ACIDS & BASES Chapter 15

15-1 Brønsted-Lowry Acid-Base Systems Objectives  To define Brønsted-Lowry acids and bases  To differentiate between Brønsted-Lowry and Arrhenius acids and bases  To identify acid-base conjugate pairs  To define an acid-base neutralization reaction

Summary of Acid-Base Definitions

15.1 Brønsted-Lowry Acid-Base Systems: Acid-Base Conjugate Pairs  When a Br ø nsted-Lowry acid transfers a proton, it also forms a base because the remaining species can accept a proton  HF (aq) + H 2 O  H 3 O + (aq) + F - (aq) acid base conjugate conjugate acid base

15.1 Brønsted-Lowry Acid-Base Systems: Acid- Base Conjugate Pairs

 A substance that can act either as an acid or a base is said to be amphoteric  Examples: water, HSO 4 -  H 2 O can donate a proton and become OH - or H 2 O can accept a proton and become H 3 O +

15.1 Brønsted-Lowry Acid-Base Systems: Acid-Base Neutralization Reactions  Acids and bases react quickly with each other and reach equilibrium at speeds that are often limited by how fast the solution is stirred  Arrhenius acids and bases react to form water and salts acid + base → water + salt HCl + NaOH → H 2 O + NaCl

15.1 Brønsted-Lowry Acid-Base Systems: Acid-Base Neutralization Reactions  Brønsted-Lowry acids and bases are not limited to this type of reaction. Water and salt do not have to be formed as products. acid 1 + base 1 → base 2 + acid 2 HCl + H 2 O → Cl - + H 3 O + HF + NH 3 → F - + NH 4 +

15.1 Brønsted-Lowry Acid-Base Systems: Acid-Base Neutralization Reactions  Identify the acid-base conjugate pairs: a) H 2 SO 4 (aq) + H 2 O (l) → HSO 4 - (aq) + H 3 O + (aq) b) H 2 O (l) + F - (aq)  OH - (aq) + HF (aq) Answers: a) Acid = H 2 SO 4 conjugate base = HSO 4 - a) Base = H 2 O conjugate acid = H 3 O + b) Acid = H 2 O conjugate base = OH - b) Base = F - conjugate acid = HF

15-2 Autoionization of Water Objectives  To relate hydrogen ion concentration to hydroxide ion concentration in aqueous solutions  To define pH and use it to express concentrations

15-2 Autoionization of Water  Water can react with itself  One water molecule acts as an acid  One water molecule acts as a base as shown on the following slide…

Self-Ionization of Water H 2 O + H 2 O  H 3 O + + OH - At 25 , [H 3 O + ] = [OH - ] = 1 x K w is a constant at 25  C: K w = [H 3 O + ][OH - ] K w = (1 x )(1 x ) = 1 x

15-2 Autoionization of Water: Calculating Hydrogen and Hydroxide Ion Concentration  Like any equilibrium constant, the autoionization constant for water (K w ) is sensitive to changes in temperature  Only the K w at 25°C needs to be memorized  If not stated, assume the temperature is 25°C

Calculating pH, pOH pH = -log 10 (H 3 O + ) pOH = -log 10 (OH - ) Relationship between pH and pOH pH + pOH = 14 Finding [H 3 O + ], [OH - ] from pH, pOH [H 3 O + ] = 10 -pH [OH - ] = 10 -pOH

pH and pOH Calculations

pH Scale

15-2 Autoionization of Water: Concentration Scales  A note about significant figures…  One decimal place in a pH value such as 3.2 suggests a single significant figure  Two decimal places in a pH value such as 4.67 suggests there are two significant figures  The numbers before the decimal do not count as significant figures and simply serve to locate the decimal point for the hydrogen ion concentration

15-2 Autoionization of Water: The Relationship Between pH and pOH  Calculate the pH of a M NaOH solution  A: [OH-] = M so [H+] = 1x /0.035 M = 2.86x M and pH = -log(2.86x ) =  Calculate the hydrogen ion concentration in a solution that has a pH of 5.37  A: [H+] = 10 -pH = = 4.3x10 -6 M

15-3 Strong Acids and Bases Objectives  Define strong acids and bases  List the species that are strong acids and bases  Calculate the concentrations of species, the pH, and the pOH in solutions of strong acids and bases

15-3 Strong Acids and Bases  Compounds that ionize or dissociate completely are called strong electrolytes  Compounds that dissociate only partially are called weak electrolytes  Strong acids ionize completely in solution HCl (g) + H 2 O (l) → Cl - (aq) + H 3 O + (aq) note the arrow!!! reaction goes essentially to completion cannot detect unionized HCl in solution

Dissociation of Strong Acids Strong acids are assumed to dissociate completely in solution. Large K a or small K a ? Reactant favored or product favored?

p. 655

Table 15-5, p. 639

Dissociation Constants: Strong Acids AcidFormula Conjugate Base KaKa Perchloric HClO 4 ClO 4 - Very large Hydriodic HI I - Very large Hydrobromic HBr Br - Very large Hydrochloric HCl Cl - Very large Nitric HNO 3 NO 3 - Very large Sulfuric H 2 SO 4 HSO 4 - Very large Hydronium ion H 3 O + H 2 O 1.0

15-3 Strong Acids and Bases:  WARNING! Do not equate the term “strong acid” with “more reactive”  The terms strong and weak refer to the degree of ionization, not the reactivity or corrosiveness  Ex: HF is a weak acid that cannot be stored in glass. HF is used to etch glass and can still cause severe skin irritation.

Dissociation of Strong Bases  Strong bases are metallic hydroxides  Group I hydroxides (NaOH, KOH) are very soluble  Group II hydroxides (Ca, Ba, Mg, Sr) are less soluble but what does tend to dissociate completely  pH of strong bases is calculated directly from the concentration of the base in solution MOH(s)  M + (aq) + OH - (aq)

15-4 Qualitative Aspects of Weak Acids and Weak Bases Objectives  Define weak acids and bases  Relate K a to the competition of different bases for protons

15-4 Qualitative Aspects of Weak Acids and Weak Bases  Weak Acid  Acid that does not ionize completely when dissolved in water  Weak acids have conjugate bases that are relatively strong  Weak Base  Base that does not dissolve completely when dissolved in water  Weak bases have conjugate acids that are relatively strong

15-5 Weak Acids Objectives  Write the chemical equation for the ionization of a weak acid  Define analytical concentration  Relate the fraction ionized of a weak acid to the acid ionization constant  Calculate acid ionization constants from experimental data  Calculate the concentrations of the species present in a weak acid solution  Calculate percentage ionization from K a and concentration

Acid Dissociation Acid Dissociation HA  H + + A - Acid Proton Conjugate base Alternately, H + may be written in its hydrated form, H 3 O + (hydronium ion)

Dissociation of Weak Acids Weak acids are assumed to dissociate only slightly (less than 5%) in solution. Large K a or small K a ? Reactant favored or product favored?

Dissociation Constants: Weak Acids AcidFormula Conjugate Base KaKa Iodic HIO 3 IO x Oxalic H 2 C 2 O 4 HC 2 O x Sulfurous H 2 SO 3 HSO x Phosphoric H 3 PO 4 H 2 PO x Citric H 3 C 6 H 5 O 7 H 2 C 6 H 5 O x Nitrous HNO 2 NO x Hydrofluoric HF F x Formic HCOOH HCOO x Benzoic C 6 H 5 COOH C 6 H 5 COO x Acetic CH 3 COOH CH 3 COO x Carbonic H 2 CO 3 HCO x Hypochlorous HClO ClO x Hydrocyanic HCN CN x

A Weak Acid Equilibrium Problem What is the pH of a 0.50 M solution of acetic acid, HC 2 H 3 O 2, K a = 1.8 x ? Step #1: Write the dissociation equation HC 2 H 3 O 2  C 2 H 3 O H +

A Weak Acid Equilibrium Problem What is the pH of a 0.50 M solution of acetic acid, HC 2 H 3 O 2, K a = 1.8 x ? Step #2: ICE it! HC 2 H 3 O 2  C 2 H 3 O H + I C E x +x x xx

A Weak Acid Equilibrium Problem What is the pH of a 0.50 M solution of acetic acid, HC 2 H 3 O 2, K a = 1.8 x ? Step #3: Set up the law of mass action HC 2 H 3 O 2  C 2 H 3 O H xxx E

A Weak Acid Equilibrium Problem What is the pH of a 0.50 M solution of acetic acid, HC 2 H 3 O 2, K a = 1.8 x ? Step #4: Solve for x, which is also [H + ] HC 2 H 3 O 2  C 2 H 3 O H xxx E [H + ] = 3.0 x M

A Weak Acid Equilibrium Problem What is the pH of a 0.50 M solution of acetic acid, HC 2 H 3 O 2, K a = 1.8 x ? Step #5: Convert [H + ] to pH HC 2 H 3 O 2  C 2 H 3 O H xxx E

15-6 Solutions of Weak Bases & Salts Objectives  Write the chemical reaction that occurs when a weak base dissolves in water  Calculate the concentrations of species present in a solution of a weak base  Relate K b for a base to K a of its conjugate acid  Calculate the pH of a salt solution

Reaction of Weak Bases with Water The base reacts with water, producing its conjugate acid and hydroxide ion: CH 3 NH 2 + H 2 O  CH 3 NH OH - K b = 4.38 x 10 -4

K b for Some Common Weak Bases BaseFormula Conjugate Acid KbKb Ammonia NH 3 NH x Methylamine CH 3 NH 2 CH 3 NH x Ethylamine C 2 H 5 NH 2 C 2 H 5 NH x Diethylamine (C 2 H 5 ) 2 NH (C 2 H 5 ) 2 NH x Triethylamine (C 2 H 5 ) 3 N (C 2 H 5 ) 3 NH x Hydroxylamine HONH 2 HONH x HydrazineH 2 NNH 2 H 2 NNH x Aniline C 6 H 5 NH 2 C 6 H 5 NH x Pyridine C 5 H 5 N C 5 H 5 NH x Many students struggle with identifying weak bases and their conjugate acids.What patterns do you see that may help you?

Reaction of Weak Bases with Water The generic reaction for a base reacting with water, producing its conjugate acid and hydroxide ion: B + H 2 O  BH + + OH - (Yes, all weak bases do this – DO NOT endeavor to make this complicated!)

A Weak Base Equilibrium Problem What is the pH of a 0.50 M solution of ammonia, NH 3, K b = 1.8 x ? Step #1: Write the equation for the reaction NH 3 + H 2 O  NH OH -

A Weak Base Equilibrium Problem What is the pH of a 0.50 M solution of ammonia, NH 3, K b = 1.8 x ? Step #2: ICE it! I C E x +x x xx NH 3 + H 2 O  NH OH -

A Weak Base Equilibrium Problem Step #3: Set up the law of mass action xxx E What is the pH of a 0.50 M solution of ammonia, NH 3, K b = 1.8 x ? NH 3 + H 2 O  NH OH -

A Weak Base Equilibrium Problem Step #4: Solve for x, which is also [OH - ] xxx E [OH - ] = 3.0 x M NH 3 + H 2 O  NH OH - What is the pH of a 0.50 M solution of ammonia, NH 3, K b = 1.8 x ?

A Weak Base Equilibrium Problem Step #5: Convert [OH - ] to pH xxx E What is the pH of a 0.50 M solution of ammonia, NH 3, K b = 1.8 x ? NH 3 + H 2 O  NH OH

Acid-Base Properties of Salts Type of SaltExamplesCommentpH of solution Cation is from a strong base, anion from a strong acid KCl, KNO 3 NaCl NaNO 3 Both ions are neutral Neutral These salts simply dissociate in water: KCl(s)  K + (aq) + Cl - (aq)

Acid-Base Properties of Salts Type of SaltExamplesCommentpH of solution Cation is from a strong base, anion from a weak acid NaC 2 H 3 O 2 KCN, NaF Cation is neutral, Anion is basic Basic C 2 H 3 O H 2 O  HC 2 H 3 O 2 + OH- base acid acid base The basic anion can accept a proton from water:

Acid-Base Properties of Salts Type of SaltExamplesCommentpH of solution Cation is the conjugate acid of a weak base, anion is from a strong acid NH 4 Cl, NH 4 NO 3 Cation is acidic, Anion is neutral Acidic NH 4 + (aq)  NH 3 (aq) + H + (aq) Acid Conjugate Proton base The acidic cation can act as a proton donor:

Acid-Base Properties of Salts Type of SaltExamplesCommentpH of solution Cation is the conjugate acid of a weak base, anion is conjugate base of a weak acid NH 4 C 2 H 3 O 2 NH 4 CN Cation is acidic, Anion is basic See below  IF K a for the acidic ion is greater than K b for the basic ion, the solution is acidic  IF K b for the basic ion is greater than K a for the acidic ion, the solution is basic  IF K b for the basic ion is equal to K a for the acidic ion, the solution is neutral

Acid-Base Properties of Salts Type of SaltExamplesCommentpH of solution Cation is a highly charged metal ion; Anion is from strong acid Al(NO 3 ) 2 FeCl 3 Hydrated cation acts as an acid; Anion is neutral Acidic Step #1: AlCl 3 (s) + 6H 2 O  Al(H 2 O) 6 3+ (aq) + Cl - (aq) Salt water Complex ion anion Step #2: Al(H 2 O) 6 3+ (aq)  Al(OH)(H 2 O) 5 2+ (aq) + H + (aq) Acid Conjugate base Proton

15-7 Mixtures of Acids Objectives  Calculate the pH of a solution that contains both strong and weak acids  Determine the pH of a solution that contains a mixture of weak acids (or bases) with different ionization constants

15-7 Mixtures of Acids Finding the pH of a mixture of acids is not too complicated as long as the strengths of the acids are quite different (K a values differ by a factor of 100) The influence of a weak acid on the pH of a solution is generally much less than that of a strong acid In a solution of weak acids, only the strongest one is important to determining the pH of the solution If a solution lacks either strong acids or weak acids, then water determines the pH of the resulting solution Same holds true for mixtures of bases ….

Are strong acids present? Is a single weak acid dominant? Are multiple weak acids present? Hydrogen ion concentrations calculated from the analytical concentrations of the strong acids pH determined by autoionization of water Solve equilibrium of dominant weak acid system Yes No Systems of multiple weak acids presented in later chemistry courses

15-7 Mixtures of Acids  Explain how to calculate the pH of: a) A solution that is 0.25 M KOH and 1.00 M ammonia b) A solution that is 0.40 M HCl, 0.20 M HBr, 0.10 M HCOOH, and 0.20 M HF c) A solution that is 0.80 M HCOOH, and 0.50 M HOCl (K a formic acid = 1.8 x & K a for hypochlorous acid is 3 x )

15-8 Influence of Molecular Structure on Acid Strength Objectives  Relate the acid ionization constants of a series of related binary acids to their structure and bonding  Define oxyacid and list several common oxyacids  Explain how fundamental properties such as size and electronegativity affect the strengths of acids

15-8 Influence of Molecular Structure on Acid Strength  Ionization of an acid is influenced by:  the strength of the bond that holds the proton  the bond polarity  changes in the strengths of other bonds that accompany the loss of a proton  solvation of the ions produced in the reaction

15-8 Influence of Molecular Structure on Acid Strength Trends for Binary Hydrides Binary Hydrides are compounds composed of two elements, one of which is hydrogen Acidity is related to the strength of the H-A bond and the stability of the A - anion in solution

15-8 Influence of Molecular Structure on Acid Strength Trends for Binary Hydrides Orders of bond strength in the series of hydrogen halides is HF>HCl>HBr>HI The same series describes weak → strong acids  The stronger the H-A bond the weaker the acid!!  The same trend is observed down any group in the periodic table. As you move down a column, the strength of the acid increases as the bond strength decreases.

p. 663 As you move down a column, the strength of the acid increases as the bond strength decreases.

15-8 Influence of Molecular Structure on Acid Strength:  The more electronegative the “A” atom in HA, the stronger the acid. Strength of an acid increases as you move across a row.

15-8 Influence of Molecular Structure on Acid Strength Trends for Oxyacids  Oxyacids contain hydrogen, oxygen, and a third element  Examples: HNO 2 HNO 3 H 2 CrO 4 CH 3 COOH

15-8 Influence of Molecular Structure on Acid Strength Trends for Oxyacids  The hydrogen atoms that ionize in oxyacids are always bonded to an oxygen atom  The strengths of oxyacids (HOX) increase with increasing electronegativity of X

15-8 Influence of Molecular Structure on Acid Strength Trends for Oxyacids  Strength of acid also increases if the number of oxygen atoms increases in a series due to increases in the stability of the anion produced

15-8 Influence of Molecular Structure on Acid Strength

15-9 Lewis Acids and Bases Objectives  To define Lewis acids and bases  To identify Lewis acids, Lewis bases, and their reaction products

15-9 Lewis Acids and Bases  Lewis Acid  any substance that can accept a pair of electrons  Lewis Base  any substance that can donate a pair of electrons  A Lewis acid must have an empty orbital available to accept the pair of electrons donated by the base  Examples:  NH 3 (aq) + HCl(aq) → NH 4 + (aq) + Cl - (aq)  BF 3 + NH 3 → BF 3 NH 3

Summary of Acid-Base Definitions