) (b) The NH4Cl solution is acidic (pH = 3.5) (c) The NaClO solution is basic (pH = 9.5)
Acid-base properties of salt solutions • Anions • Conjugate base of strong acid DOES NOT react with water (Cl-) • Conjugate base of weak acid reacts with water (CH3COO-) • Cations • Conjugate acid of weak base reacts with water (NH4+) • Most metals can react with water (Al3+, Cr3+, Fe3+, Bi3+, Be2+) • Ions of alkali metals and heavier alkaline earth metals DO NOT react with water (Na+)
Acid-base properties of salt solutions • Anions • Conjugate base of strong acid DOES NOT react with water (Cl-) • Conjugate base of weak acid reacts with water (acetate) • Cations • Conjugate acid of weak base reacts with water (NH4+) • Most metals can react with water (Al3+, Cr3+, Fe3+, Bi3+, Be2+) • Ions of alkali metals and heavier alkaline earth metals DO NOT react with water (Na+) (a) The NaCl solution is neutral (pH = 7.0) (b) The NH4Cl solution is acidic (pH = 3.5) (c) The NaClO solution is basic (pH = 9.5)
Acid-base properties of salt solutions • Examples where either the cation or the anion of the salt reacts -but not both! (a) The NaCl solution is neutral (pH = 7.0) (b) The NH4Cl solution is acidic (pH = 3.5) (c) The NaClO solution is basic (pH = 9.5)
Combined effect of cation and anion in solution • If salt solution contains an anion that reacts with water, we expect solution to be basic • If salt solution contains a cation that reacts with water, we expect solution to be acidic • What happens when salt is made from a cation and anion that both react with water? • Whether solution is basic, neutral, or acidic depends on the relative abilities of the ions to react with water • When a solution contains both the conjugate base of a weak acid and the conjugate acid of a weak base, the ion with the larger equilibrium constant, Ka or Kb, will have the greater influence on the pH • Stated another way: When a solution contains a reactive anion and a reactive cation (toward water), the ion with the larger equilibrium constant, Ka or Kb, will have the greater influence on the pH
Combined effect of cation and anion in solution • When a solution contains both the conjugate base of a weak acid and the conjugate acid of a weak base, the ion with the larger equilibrium constant, Ka or Kb, will have the greater influence on the pH • Take NH4F as an example • Both NH4+ and F- will react with water • Ka for NH4+ is 5.6 x 10-10 • Kb for F- is 1.4 x 10-11 • Therefore, Ka is larger, and solution will be acidic
Anions that act as both an acid and a base • Bicarbonate ion can ionize (rxn 1) or undergo hydrolysis (rxn 2) • Ionization involves bicarbonate acting as an acid; hydrolysis involves bicarbonate acting as a base • Because Kb is larger, we predict that hydrolysis will predominate, yielding a basic solution
CO2 (g) + H2O (l) H2CO3 (aq) N2O5 (g) + H2O (l) 2HNO3 (aq) Acidic, Basic and Amphoteric Oxides, p. 264 (shown in highest oxidation states) 16.11
Lewis Acids • Lewis acids are defined as electron-pair acceptors. • Atoms with an empty valence orbital can be Lewis acids.
Lewis Bases • Lewis bases are defined as electron-pair donors. • Anything that could be a Brønsted–Lowry base is a Lewis base. • Lewis bases can interact with things other than protons, however (broadest definition).
NaHCO3 (aq) + HCl (aq) NaCl (aq) + H2O (l) + CO2 (g) Mg(OH)2 (s) + 2HCl (aq) MgCl2 (aq) + 2H2O (l) Chemistry In Action: Antacids and the Stomach pH Balance