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) 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 (K a = 1.8 x ), 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 (K a = 5.0 x )

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 NH 4 Cl 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 (CH 3 COO - ) Cations –Conjugate acid of weak base reacts with water (NH 4 + ) –Most metals can react with water (Al 3+, Cr 3+, Fe 3+, Bi 3+, Be 2+ ) –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 (NH 4 + ) –Most metals can react with water (Al 3+, Cr 3+, Fe 3+, Bi 3+, Be 2+ ) –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, K a or K b, 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, K a or K b, 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, K a or K b, will have the greater influence on the pH Take NH 4 F as an example –Both NH 4 + and F - will react with water –K a for NH 4 + is 5.6 x –K b for F - is 1.4 x –Therefore, K a 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 K b is larger, we predict that hydrolysis will predominate, yielding a basic solution

Acid-Base Properties of Salts 16.10

Acidic, Basic and Amphoteric Oxides, p. 264 (shown in highest oxidation states) CO 2 (g) + H 2 O (l) H 2 CO 3 (aq) N 2 O 5 (g) + H 2 O (l) 2HNO 3 (aq)

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).

Chemistry In Action: Antacids and the Stomach pH Balance NaHCO 3 (aq) + HCl (aq) NaCl (aq) + H 2 O (l) + CO 2 (g) Mg(OH) 2 (s) + 2HCl (aq) MgCl 2 (aq) + 2H 2 O (l)