ACIDS AND BASES CHAPTER 15

I. Arrhenius Acids and Bases (What we have been using to this point) Arrhenius Acid is a substance that, when dissolved in water, increases the concentration of hydrogen ion, H + or (hydronium ion H 3 O + ). Arrhenius Base is a substance that, when dissolved in water, increases the concentration of hydroxide ions, OH -.

II. Bronsted-Lowry Acids & Bases A. Definitions 1. Bronsted Acid: H + (proton) donor Ionizable hydrogen in the acid structure is usually bonded to an electronegative atom. 2. Bronsted Base: H + (proton) acceptor Base structure must have an unshared pair of electrons.

B. Broader Definition For Acid-Base Reactions Consider: HCl + NH 4 OH  NH 4 Cl + H 2 O Consider: HF + HCO 3 -  H 2 CO 3 + F -

C. Amphiprotic Compounds Some compounds can act as a Bronsted acid in one case and a Bronsted base in another case. It depends on the molecules the amphiprotic compound is reacting with. Example: Water HCl + H 2 O  H 3 O + + Cl - NH 3 + H 2 O  NH OH -

D. Conjugate Acid-Base Pairs A conjugate acid-base pair consists of two species in an acid-base reaction, one acid and one base, that differ by the loss or gain of a proton. Conjugate acid- species formed when a base has accepted a proton. Conjugate base- species formed when an acid has donated or removed a proton. Example: Acetic Acid CH 3 COOH ⇌ CH 3 COO - + H + CH 3 COOH + H 2 O ⇌ CH 3 COO - + H 3 O +

Example Problems 1. What is the conjugate base of HNO 3 ? 2. What is the conjugate acid of NH 2 - ? 3. Label all species in the following reaction. H 2 PO HCO 3 - ⇌ H 2 CO 3 + HPO 4 2-

E. Strong vs. Weak Acids and Bases 1. Strong Acids and Bases Essentially go to 100% ionization Rxn normally shown with single headed arrow. Strong AcidsStrong Bases HClLiOH HBrNaOH HIKOH HNO 3 Ca(OH) 2 H 2 SO 4 Ba(OH) 2 HClO 4 Sr(OH) 2 HNO 3 → H + + NO 3 -

2. Weak Acids and Bases Partially dissociate or ionize, reaching some dynamic equilibrium state. Rxn normally shown with double headed arrow. HCN ⇌ H + + CN - Weak acids and bases significant in biological systems.

F. Examples of Organic Weak Acids and Bases Carboxylic Acids: (weak acids) RCO 2 H or RCOOH Anion formed by ionization is resonance stabilized Examples: pyruvic acid, lactic acid, acetic acid Amines: (weak bases) Based on ammonia, with differing # of “R” groups Primary, secondary, tertiary

G. Polyprotic Acids and Bases Polyprotic acids can donate more than one proton sequentially. H 2 SO 4 H 3 PO 4 H 2 S Polyprotic bases can accept more than one proton sequentially. SO 4 2- PO 4 3- S 2-

III. Self-Ionization of Water Description Pure water slightly ionizes on its own (autoionizes) with equilibrium reactant favored. H 2 O(l) + H 2 O(l) ⇌ H 3 O + (aq) + OH - (aq) Determine the K c value.

K c for water ionization = K w K w = ionization constant for water or = ion product constant for water K w = [H + ] [OH - ] = 1 x at 25 o C What are [H + ] and [OH - ] for pure water at 25 o C?

True for Water and Dilute Aqueous Solutions (Including Acid and Base Solutions of Interest) [H + ] [OH - ] = 1 x or [H 3 O + ] [OH - ] = 1 x Neutral Solution[H + ] = [OH - ] Acidic Solution[H + ] > [OH - ] Basic Solution [H + ] < [OH - ] or [H 3 O + ]

IV. pH Scale In most aqueous weak acid and base solutions, the H + concentrations are very small. Easier to express concentrations using a logarithmic relationship. p = - log Thus: pH = -log [H + ] pOH = -log [OH - ]

What is the relationship between pH and pOH? [H + ] [OH - ] = 1 x (-log [H + ]) + (-log [OH - ]) = -log (1 x ) pH + pOH = 14

The pH Scale Since pH is a logarithmic scale, cola drinks (pH about 2.5) are about ____ times as acidic as tomatoes (pH about 4.5)

Recall for problem solving: ****[H + ] is the same as [H 3 O + ] 1)pH = -log [H + ] 2) [H + ] = 10 -pH or antilog (-pH) = [H + ] 3) pOH = -log [OH - ] 4) [OH - ] = 10 -pOH or antilog (-pOH - ) = [OH - ] 5) [H + ] [OH - ] = 1 x ) pH + pOH = 14

Problem Solving: 1) A blood sample (considered a dilute aqueous solution) contains 7.2 x mol H + per liter. a. What is the pH of the blood sample? b. What is the [OH - ] of the blood sample? c. Is the blood sample acidic, basic, or neutral? 2) The pH of an aqueous solution is 8.7 a. What is the [H + ]? b. What is the pOH?

V. Strong Acids and Bases Strong acids and bases undergo essentially complete dissociation (ionization). Equilibrium expressions not needed for problem solving. Example: What is the pH and [H + ] for a 0.05 M HCl solution?

Problem Solving for Weak Acids and Bases 1.They only dissociate or ionize partially in solution. 2.This means we have an equilibrium situation, and can solve problems using the techniques we learned earlier for dealing with equilibrium problems. (ICE tables…) 3.Equilibrium Constants (K c ) K a for weak acids K b for weak bases

VI. Weak Acid Equilibrium Rxns A. Acid Ionization Constant 1. Definition Give the acid ionization constant expression for the acid: HA(aq) + H 2 O(l) ⇌ H 3 O + (aq) + A - (aq) or HA(aq) ⇌ H + (aq) + A - (aq) K a = ?

2. What does the K a tell you? Larger K a  stronger acid (more of the acid ionizes in water to form H +) See Table 15.5, for examples Which is the strongest acid? Acetic acid K a = 1.8 x Boric Acid K a = 7.3 x

B. K a From Equilibrium Concentrations A 0.10 M solution of phenol (weak acid) has a pH of Calculate the K a for phenol.

C. pH Calculations from K a of Weak Acid The weak monoprotic acid, parahydroxybenzoic acid, has a K a of 2.6 x Answer the following questions for a M solution of parahydroxybenzoic acid. 1. What is the pH of the solution? 2. What percent of the parahydroxybenzoic acid has ionized in this solution?

D. Comment on Polyprotic Acids Example: H 3 PO 4 K a ’s = 7.5 x 10 -3, 6.2 x 10 -8, 3.6 x Show the three acid ionization equations with their accompanying K a values. 2. Which is the strongest acid? 3. How would you go about calculating the pH of a given H 3 PO 4 solution?

VII. Weak Base Equilibrium Rxns A.Base Ionization Constant (base will accept proton from H 2 O, forming OH - ) 1. Definition Give the base ionization constant expression for the base: B(aq) + H 2 O(l) ⇌ BH + (aq) + OH - (aq) K b = ?

2. Relative Strength of Weak Bases a. The K b for ammonia is 1.8 x The K b for phosphate ion is 2.8 x Which is the strongest base? b. Which will have a higher pH: a 1.0 M ammonia solution, or a 1.0 M phosphate ion solution

B. K b From Equilibrium Concentrations The pain killer, morphine, is a weak base. A 0.01 M morphine solution has a pH of Calculate the K b for morphine.

C. pH Calculations from K b of Weak Base The weak base methylamine (CH 3 NH 2 ) has a K b of 5.0 x Answer the following questions for a M aqueous solution of methylamine. a. Write the chemical equation. b. Write the base ionization equilibrium expression. c. Calculate the [OH - ], pOH, and pH for solution.

VIII. Relationship of K a and K b A. Conjugate Acid and Base Reaction May Be Written in Both Directions. For Example: HA + H 2 O ⇌ H 3 O + + A - K a = A - + H 2 O ⇌ HA + OH - K b = ** Reaction will go in direction of stronger acid and base to the weaker acid and base. ** Compare K a and K b to decide direction.

B. Mathematical Relationship of K a and K b (for previous equations) Thus: K a x K b = [H + ] [OH - ] = K w K a x K b = 1 x andpK a + pK b = 14

C. Example Problems Consider the dihydrogen phosphate ion, H 2 PO K a = 6.2 x What is its conjugate base? 2. Show the chemical reaction to which K a applies for H 2 PO Determine the K b value for HPO Which direction is the reaction favored?

IX. Acid-Base Reactions of Salts (Ions as Acids and Bases) A. Salts 1. Ionic compounds (salts) formed from acid-base reaction. HA + MOH  M + A - + H 2 O acid base salt 2. The salt (MA) could leave the resulting aqueous solution to be neutral, acidic, or basic.

B. Hydrolysis of Salts Salts may react with water (hydrolysis reaction involving the splitting of a water molecule) to produce acidic or basic solutions. For hypothetical ionic compound MA, consisting of M x+ cation and A x- anion, consider possible hydrolysis reactions and possible consequences: M x+ + H 2 O  M(OH) (x-1)+ + H + acidic solution A x- + H 2 O  HA (x+1)- + OH - basic solution If both cation and anion hydrolyze, compare K a and K b

C. Predicting If Hydrolysis Occurs 1. Write the hypothetical hydrolysis rxns for salt. (one for cation and one for anion) 2. Look at the hypothetical hydrolysis products. 3. If a strong acid or strong base was hypothetically produced, these hydrolysis reactions DO NOT occur. No acid or base properties upon hydrolysis. Recall Table of Strong Acids and Bases 4. If a weak acid or base was produced, these hydrolysis reactions DO occur and produce acid or base properties. For hydrolysis products: H + : acidicOH - : Basic

D. Example Problems Will an aqueous solution of the salt be acidic, basic, or neutral? 1) NaCl 2) NH 4 Cl 3) NH 4 CN

Cumulative Example Problem The cyanate ion, OCN -, is a weak base. The K a for HOCN (cyanic acid) is 3.4 x Calculate the pH of a M solution of NaOCN.

X. Lewis Acids and Bases A. Some Acid-Base Reactions Do Not Fit Either Arrehenius or Bronsted –Lowry Definitions. Consider: HCl + NaOH  NaCl + H 2 O Al H 2 O  Al (H 2 O) 6 3+

B. Lewis Acids and Bases 1. Lewis Acid: electron pair acceptor (Accepts a pair of electrons to form a new bond) ** Usually has an incomplete octet of electrons 2. Lewis Base: electron pair donor (Donates a pair of electrons to form a new bond) ** Must have at least one lone pair of electrons C. Identify as Lewis acid or Lewis base? H 2 O Cr 3+ BF 3

XI. Acid Rain Read Section What is acid rain and what are its environmental effects?