Slide 1 / 123 Slide 2 / 123 New Jersey Center for Teaching and Learning Progressive Science Initiative Acids and Bases This material is made freely available at www.njctl.org and is intended for the non-commercial use of students and teachers. These materials may not be used for any commercial purpose without the written permission of the owners. NJCTL maintains its website for the convenience of teachers who wish to make their work available to other teachers, participate in a virtual professional learning community, and/or provide access to course materials to parents, students and others. www.njctl.org Click to go to website: www.njctl.org Slide 3 / 123 Slide 4 / 123 Auto-ionization of Water Acids and Bases In any sample of water, a small number of water molecules will dissociate into H+ and OH- ions. Lactic acid is one of many metabolities produced when H 2 O(l) -------> OH - (aq) + H + (aq) we exercise. It generally loses an H+ ion to from the lactate O ion (one of the chemicals that H O H H causes burning sensations in H - our muscles.) + - + The H+ ion then typically binds to a lone pair of electrons on another water molecule to make the hydronium ion: H 3 O + 2H 2 O(l) -------> OH - (aq) + H 3 O + (aq) H - O O O O H H H H H lactate H H lactic acid - + Slide 5 / 123 Slide 6 / 123 Auto-ionization of Water 1 What is the concentration of hydronium ions (H 3 O+) in pure water? In 1909, a device was invented that could H 3 O + (aq) = 1.0 x 10 -7 M measure the H+ or H 3 O A 1.0 x 10 -2 M @ 25 C + concentration in an B 1.0 x 10 -5 M aqueous solution. C 1.0 x 10 -7 M Using this data, the equilibrium constant for the auto-ionization of water can be calculated. D 1.0 x 10 -10 M 2H 2 O(l) --> H 3 O + (aq) + OH - (aq) E 1.0 x 10 -14 M Recalling our equilibrium concepts...... Kw = [H 3 O + ][OH - ] answer Since equal amounts of H 3 O+ and OH- are created... [H 3 O+] = [OH-] = 1.0 x 10 -7 M Kw = (1.0 x 10 -7 )(1.0 x 10 -7 ) = 1.0 x 10 -14 M Clearly, the equilibrium lies far to the left! Water does NOT like to dissociate.
Slide 7 / 123 Slide 8 / 123 2 Which of the following is the value of Kw for 3 Which of the following would be true in pure water? water? A 1.0 x 10 -2 A [H 3 O+] = [OH-] B [H 3 O+] < [OH-] B 1.0 x 10 -4 C [OH-] = 1 x10 -7 M C 1.0 x 10 -7 answer D A and C D 1.0 x 10 -9 answer E B and C E 1.0 x 10 -14 Slide 9 / 123 Slide 10 / 123 4 The magnitude of K w indicates that _________ 5 The molar concentration of hydronium ion, [H 3 O + ], in pure water at 25 °C is ___________. A 0 A water ionizes to a very small extent B 1 B the autoionization of water is exothermic C water ionizes very quickly C 7 answer D water ionizes very slowly D 10 -7 answer E 10 -14 Slide 11 / 123 Slide 12 / 123 Calculating H 3 O + or OH - Calculating H 3 O + or OH - In the natural world, we do not find pure water. There are always Let's do some examples! things dissolved in it that influence the concentrations of hydronium and hydroxide ions. #1 What is the [OH-] in a solution with [H 3 O+] = 3.4 x 10 -5 M? Kw = [H 3 O + ][OH - ] rearranged to find [OH-] = Kw/[H3O+] = 1.0 x 10 -14 / 3.4 x 10 -5 = 2.9 x 10 -10 = [OH-] move for answer #2 What is the [H 3 O+] in a solution with [OH-] = 1.2 x 10 -12 The hydronium or hydroxide concentration in a solution can be determined easily if one knows one or the other. Kw = [H 3 O + ][OH - ] rearranged to find [H 3 O+] = Kw/[OH-] Kw = [H 3 O + ][OH - ] = 1.0 x 10 -14 = 1.0 x 10 -14 / 1.2 x 10 -12 = 8.3 x 10 -3 = [H 3 O+] move for answer Rearranged for [H 3 O+] Rearranged for [OH-] [H 3 O+] = 1.0 x 10 -14 /[OH-] [OH-] = 1.0 x 10 -14 /[H 3 O+]
Slide 13 / 123 Slide 14 / 123 Calculating H 3 O + or OH - 6 What is the [H 3 O+] in an aqueous sample with an Application: [OH-] equal to 3.4 x 10 -3 M? Tap water is NOT pure water. There are many things dissolved in it that affect the amount of [H 3 O+] and [OH-] in the water sample. Can A 3.4 x 10 -3 M you think of some things that might chloride (Cl-), carbonate (CO 32- ) be dissolved in tap water? B 2.9 x 10 -12 M C 1.0 x 10 -7 M move for answer Flouride (F-), calcium ions (Ca 2+ ), c D 9.4 x 10 -7 M answer The average concentration of H 3 O+ in New York City tap water is E 3.4 x 10 11 M 5.01 x 10 -8 M. What is the average [OH-]? Kw = [H 3 O + ][OH - ] rearranged to find [OH-] = Kw/[H3O+] move for answer = 1.0 x 10 -14 /5.01x 10 -8 = 1.99x 10 -7 M Slide 15 / 123 Slide 16 / 123 7 Which of the following would have the smallest 8 The pacific ocean off the coast of Hawaii has [OH-]? a [OH-] = 8.32 x 10 -9 M. A solution with [H 3 O+] = 2.4 x 10 -1 What is the [H 3 O+]? B solution with [H 3 O+] = 2.4 x 10 -11 C solution with [H 3 O+] = 2.4 x 10 -6 D solution with [OH-] = 2.4 x 10 -3 E solution with [OH-] = 2.4 x 10 -12 answer answer Slide 17 / 123 Slide 18 / 123 Arrhenius Definition of Acids and Bases Arrhenius Definition of Acids and Bases In 1884, Swedish scientist Svante Arrhenius decided to As we have learned, when certain substances are added to create definitions for substances water, the H 3 O+ concentration changes. that changed the [H 3 O+] in an aqueous solution. Furthermore, if the [H 3 O+] changes, it would influence the [OH-]. Arrhenius labeled anything that increased the [H 3 O+] an acid Arrhenius labeled anything that increased the [OH-] a base Kw = [H 3 O + ] [OH - ] = 1.0x 10 -14 By measuring the [H 3 O+] of a water solution after a substance had been added, he could see if the substance was acidic or basic!
Slide 19 / 123 Slide 20 / 123 Arrhenius Definition of Acids and Bases Arrhenius Definition of Acids and Bases By measuring the [H 3 O+] of a water solution after a substance had been added, he could see if the substance was acidic or basic! Example 1: Let's add some HCN(aq) Example 2: Let's add some NaOH(s) HCN(aq) NaOH(s) H 3 O + (aq) = 2.3x 10 -6 M @ 25 C H 3 O + (aq) = 4.1x10 -11 M @ 25 C Remember that pure water has an [H 3 O+] = 1.0 X 10 -7 M. Since the [H 3 O+] is lower than 1.0 x 10 -7 M thereby making the Since the [H 3 O+] is higher than 1.0 X 10 -7 M, Arrhenius would [OH-] higher than 1.0 x 10 -7 M, Arrhenius would have described have described HCN as an acid! NaOH as a base! Slide 21 / 123 Slide 22 / 123 10 Vinegar has a [H 3 O+] of around 3.4 x10 -3 M. Which of 9 Which of the following solutions would be the following solutions would be considered by considered by Arrhenius to be the most basic? Arrhenius to be MORE acidic than vinegar? A 0.1 M NH 3 [H 3 O+] = 3.4x10 -10 M A 0.1 M NaOH [H 3 O+] = 1x10 -13 M B 0.1 M NaOH [H 3 O+]= 1x10 -13 M B 0.1 M HCl [OH-] =1.0 x10 -13 M C 0.1 M HCl [H 3 O+] =1x10 -1 M C 0.1 M NaCN [OH-] = 2.6x10 -4 M D 0.1 M HCN [H 3 O+]= 2.3x10 -6 M answer D 0.1 M NH 3 [H 3 O+] = 7.6x10 -9 M E Pure water [H 3 O+]=1x10 -7 M E pure water [OH-] = 1.0 x10 -7 M answer Slide 23 / 123 Slide 24 / 123 Bronsted Lowry Definition of an Acid Bronsted/Lowry Definition of Base At this time, most scientists explained Arrhenius acids as possessing At this time, most scientists explained Arrhenius bases as possessing H+ ions that could be added to water to produce [H 3 O+] OH- ions that would increase the [OH-] and decrease the [H3O+]. Arrhenius acids in action Arrhenius base in action HF(aq) + H 2 O(l) --> F - (aq) + H 3 O + (aq) NaOH(aq) --> Na+(aq) + OH-(aq) [OH-] causes [H 3 O+] Here, the hydroflouric acid (HF) donates one of it's H+ ions to a water molecule increasing the [H 3 O+](aq) Unfortunately, this view required that all bases had to possess the hydroxide ion. This was clearly not the case. Many substances, like ammonia (NH 3 ) or sodium phosphate (Na 3 PO 4 ), were known to be Two scientists - Bronsted and Lowry, working independently, decided basic but did NOT have any hydroxide ions! a more appropriate definition of an acid would be that of an H+ donor .
Slide 25 / 123 Slide 26 / 123 Bronsted Lowry Definition of Base Bronsted Lowry Definition of Acid and Bases Summary Bronsted and Lowry proposed that, insteading of possessing hydroxide ions, a base was a substance that accepted an H+ from water to produce OH- ions! Acids are defined as H+ (proton) donors. HC 3 H 6 O 3 (aq) + H 2 O(l) --> C 3 H 6 O 3 -(aq) + H 3 O+(aq) Bronsted base in action lactic acid NH 3 (g) + H 2 O(l) --> NH 4 +(aq) + OH-(aq) Bases are defined as H+ (proton) acceptors. When ammonia, NH 3 , accepts the H+ from the water, the water CN-(aq) + H 2 O(l) --> HCN(aq) + OH-(aq) turns into OH- making the solution basic. cyanide base Slide 27 / 123 Slide 28 / 123 11 A Bronsted acid is a substance that... 12 Which of the following could NOT act as a Bronsted acid? A accepts H+ ions A HCN B donates OH- ions B H 2 SO 4 C increases the concentration of OH- ions C NH 4 + D donates H+ ions D H 3 O+ answer E accepts OH- ions answer E BF 3 Slide 29 / 123 Slide 30 / 123 13 A Bronsted-Lowry base is defined as a 14 Which of the following compounds could never substance that __________. act as a Bronsted acid? SO 4 2- A A increases [H + ] when placed in H 2 O answer B HSO 4 - B decreases [H + ] when placed in H 2 O C H 2 SO 4 C increases [OH - ] when placed in H 2 O D NH 3 D acts as a proton acceptor E CH 3 COOH answer E acts as a proton donor
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