Bronsted Acids and Bases (In Depth) Identifying an acid or a base Identify which reactant behaves as an acid and which behaves as a base in the following reaction! H 2 O(l) + CH 3 NH 3 + (aq) > CH 3 NH 2 (aq) + H 3 O + (aq) CH 3 NH 3 + (aq) donated an H + to become CH 3 NH 2 = It's an acid! H2O(aq) accepted an H+ to become H3O+ = It's a base! move for answer 33
15 According to the following reaction, which reactant molecule is acting as an acid? H 2 O + H 2 SO 4 → H 3 O + + HSO 4 A H 2 SO 4 answer B H 2 O C H 3 O + D HSO 4 None of the above E 34
16 According to the following reaction, which reactant molecule is acting as a base? H 2 O + H 2 SO 4 → H 3 O + + HSO 4 A H 2 SO 4 answer B H 2 O C H 3 O + D HSO 4 E None of the above 35
17 According to the following reaction, which reactant molecule is acting as a base? H 3 O + + HSO 4 → H 2 O + H 2 SO 4 A H 2 SO 4 B H 2 O answer H 3 O + C HSO 4 D E None of the above 36
18 For the following reaction, identify whether the circled compound is behaving as an acid or a base. H 3 PO 4 + H 2 O ⇌ H 2 PO 4 + H 3 O + A Acid answer B Base C Neither D Both E None of the above 37
19 For the following reaction, identify whether the circled compound is behaving as an acid or a base. H 3 PO 4 + H 2 O ⇌ H 2 PO 4 + H 3 O + A Acid B Base answer C Neither D Both E None of the above 38
Bronsted Acids and Bases (In Depth) Identifying an acid or a base in reversible reactions Reactions are reversible so we must be able to identify acids and bases based on the reverse reaction. Example F (aq) + H 2 O(l) <> HF(aq) + OH (aq) HF(aq) donates an H + ion to become F (aq) = It's an acid OH (aq) accepts an H+ to become H 2 O(l) = It's a base 39
20 For the following reaction, identify whether the circled compound is behaving as an acid or a base. H 3 PO 4 + H 2 O ⇌ H 2 PO 4 + H 3 O + A Acid B Base answer C Neither D Both E None of the above 40
21 For the following reaction, identify whether the circled compound is behaving as an acid or a base. H 3 PO 4 + H 2 O ⇌ H 2 PO 4 + H 3 O + answer A Acid B Base C Neither D Both E None of the above 41
Conjugate Acids and Bases The term conjugate comes from the Latin word “conjugare,” meaning “to join together.” Reactions between acids and bases always yield their conjugate bases and acids. donates H + HNO 2 (aq) + H 2 O(l) NO 2 (aq) + H 3 O + (aq) conjugate Conjugate Acid Base acid base accepts H+ 42
Conjugate Acids and Bases To find an acid or bases conjugate in a reaction, simply write the formula for the substance left after the H+ has been donated or accepted. Example: What is the conjugate base of HSO 4 (aq)? Since we are looking for a conjugate base, HSO 4 must be an acid so let's have it donate an H + 2 (aq) + H + (aq) HSO 4 (aq) > SO 4 conjugate base 43
Conjugate Acids and Bases 2 (aq)? Example: What is the conjugate acid of CO 3 2 must be a base Since we are looking for a conjugate acid, CO 3 so let's have it accept an H+ 2 (aq) + H + > HCO 3 CO 3 (aq) conjugate acid Dealing with charges If you accept an H+, you become more positive If you donate an H+, you become more negative 44
22 Which of the following would be the conjugate base of HNO 2 ? A NO 2 answer B H 2 NO 2 C NO 2 D NO 2 2 E HNO 2 45
23 Which would be the conjugate acid of HCO 3 (aq)? A CO 3 2 B HCO 3 C CO 3 answer D H 2 CO 3 E H 2 CO 3 46
24 What would be the an acid/conjugate pair in the following reaction? NH 2 + H 2 O > NH 3 + OH A NH 2 /H 2 O B NH 2 /NH 3 answer C H 2 O/OH D H 2 O/NH 3 E None of these 47
Lewis Acids and Bases Definition Scientists noticed that some substances could create acidic solutions despite not having any H+ ions to donate. An example of this was the Ca 2+ ion. G.N. Lewis proposed a mechanism for this O Ca 2+ + > Ca (OH) + + + H H H The metal ion accepted a pair of electrons from the water molecule, resulting in the donation of one of the water's H+ ions. 48
Lewis Acids and Bases A Lewis acid is an electron pair acceptor . Metal ions or molecules with incomplete octets (BF 3 ) are good examples. A Lewis base is an electron pair donor . Molecules with unbonded electrons (NH 3 , CN, OH, H 2 O) are good examples. Lewis Acid Lewis Base (e pair (e pair donor) acceptor) 49
25 A lewis base is a substance that... A Accepts H+ ions answer B Donates H+ ions C Accepts e pairs D Donates e pairs E Decreases the concentration of [OH] 50
26 Which of the following would likely act as a lewis acid? answer A NH 3 B OH C CN D H 2 O E Fe 3+ 51
What are Acids and Bases? Definition Type Acid Base substance that produces substance that decreases Arrhenius (traditional) H 3 O + ions in aqueous H 3 O + ions in aqueous solution solution substance that donates H+ substance that accepts H+ Bronsted Lowry ions in reaction ions in reaction substance that accepts an substance that donates an Lewis electron pair in reaction electron pair in reaction 52
Class Discussion Evolution of a definition Question 1: Can you think why the Arrhenius definition was considered insuffienct? It could not explain how a substance without hydroxide could make a move for answer solution basic Question 2: Can you explain why Lewis felt that the Bronsted definition was insufficient? It required an acid to be in possession of a hydrogen atom. move for answer 53
What are Acids and Bases? The lewis definition is generally considered the most broad. All acids are Lewis acids, most are also Bronsted acids, and many are Arrhenius acids Lewis Bronsted Arrhenius 54
Amphoteric Substances If a substance can act both as an acid and base, it is known as amphoteric. For example, water can act as a base or acid depending on the situation. HCl + H 2 O Cl + H 3 O + Above, water accepts a proton, thus acting as a base. NH 3 +H 2 O NH 4 + + OH Above, water donates a proton, thus acting as an acid Because of water's amphoteric nature, it makes the perfect solvent for most acid base reactions. Its nature allows for easier exchange of protons between acids and bases. 55
Acid and Base Strength Acid Base 100% HCl Cl Strong acids are completely S Negligible ionized t r H 2 SO 4 HSO 4 o in H 2 O n ionized in water (They all donate Base strength increases g HNO 3 NO 3 their H+ ions). H 3 O + H 2 O HSO 4 SO 42 H 3 PO 4 H 2 PO 4 Their conjugate bases are very HF F HC 2 H 3 O 2 C 2 H 3 O 2 weak. Acid strength increases Weak H 2 CO 3 HCO 3 Weak H 2 S HS H 2 PO 4 HPO 42 NH 4+ NH 3 HCO 3 CO 32 HPO 42 PO 43 Negligible H 2 O OH 100% OH O 2 Strong protonated H 2 H in H 2 O CH 4 CH 3 56
Acid and Base Strength Acid Base 100% HCl Cl Strong Negligible ionized H 2 SO 4 HSO 4 in H 2 O Base strength increases HNO 3 NO 3 H 3 O + H 2 O Weak acids only ionize partially HSO 4 SO 42 H 3 PO 4 H 2 PO 4 in water. HF F HC 2 H 3 O 2 C 2 H 3 O 2 Acid strength increases Weak H 2 CO 3 HCO 3 Weak Their conjugate bases are H 2 S HS H 2 PO 4 HPO 42 weak bases. NH 4+ NH 3 HCO 3 CO 32 HPO 42 PO 43 N H 2 O OH 100% e g OH O 2 Strong l protonated i g H 2 H i b in H 2 O l e CH 4 CH 3 57
Acid and Base Strength Acid Base 100% S HCl Cl Negligible ionized t r o H 2 SO 4 HSO 4 in H 2 O n Base strength increases g HNO 3 NO 3 H 3 O + H 2 O HSO 4 SO 42 H 3 PO 4 H 2 PO 4 HF F Substances with negligible HC 2 H 3 O 2 C 2 H 3 O 2 Acid strength increases Weak H 2 CO 3 HCO 3 acidity do not ionize in water. Weak H 2 S HS They will not readily give up H 2 PO 4 HPO 42 protons. NH 4+ NH 3 HCO 3 CO 32 HPO 42 PO 43 Their conjugate bases are H 2 O OH Negligible 100% OH O 2 Strong protonated exceedingly strong. H 2 H in H 2 O CH 4 CH 3 58
Strong Acids There are seven strong acids: 3 contain a H bound to the very electronegative halogens: HCl hydrochloric acid HBr hydrobromic acid HI hydroiodic acid HF, or hydrofloric acid, is a weak acid. Although flourine is very electronegative, the bond strength between flourine and hydrogen is too strong for HF to easily give up H + . 59
27 Which of the following is NOT a strong acid? A HBr B HF answer C HI D HCl E A and C 60
Strong Acids There are seven strong acids: 4 are from the very electron drawing oxyanions: HNO 3 nitric acid H 2 SO 4 sulfuric acid HClO 3 chloric acid HClO 4 perchloric acid Each of these anions has a central atom that is highly electronegative compared to hydrogen. The oxygens that are bonded to that central atom draw more electrons from it making it even more electronegative and likely to take electrons from hydrogen forming H + . 61
Strong Acids The seven strong acids are: HCl hydrochloric acid HBr hydrobromic acid HI hydroiodic acid HNO 3 nitric acid H 2 SO 4 sulfuric acid HClO 3 chloric acid HClO 4 perchloric acid 62
Monoprotic Acids The seven strong acids are strong electrolytes because they are 100% ionized. In other words, these compounds exist totally as ions in aqueous solution. For the monoprotic strong acids (acids that donates only one proton per molecule of the acid), the hydronium ion concentration equals the acid concentration. [H 3 O + ] = [acid] So, if you have a solution of 0.5 M HCl, then [H 3 O + ] = 0.5 M 63
Strong Bases All strong bases are group of compounds called "metal hydroxides." All alkali metals in Group I form hydroxides that are strong bases: LiOH, NaOH, KOH, etc. Only the heavier alkaline earth metals in Group II form strong bases: Ca(OH) 2 , Sr(OH) 2 , and Ba(OH) 2 . Again, these substances dissociate completely in aqueous solution. In other words, NaOH exists entirely as Na + ions and OH ions in water. 64
28 What would be the [H3O+] in a 0.005 M HBr solution? answer 65
Acid and Base Strength In any acidbase reaction, the proton Acid Base moves toward the stronger base. In 100% HCl Cl S Negligible ionized t r o H 2 SO 4 HSO 4 other words, a stronger base will "hold in H 2 O n Base strength increases g HNO 3 NO 3 onto" its proton whereas a strong acid H 3 O + H 2 O HSO 4 SO 42 easily releases its proton(s). H 3 PO 4 H 2 PO 4 HF F HC 2 H 3 O 2 C 2 H 3 O 2 Acid strength increases Weak H 2 CO 3 HCO 3 Weak H 2 S HS HCl (aq) + H 2 O (l) > H 3 O + (aq) + Cl (aq) H 2 PO 4 HPO 42 acid base conj. acid conj. base NH 4+ NH 3 HCO 3 CO 32 HPO 42 PO 43 H 2 O is a much stronger Negligible H 2 O OH 100% OH O 2 g base than Cl , so the proton protonated n o H 2 H in H 2 O r t S moves from HCl to H 2 O. CH 4 CH 3 66
29 What would be the [OH] in a 0.034 M NaOH solution? answer 67
Acid and Base Strength Acetic acid is a weak acid. This means that only a small percent of the acid will dissociate. The double headed arrow is used only in weak acid or weak base dissociation equations since the reaction can proceed with both the forward and reverse reactions. H 3 O +(aq) + CH 3 CO 2(aq) CH 3 CO 2 H (aq) + H 2 O (l) A single arrow is used for strong acid or strong bases which dissociate completely since the forward reaction is much more favorable than the reverse reaction. NaOH Na + (aq) + OH (aq) 68
30 Strong acids have ___________ conjugate bases. A strong B weak C neutral D negative answer 69
31 HBr, hydrobromic acid is a strong acid. This means that _______________. aqueous solutions of HBr contain equal concentrations A of H + and OH does not dissociate at all when it is dissolved in water B cannot be neutralized by a base C dissociates completely to H + and Br when it dissolves in water D answer 70
pH pH is defined as the negative base10 logarithm of the concentration of hydronium ion. pH = log [H 3 O + ] It is a measure of hydrogen ion concentration, [H + ] in a solution, where the concentration is measured in moles H + per liter, or molarity. The pH scale ranges from 014. Generally when calculating pH we round to two decimal places. 71
pH Recall that in pure water, the ionproduct is K w = 1.0*10 14 = [H 3 O+][OH] In pure water, the hydronium ion concentration and hydroxide ion concentrations are equal: [H 3 O + ] = [OH ] Therefore, in pure water, 3 O + ] = [OH] = 1.0 x 10 14 = 1.0 x 10 7 M [H pH = Log[H 3 O+] = Log(1.0 x10 7 ) = 7 A pH of 7 is considered neutral on the pH scale 72
Calculating pH What is the pH of the solution with hydrogen ion concentration of 5.67x10 8 M (molar)? pH = log [H + ] First, take the log of 5.67x10 8 = 7.25 Now, change the sign from to + Answer: pH = 7.25 Note: If you take the log of The order of operations: 5.67x10 8 M, 1. Take the log you will end up 2. Switch the sign with an incorrect answer. 73
32 What is the pH of a solution with hydrogen ion concentration of 1.0 x 10 5 M? A 1.0 x 10 5 B 5.00 C 5.00 D 9.00 E 9.00 answer 74
33 What is the pH of a solution with hydrogen ion concentration of 1.0 x 10 12 M? A 1.0 x 10 12 B 12.00 answer C 2.00 12.00 D 75
34 What is the pH of a solution whose hydronium ion concentration is 7.14 x 10 3 M? answer 76
35 What is the pH of a solution whose hydronium ion concentration is 1.92 x 10 9 M? answer 77
36 What is the pH of a 0.34 M solution of the strong acid HI? (Remember that strong acids ionize completely) answer 78
pH Application In order for proteins to be digested in the stomach, the pH must be lower than 2.7. If the pH is too high, proteins will not be broken down and may cause a food allergy or indigestion. A patient complains of indigestion and a sample of stomach fluid is taken and the [H 3 O+] is found to be 3.4 X10 3 M. Is there a problem with the pH? 79
pH What is the relationship between [H 3 O+] and the pH value? Below are three different [H 3 O+]. Find the pH of each. pH = log [H 3 O + ] Hydrogen ion concentration, [H3O+] pH in moles/Liter 1.0 x 101 1.0 x 102 1.0 x 1010 Clearly, the lower the [H3O+], the _____ the pH. 80
pH What is the relationship between [H 3 O+], the pH value, and the acidity and basicity of a solution? high OH basic low H 3 O+ basic neutral acidic acidic low OH High H 3 O+ 81
pH 0 Battery acid 0 More acidic 90° 1 gastric fluid These are the lemon juice carbonated 2 pH values for beverages 1 several common vinegar 3 orange juice substances. 4 beer coffee 5 egg yolks 2 pure rain or water milk 6 distilled water 7 blood sea water 3 baking soda 8 9 10 4 milk of magnesia household ammonia 11 More basic 12 household bleach 5 13 household lye 14 82
How Do We Measure pH? For more accurate measurements, one uses a pH meter, which measures the voltage in the solution. 83
How Do We Measure pH? For less accurate measurements, one can use Litmus paper “Red” paper turns blue above ~pH = 8 “Blue” paper turns red below ~pH = 5 Or an indicator (usually an organic dye) pH range for color change 8 0 2 6 4 10 12 14 Methyl violet Thymol blue Methyl orange Methyl red Bromothymol blue Phenolphthalein Alizarin yellow R 84
pH BASE ACID [H+] > [OH] [H+] < [OH] There are excess There are excess hydrogen ions in hydroxide ions in solution. solution. Solution type [H +](M) [OH] (M) pH value Acidic > 1.0x10 7 <1.0x10 7 <7.00 Neutral =1.0x10 7 =1.0x10 7 =7.00 Basic <1.0x10 7 > 1.0x10 7 >7.00 85
37 Which of the following solutions would be most acidic? A pH = 3 B pH = 2 answer C pH = 11 D pH = 14 E pH = 1 86
38 Which of the following (M) solutions would be LEAST acidic? A [H 3 O+] = 2.3x107 B [H 3 O+] = 9.1 x10 3 answer C [H 3 O+] = 1.3 x10 2 D [H 3 O+] = 7.8 x10 9 E [H 3 O+] = 4.5 x10 4 87
39 Which of the following solutions would have the highest pH? A [OH] =3.4 x10 3 answer B [H 3 O+] = 5.4 x10 11 C [OH] = 3.4 x10 12 D [H 3 O+] =5.4 x10 2 E [OH] =3.4 x10 1 88
40 Which solution below has the highest concentration of hydroxide ions? A pH = 3.21 B pH = 7.00 C pH = 8.93 answer D pH = 12.60 89
41 Which solution below has the lowest concentration of hydrogen ions? A pH = 11.40 B pH = 8.53 answer C pH = 5.91 D pH =1.98 90
42 For a basic solution, the hydrogen ion concentration is ______________ than the hydroxide ion concentration. A greater than B less than answer C equal to D Not enough information. 91
43 For an acidic solution, the hydroxide ion concentration is ______________ than the hydrogen ion concentration. A greater than answer B less than C equal to D Not enough information. 92
44 Which of the following would turn blue litmus paper red? A Solution with [OH] = 2.3 E7 M B Solution with pH = 4 C Solution with pOH = 2 D A and C E B and C answer 93
Understanding a Log Based Scale Because of the base10 logarithm, each 1.0point value on the pH scale differs by a value of ten. A solution with pH = 9 has a hydrogen ion concentration, [H + ], that is 10 times more than a pH = 10 solution. A solution with pH = 8 has a hydrogen ion concentration, [H + ], that is 10 2 or 100 times more than a pH = 10 solution. 94
45 A solution with pH = 3 has a hydrogen ion concentration that is __________than a solution with pH = 5. A 2x more B 2x less C 100x more D 100x less answer 95
46 A solution with pH = 14 has a hydrogen ion concentration that is __________than a solution with pH = 11. A 3x more B 3x less answer C 1000x more D 1000x less 96
pOH Just as the pH of a solution can be calculated by: pH = log[H 3 O+] The pOH of a solution can be calculated by: pOH = log[OH] Recall that the [OH] and [H 3 O+] are inversly related so pH and pOH are as well. high pH low pH 0 7 14 low pOH high pOH 14 7 0 97
Calculating pOH What is the pOH of a solution that has a [OH] = 2.3 E5 M? pOH = log[OH] pOH = log(2.3 E5) = 4.63 98
47 What is the pOH of a solution with a [OH] = 2.7 x10 2 M? A 2.7 B 12.43 C 1.57 D 1.57 answer E 2.7 99
pOH Once we have calculated pOH, it is very easy to calculate pH. Remember that our solvent for all of our reactions is Water. We also know that we have a K w value for water of 1 x 10 14 . This is ALWAYS true for water. We can also determine the following equations: K w =[H + ][OH ] Throwing in our logarithms for pH, pOH and pK w we end up with this: pK w = pH + pOH Remember that Kw is a constant and if we that the negative log of that constant we get 14 so..... 14 = pH + pOH 100
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