Properties of Acids and Bases

Chapter 15 Aqueous Equilibria: Acids and Bases Properties of Acids and Bases Generally, an acid is a compound that releases hydrogen ions, H +, into water. Blue litmus is used to test for acids. Blue litmus paper turns red in the presence of hydrogen ions. Acids are generally sour in taste and can be corrosive. Generally, a base is a compound that releases hydroxide ions, OH, into water. Red litmus is used to test for bases. Red litmus paper turns blue in the presence of hydroxide ions. Bases have a bitter taste and a slippery, soapy feel. Chapter 15 2 1

Definitions of an Acid and Base There are three definitions used to describe an acid or a base: Arrhenius Acids and Bases Brønsted-Lowry Acids and Bases Lewis Acids and Bases Chapter 15 3 Arrhenius Acids and Bases An Arrhenius acid is a substance that ionizes in water to produce hydrogen ions (H + ). HA (aq) H + (aq) + A - (aq) An Arrhenius base is a substance that ionizes in water to release hydroxide ions (OH - ). MOH (aq) OH - (aq) + M + (aq) Chapter 15 4 2

Brønsted-Lowry Acids & Bases The Brønsted-Lowry definitions of acids and bases are broader than the Arrhenius definitions. A Brønsted-Lowry acid is a substance that donates a hydrogen ion (H + ) to any other substance. It is a proton donor. All Arrhenius acids are classified as acids by the B-L definition Chapter 15 5 Brønsted-Lowry Acids & Bases A Brønsted-Lowry base is a substance that accepts a hydrogen ion (H + ). It is a proton acceptor (So no OH - required!). All Arrhenius bases are classified as bases by the B-L definition Chapter 15 6 3

Conjugate Acid-Base Pairs When a pair of molecules are related by the loss or gain of one H +, they are called a conjugate pair A conjugate acid-base pair is the proton donor and the ion formed by the loss of the proton. A conjugate base-acid pair is the proton acceptor and the ion formed by the gain of the proton. Chapter 15 7 Conjugate Acid-Base Pairs HF (g) + H 2 O (l) F - (aq) + H 3 O + (aq) NH 3 (g) + H 2 O (l) NH 4+ (aq) + OH - (aq) Chapter 15 8 4

Conjugate Acid-Base Pairs Let s practice writing and identifying conjugate pairs What is the conjugate acid or base for the following: Acid HCl H 2 O Conjugate Base Base HCO 3 - H 2 O Conjugate Acid HNO 2 HSO 4 - NH 3 CN - Identify the acid and base and their conjugates: HCN + H 2 O CN - + H 3 O + NO 2 - + H 2 Se HSe - + HNO 2 Chapter 15 9 Brønsted-Lowry Acids & Bases Write balanced equations for the dissociation of each of the following Brønsted Lowry acids in water. (a) H 2 SO 4 (b) HSO 4 (c) H 3 O + For the above reactions, identify the conjugate pairs and the acid and base in each pair. Chapter 15 10 5

Strengths of Acids and Bases B-L Acids and bases have varying strengths. The strength of an acid is measured by the amount of H + that is produced for each mole of acid that dissolves Strong Acids fully ionize in water ALL Products The strength of a base is measured by the degree of dissociation in solution. Strong Bases fully dissociate in solution ALL Products Chapter 15 11 Sulfuric Acid Strong Acids and Bases Hydrochloric Acid Nitric Acid Strong Acids Hydroiodic Acid Hydrobromic Acid Perchloric Acid H 2 SO 4 HI HBr HCl HClO 4 HNO 3 Only a few acids are considered strong acids. Other acids have varying degrees of weakness You need to know these!! Bases made from Groups IA and IIA metals and hydroxide are strong bases: LiOH, NaOH, KOH, Ca(OH) 2, etc. Most other bases are weak bases NH 3, NH 4 OH, Al(OH) 3, etc. Chapter 15 12 6

Strengths of Conjugate Acids and Bases Chapter 15 13 Strengths of Conjugate Acids and Bases Adcock, J Chem Ed (2001) 78, 1495-1496. Chapter 15 14 7

Strengths of Conjugate Acids and Bases If you mix equal concentrations of reactants and products, which of the following reactions proceed to the right and which proceed to the left? HF (aq) + NO 3- (aq) HNO 3 (aq) + F - (aq) NH 4+ (aq) + CO 3- (aq) HCO 3 (aq) + NH 3 (aq) Chapter 15 15 Lewis Acids and Bases The Lewis definition of acids and bases is the broadest of the three. A Lewis Acid is an electron pair acceptor. These are generally cations and neutral molecules with vacant valence orbitals. A Lewis Base is an electron pair donor. These are generally anions and neutral molecules with available pairs of electrons (lone pairs!) The bond formed is a coordinate covalent bond! Chapter 15 16 8

O Lewis Acids and Bases Examples of Lewis acids include: Halides of Group 3A elements Oxides of Nonmetals (SO 2, CO 2 and SO 3 ) δ + δ O S O Lewis Acid H + O H O S Lewis Base The S=O bond is polar with a δ + on the less electronegative S. This atom attracts the lone pair from the oxygen of water O O Chapter 15 17 H O H O O S O H O H Sulfuric Acid Lewis Acids and Bases Identify the Lewis acid and Lewis base in each of the following reactions: SnCl 4 (s) + 2 Cl (aq) SnCl 6 2 (aq) Hg 2+ (aq) + 4 CN (aq) Hg(CN) 4 2 (aq) Co 3+ (aq) + 6 NH 3 (aq) Co(NH 3 ) 6 3+ (aq) SO 2 (aq) + OH - (aq) HSO 3- (aq) Chapter 15 18 9

Summary of Acids & Bases Arrhenius Acids/Bases Brønsted-Lowry Acids/Bases Lewis Acids/Bases Chapter 15 19 Hydrated Protons In aqueous solutions, the bare proton (H + ) is too reactive to exist alone, so it bonds to the oxygen atom of a solvent water molecule to yield the hydronium ion (H 3 O + ). As all of the acids we will be dealing with will be in aqueous solution, the symbols H + and H 3 O + are interchangable Chapter 15 20 10

Autoionization of Water Water undergoes an autoionization reaction. H 2 O(l) + H 2 O(l) H 3 O + (aq) + OH - (aq) OR H 2 O(l) H + (aq) + OH - (aq) Only about 1 in 5 million water molecules is present as ions so water is a weak electrolyte. The concentration of hydrogen ions, [H + ], in pure water is about 1 10-7 mol/l at 25 C. Chapter 15 21 Autoionization of Water The molar ratio of H + to OH - in the reaction is 1 to 1, so if the [H + ] = 1 10-7 mol/l at 25 C, then the [OH - ] must also be 1 10-7 mol/l at 25 C: H 2 O (l) H + (aq) + OH - (aq) [H + ] [OH - ] = (1 10-7 )(1 10-7 ) = 1.0 10-14 This value is the ion product constant of water, K w. K W = [H + ][OH - ] We can use this value to calculate [H + ] or [OH - ] Chapter 15 22 11

[H + ] and [OH - ] Relationship The concentration of OH ions in a certain household ammonia cleaning solution is 0.00250 M. Calculate the concentration of H + ions. Calculate the concentration of OH ions in a HCl solution whose hydrogen ion concentration is 1.30 M. Chapter 15 23 The ph Scale ph is a measure of the acidity of a solution Acidic solutions: [H + ] > 1.0 x 10 7 M, ph < 7.00 Neutral solutions: [H + ] = 1.0 x 10 7 M, ph = 7.00 Basic solutions: [H + ] < 1.0 x 10 7 M, ph > 7.00 Chapter 15 24 12

The ph Concept Mathematically: ph = log [H + ] OR ph = log [H 3 O + ] Nitric acid (HNO 3 ) is used in the production of fertilizer, dyes, drugs, and explosives. Calculate the ph of a HNO 3 solution having a hydrogen ion concentration of 0.76 M. The ph of a certain orange juice is 3.33. Calculate the H + ion concentration. Chapter 15 25 The poh Concept poh is similar to ph except it is a measure of basicity of a solution. The lower the value, the more basic the solution poh = - log [OH - ] There is also a relationship between ph and poh: ph + poh = 14 So, if you know the [H + ] you can determine the ph, [OH - ] and poh Chapter 15 26 13

ph, poh, [H + ] and [OH - ] Calculations [H + ] [OH - ] ph poh Acidic, Basic or Neutral? 6.15 x 10-4 M 2.61 5.28 x 10-8 M 3.45 ph = - log [H + ] poh = - log [OH - ] [H + ] = 10 ph [OH - ] = 10 poh [H+] [OH-] = K W ph + poh = 14.00 Chapter 15 27 Measuring ph The approximate ph of a solution can be determined by using an acid-base indicator An indicator is a chemical substance that changes color in a specific ph range. Chapter 15 28 14

Measuring ph Indicators let you determine the ph of your solution within approximately ± 1 ph unit. More accurate methods, such as a ph meter, are also available in most labs Chapter 15 29 ph of Strong Acids and Bases We know that strong acids and bases fully dissociate in solution Therefore, to determine ph, you need to do the following: Write and balance the dissociation reaction Determine the amount of H + or H 3 O + (or OH - if it s a base) based on the concentration of the acid (or base) and the mole ratio. Calculate the ph using the [H + ] or [H 3 O + ] (or [OH - ] if it s a base) Calculate the ph of the following solutions: 0.050 M HClO 4 6.0 M KOH 0.125 M H 2 SO 4 0.0357 M Ba(OH) 2 Chapter 15 30 15

Equilibria in Solutions of Weak Acids We know that strong acids fully dissociate in solution Solutions of weak acids contain both ionized and non-ionized species in equilibrium HA (aq) + H 2 O (l) H 3 O + (aq) + A - (aq) + [H O ][A = [HA] 3 K a K a is the Acid Dissociation Constant This value can also be expressed as a pk a : pk a = -log K a ] Chapter 15 31 Equilibria in Solutions of Weak Acids Note that the larger the K a value, the stronger the acid WHY?? Chapter 15 32 16

Equilibria in Solutions of Weak Acids (a) Arrange the three acids in order of increasing value of K a. (b) Which acid, if any, is a strong acid? (c) Which solution has the highest ph, and which has the lowest? Chapter 15 33 Determination of the STEP 1 Write the balanced equation for the reaction ph of a Weak Acid STEP 2 Under the balanced equation, set up your ICE table Here, a good rule of thumb: If K a is significantly smaller (> 1000 time) than [HA] the assumption can be made. STEP 3 Write the K a expression and substitute the equilibrium concentrations into the expression STEP 4 Solve the K a expression for X. If you have made an assumption, make sure it is valid. If it is not, you must use the quadratic equation! STEP 5 Go back to your ICE table and calculate the Equilibrium concentrations using X STEP 6 Use the equilibrium concentration of H + or H 3 O + to determine ph Chapter 15 34 17

Determination of the ph of a Weak Acid Calculate the ph of a 0.036 M nitrous acid (HNO 2 ) solution. What is the ph of a 0.122 M monoprotic acid whose K a is 5.7 x 10 4? The ph of a 0.060 M weak monoprotic acid is 3.44. Calculate the K a of the acid. Chapter 15 35 Percent Dissociation in Solutions of Weak Acids The Percent Dissociation is a measure of the strength of an acid. + [H ] % Dissociation = 100% [HA] Stronger acids have higher percent dissociation. Percent dissociation of a weak acid decreases as its concentration increases. Calculate the % dissociation of HF (K a = 3.5 x 10-4 ) in 0.050 M HF Chapter 15 36 18

Equilibria in Solutions of Weak Bases We know that strong bases fully dissociate in solution Solutions of weak bases contain both ionized and non-ionized species in equilibrium B (aq) + H 2 O (l) BH + (aq) + OH - (aq) K b [OH ][BH = [B] K b is the Base Dissociation Constant This value can also be expressed as a pk b : pk b = -log K b + ] Chapter 15 37 Equilibria in Solutions of Weak Acids Again, the larger the K b value, the stronger the base WHY?? Chapter 15 38 19

Determination of the ph of a Weak Base Strychnine (C 21 H 22 N 2 O 2 ), a deadly poison used to kill rodents, is a weak base with a K b = 1.8 x 10-6. Calculate the ph of a saturated solution of strychnine (16.0 mg/100 ml). Chapter 15 39 Relationship between K a and K b Product of K a and K b : multiplying out the expressions for K a and K b equals K w. K a K b = K w What is the K a of an ammonia solution (K b = 1.8 x 10-5 )? Calculate the ph of a 0.26 M methylamine solution. Chapter 15 40 20

Polyprotic Acids Polyprotic acids yield more than one hydrogen ion per molecule. Protons are lost sequentially, one at a time. The conjugate base of first step is acid of second step. Ionization constants (K a ) decrease as protons are removed. H 2 CO 3 (aq) + H 2 O(l) H 3 O + (aq) + HCO 3- (aq) [HCO ][H O ] K = [H CO ] + 3 3-7 = 4.3 x 10 a1 2 3 HCO 3- (aq) + H 2 O(l) H 3 O + (aq) + CO 3 2- (aq) [CO ][H O ] K = [HCO ] 2 + 3 3-11 = 5.6 x 10 a2 3 Chapter 15 41 Polyprotic Acids Calculate the concentration of all species present in a 0.10 M solution of oxalic acid (C 2 H 2 O 4 ). Determine the ph of the solution. Chapter 15 42 21

Molecular Structure and Acid Strength The strength of an acid depends on its tendency to ionize. The better it is at ionizing, the stronger the acid For general acids of the type H X: The stronger the bond between H and X, the weaker the acid. The more polar the bond, the stronger the acid. For the hydrohalic acids, bond strength plays the key role giving: HF < HCl < HBr < HI Chapter 15 43 Molecular Structure and Acid Strength For binary acids in the same group, H A bond strength decreases with increasing size of A, so acidity increases. For binary acids in the same row, H A polarity increases with increasing electronegativity of A, so acidity increases. Chapter 15 44 22

Molecular Structure and Acid Strength For oxoacids bond polarity is more important. If we consider the main element (Y): Y O H + H Y O - 2 O + H 3 O + If Y is an electronegative element, or in a high oxidation state, the Y O bond will be more covalent. This causes the O H bond more polar and the acid stronger. Think of it the oxygen trying to hold onto electrons on both sides Chapter 15 45 Molecular Structure and Acid Strength For oxoacids with different central atoms that are from the same group of the periodic table and that have the same oxidation number, acid strength increases with increasing electronegativity. Chapter 15 46 23

Molecular Structure and Acid Strength For oxoacids having the same central atom but different numbers of attached groups, acid strength increases with increasing central atom oxidation number. The number of oxygen atoms increases the positive charge on the chlorine which weakens the O H bond and increases its polarity. Chapter 15 47 Molecular Structure and Acid Strength Predict the relative strengths of the following groups of acids: (a) H 2 S and H 2 Se (b) HClO, HBrO, and HIO. (c) HNO 3 and HNO 2. (d) H 3 PO 3 and H 3 PO 4. Chapter 15 48 24

Acid-Base Properties of Salt Solutions When a salt dissolves in solution, it dissociates into cations and anions. Solutions of salts can be acidic, basic or neutral. Anions and cations from strong acids and bases do not affect ph. Anions and cations from weak acids and bases do affect the ph. So, to determine whether your salt solution will be acidic, basic or neutral, you have to figure out whether the anion and cation would come from a strong or weak acid/base. Chapter 15 49 Salts that Form Neutral Solutions A solution of a salt containing the cation from a strong base and the anion from a strong acid will be neutral (ph = 7) NaCl (s) Na + (aq) + Cl - (aq) Cation of Strong Base (NaOH) Anion of Strong Acid (HCl) Both from strong Acid/Base so this solution will be neutral Chapter 15 50 25

Salts that Form Basic Solutions A solution of a salt containing the cation from a strong base and the anion from a weak acid will be basic (ph > 7) KF (s) K + (aq) + F - (aq) Cation of Strong Base (KOH) Anion of Weak Acid (HF) The anion is the conjugate of a weak acid so it is a strong base, making the solution basic Chapter 15 51 Salts that Form Acidic Solutions A solution of a salt containing the cation from a weak base and the anion from a strong acid will be acidic (ph < 7) NH 4 Br (s) NH 4 + (aq) + Br - (aq) Cation of Weak Base (NH 4 OH) Anion of Strong Acid (HBr) The cation is the conjugate of a weak base so it is a strong acid, making the solution acidic Chapter 15 52 26

Salts that Can Go Either Way For salt solutions containing the cation from a weak base and the anion from a weak acid the ph will be dependent on the K a and K b NH 4 F (s) NH 4 + (aq) + F - (aq) If K a > K b then ph is Acidic If K a < K b then ph is Basic If K a K b then ph is Neutral Cation of Weak Base (NH 4 OH) Anion of Weak Acid (HF) You have to compare the K a of NH + 4 and the K b of F - to determine ph Chapter 15 53 Acid-Base Properties of Salt Solutions Chapter 15 54 27

Acid-Base Properties of Salt Solutions Another type of acidic cation is a hydrated cation of a small, highly charged metal ion Metal Ion Hydrolysis: Chapter 15 55 Acid-Base Properties of Salt Solutions Predict whether the following solutions will be acidic, basic, or nearly neutral: (a) NH 4 I (b) CaCl 2 (c) KCN (d) Fe(NO 3 ) 3 (e) NH 4 CN Calculate the ph of a 0.15 M solution of sodium acetate (CH 3 COONa). Calculate the ph of a 0.24 M ammonium chloride solution (ZnCl 2 ). The K a = 2.5 x 10-10 for Zn(H 2 O) 6 2+. Chapter 15 56 28