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 BasesChapter 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

HCO3-(aq) + H2O H3O+ (aq) + CO3

2- (aq) Ka = 4.8 x 10-11

HCO3-(aq) + H2O H2CO3 (aq) + OH- (aq) Kb = 2.4 x 10-8

Acid-Base Properties of Salts

16.10

Acidic, Basic and Amphoteric Oxides, p. 264 (shown in highest oxidation states)

16.11

CO2 (g) + H2O (l) H2CO3 (aq)

N2O5 (g) + H2O (l) 2HNO3 (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

NaHCO3 (aq) + HCl (aq)

NaCl (aq) + H2O (l) + CO2 (g)

Mg(OH)2 (s) + 2HCl (aq)

MgCl2 (aq) + 2H2O (l)