Acids and Bases Lactic acid is one of many metabolities produced - PDF document

Slide 1 / 123 Slide 2 / 123 Chemistry Atomic Origins 2015-08-14 www.njctl.org Slide 3 / 123 Acids and Bases Lactic acid is one of many metabolities produced when we exercise. It generally loses an H+ ion to from the lactate ion (one of the chemicals that causes burning sensations in our muscles.) - lactate lactic acid

Slide 4 / 123 Auto-ionization of Water In any sample of water, a small number of water molecules will dissociate into H+ and OH- ions. H 2 O(l) -------> OH - (aq) + H + (aq) O H O H H H - + - + 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 H H - + Slide 5 / 123 Auto-ionization of 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 @ 25 C + concentration in an aqueous solution. Using this data, the equilibrium constant for the auto-ionization of water can be calculated. 2H 2 O(l) --> H 3 O + (aq) + OH - (aq) Recalling our equilibrium concepts...... Kw = [H 3 O + ][OH - ] 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 6 / 123 1 What is the concentration of hydronium ions (H 3 O+) in pure water? A 1.0 x 10 -2 M B 1.0 x 10 -5 M C 1.0 x 10 -7 M D 1.0 x 10 -10 M E 1.0 x 10 -14 M answer

Slide 7 / 123 2 Which of the following is the value of Kw for water? A 1.0 x 10 -2 B 1.0 x 10 -4 answer C 1.0 x 10 -7 D 1.0 x 10 -9 E 1.0 x 10 -14 Slide 8 / 123 3 Which of the following would be true in pure water? A [H 3 O+] = [OH-] B [H 3 O+] < [OH-] C [OH-] = 1 x10 -7 M D A and C answer E B and C Slide 9 / 123 4 The magnitude of K w indicates that _________ A water ionizes to a very small extent B the autoionization of water is exothermic C water ionizes very quickly D water ionizes very slowly answer

Slide 10 / 123 5 The molar concentration of hydronium ion, [H 3 O + ], in pure water at 25 °C is ___________. A 0 B 1 C 7 answer D 10 -7 E 10 -14 Slide 11 / 123 Calculating H 3 O + or OH - In the natural world, we do not find pure water. There are always things dissolved in it that influence the concentrations of hydronium and hydroxide ions. The hydronium or hydroxide concentration in a solution can be determined easily if one knows one or the other. Kw = [H 3 O + ][OH - ] = 1.0 x 10 -14 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 12 / 123 Calculating H 3 O + or OH - Let's do some examples! #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 Kw = [H 3 O + ][OH - ] rearranged to find [H 3 O+] = Kw/[OH-] = 1.0 x 10 -14 / 1.2 x 10 -12 = 8.3 x 10 -3 = [H 3 O+] move for answer

Slide 13 / 123 Calculating H 3 O + or OH - Application: 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 you think of some things that might chloride (Cl-), carbonate (CO 32- ) be dissolved in tap water? move for answer Flouride (F-), calcium ions (Ca 2+ ), c The average concentration of H 3 O+ in New York City tap water is 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 14 / 123 6 What is the [H 3 O+] in an aqueous sample with an [OH-] equal to 3.4 x 10 -3 M? A 3.4 x 10 -3 M B 2.9 x 10 -12 M C 1.0 x 10 -7 M D 9.4 x 10 -7 M answer E 3.4 x 10 11 M Slide 15 / 123 7 Which of the following would have the smallest [OH-]? A solution with [H 3 O+] = 2.4 x 10 -1 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

Slide 16 / 123 8 The pacific ocean off the coast of Hawaii has a [OH-] = 8.32 x 10 -9 M. What is the [H 3 O+]? answer Slide 17 / 123 Arrhenius Definition of Acids and Bases As we have learned, when certain substances are added to water, the H 3 O+ concentration changes. Furthermore, if the [H 3 O+] changes, it would influence the [OH-]. Kw = [H 3 O + ] [OH - ] = 1.0x 10 -14 Slide 18 / 123 Arrhenius Definition of Acids and Bases In 1884, Swedish scientist Svante Arrhenius decided to create definitions for substances that changed the [H 3 O+] in an aqueous solution. Arrhenius labeled anything that increased the [H 3 O+] an acid Arrhenius labeled anything that increased the [OH-] a base 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 Arrhenius Definition of Acids and Bases Example 1: Let's add some HCN(aq) HCN(aq) H 3 O + (aq) = 2.3x 10 -6 M @ 25 C Remember that pure water has an [H 3 O+] = 1.0 X 10 -7 M. Since the [H 3 O+] is higher than 1.0 X 10 -7 M, Arrhenius would have described HCN as an acid! Slide 20 / 123 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 2: Let's add some NaOH(s) NaOH(s) H 3 O + (aq) = 4.1x10 -11 M @ 25 C Since the [H 3 O+] is lower than 1.0 x 10 -7 M thereby making the [OH-] higher than 1.0 x 10 -7 M, Arrhenius would have described NaOH as a base! Slide 21 / 123 9 Which of the following solutions would be considered by Arrhenius to be the most basic? A 0.1 M NH 3 [H 3 O+] = 3.4x10 -10 M B 0.1 M NaOH [H 3 O+]= 1x10 -13 M C 0.1 M HCl [H 3 O+] =1x10 -1 M D 0.1 M HCN [H 3 O+]= 2.3x10 -6 M answer E Pure water [H 3 O+]=1x10 -7 M

Slide 22 / 123 10 Vinegar has a [H 3 O+] of around 3.4 x10 -3 M. Which of the following solutions would be considered by Arrhenius to be MORE acidic than vinegar? A 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 NaCN [OH-] = 2.6x10 -4 M D 0.1 M NH 3 [H 3 O+] = 7.6x10 -9 M E pure water [OH-] = 1.0 x10 -7 M answer Slide 23 / 123 Bronsted Lowry Definition of an Acid At this time, most scientists explained Arrhenius acids as possessing H+ ions that could be added to water to produce [H 3 O+] Arrhenius acids in action HF(aq) + H 2 O(l) --> F - (aq) + H 3 O + (aq) Here, the hydroflouric acid (HF) donates one of it's H+ ions to a water molecule increasing the [H 3 O+](aq) Two scientists - Bronsted and Lowry, working independently, decided a more appropriate definition of an acid would be that of an H+ donor . Slide 24 / 123 Bronsted/Lowry Definition of Base At this time, most scientists explained Arrhenius bases as possessing OH- ions that would increase the [OH-] and decrease the [H3O+]. Arrhenius base in action NaOH(aq) --> Na+(aq) + OH-(aq) [OH-] causes [H 3 O+] 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 basic but did NOT have any hydroxide ions!

Slide 25 / 123 Bronsted Lowry Definition of Base 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! Bronsted base in action NH 3 (g) + H 2 O(l) --> NH 4 +(aq) + OH-(aq) When ammonia, NH 3 , accepts the H+ from the water, the water turns into OH- making the solution basic. Slide 26 / 123 Bronsted Lowry Definition of Acid and Bases Summary 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) lactic acid Bases are defined as H+ (proton) acceptors. CN-(aq) + H 2 O(l) --> HCN(aq) + OH-(aq) cyanide base Slide 27 / 123 11 A Bronsted acid is a substance that... A accepts H+ ions B donates OH- ions C increases the concentration of OH- ions D donates H+ ions E accepts OH- ions answer

Slide 28 / 123 12 Which of the following could NOT act as a Bronsted acid? A HCN B H 2 SO 4 C NH 4 + D H 3 O+ answer E BF 3 Slide 29 / 123 13 A Bronsted-Lowry base is defined as a substance that __________. A increases [H + ] when placed in H 2 O B decreases [H + ] when placed in H 2 O C increases [OH - ] when placed in H 2 O D acts as a proton acceptor answer E acts as a proton donor Slide 30 / 123 14 Which of the following compounds could never act as a Bronsted acid? A SO 4 2- answer B HSO 4 - C H 2 SO 4 D NH 3 E CH 3 COOH