E4 Acids, Bases, and Salts Session One of two session lab Complete Parts 1 and 2 in lab. If time allows, start or complete Part 3. Acids and Bases Q. Are acid-base properties of substances predictable from their chemical composition? Common consumer products Pre-lab report Pre-lab report, page 112, due at start of lab. Note pre-lab report corrections (see Ctools) ousehold acids ousehold bases Part 1. Structure and Acid-Base Properties Lab Determine if compounds labeled with the formula X c O a b are acids or bases by measuring the p X c O a b compounds and Acid-Base formulas The formulas for acids are written in the form b X c O a. Information Acids and Bases usually contain O,, and another element (X). Teams will be provided with compounds labeled with the formula X c O a b (X= some element; a, b, and c = some number) 2 CO 3 (carbonic acid) 3 PO 4 (phosphoric acid) 1
X c O a b compounds and Acid-Base formulas The formulas for bases are written in the form X c (O) n. X c O a b compounds and Acid-Base formulas Q. Given formulas of common acids and bases, what distinguishes acids from bases with regard to element X? Common acids: 2 CO 3, 3 PO 4, ClO, NO 3 Common bases: NaO, Mg(O) 2, Ca(O) 2 Answer: X = non-metal in acids; X = metal in bases. NaO Mg(O) 2 Ca(O) 2 N 4 O Part 1 Objectives p measurement Determine if the p of tested compounds is related to: The number of oxygen and/or hydrogen atoms in X c O a b compounds? The position of the element X in the Periodic Table The electronegativity of element X? Disccussion questions 1-3, page 125 p paper DEMO p meter 2
p 1 2 3 4 ACID = p < 7 Universal Indicator Demo Dry ice is added to water that contains a few drops of universal indicator to produce carbonic acid 5 6 Universal indicator 7 8 9 10 14 BASE = p > 7 Dry ice = CO 2 (s) Universal indicator 2 O (l) + CO 2 (s) 2 CO 3 (aq) Water dry ice carbonic acid Universal indicator changes colors when p changes; as the solution becomes more acidic, the p drops p scale? Why? On average, at 25 o C, only 1 in 10,000,000 2 O molecules ionize: p p = - log [ 3 O + ] Example: p 7 = - log [ 1 x 10-7 ] 1 10,000,000 = 1 x 10-7 M [ 3 O + ] = mole/liter units p values are unitless 3
p and [ 3 O + ] [ 3 O + ] p 10-1 1 10-2 3 DIGIT/S LEFT of the p decimal point J = POWER of TEN by which the hydronium concentration value is multiplied. If p = 3.27, [ 3 O + ] = [? ] x 10-3 10-4 4 10-5 5 10-6 [ 3 O + ] 10-3 2 10-7 6 10-8 7 10-9 8 10 10-10 9 10-14 14 p = 1 reflects a ten fold change in [ 3 O + ]; p = 2 reflects a hundred fold change in [ 3 O + ], etc. As [ 3 O + ] increases, p decreases: p [ 3 O + ] p 10-1 1 10-2 10-3 2 10-4 3 10-5 4 10-6 5 10-7 6 10-8 7 10-9 8 9 10-10 10 10-14 14 Check p data for errors Q. Your teammate indicates that 0.001 M NO 3 = p 2.0. You correctly respond, your p reading must be wrong! Why? Assume the acid ionizes completely: NO 3 + 2 O 3 O + + NO 3-0.001 M 0.001 M 0.001 M Answer: [ 3 O + ] cannot be more (more, less) than 0.001 M and p cannot be less (more, less) than p 3.0. [ [O - 3 O + ] p ] 10-1 1 10-13 -2 10 2 10-12 10-3 3 10-11 -4 10 4 10-10 10-5 5 10-9 -6 10 6 10-8 10-7 7 10-7 10-8 8 10-6 10-9 9 10-5 10-10 10 10-4 10-14 14 10-0 K w = [ 3 O + ] [O - ] = 1.0 x 10-14 4
Q. A sample of 0.1 M N 3 has a p = 11.0. What is [O - ]? Kw = [ 3 O + ] [O - ] = 1.0 x 10-14 [O - ] = Kw = 1.0 x 10-14 [ 3 O + ] 1.0 x 10-11 Answer: [O - ] = 1.0 x 10-3 M Bronsted Definition of Acids and Bases An acid is a proton ( + ) donor - forms hydronium ions. Cl- + O Acid Base + Cl - + O A base is a proton ( + ) acceptor - forms hydroxide ions. O + : N 3 N + 4 + O - Acid Base Acid-Base Ionization Acid-base ionization is solvent and concentration dependent. acid + + O anion + O base + water cation + O - DEMO: Addition of water to glacial acetic acid [ 3 O + ] p 10-1 10-2 1 10-3 2 10-4 3 10-5 4 10-6 5 10-7 6 10-8 7 10-9 8 9 10-10 10 10-14 14 Acid Strength and p Stronger acids donate protons/ionize to a greater extent than weak acids. Stronger acids exhibit a lower p than equimolar concentrations of weaker acids. DEMO: p of 0.10 M 3 BO 3 versus 0.10 M ClO 4 5
Acid-Base Strength and Structure Acid-base strength is dependent on structure. If in a compound is a bare proton it will be an electron pair seeker and readily bond to water molecules to form [ 3 O] + Example: ClO is a bare proton when bonded to an O atom, which in turn is attached to highly electronegative Cl in ClO ClO + O 3 O + + ClO - 100% ionization O Electronegativity of X vs. p of X c O a b compounds Electronegativity of the elements* 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 IA IIA IIIB IVB VB VIB VIIB VIIIB IB IIB IIIA IVA VA VIA VIIA Q. Based strictly on electronegativity values, predict the acid 2.1 strength of NO Li Be 3 versus C 2 3 O 2. B C N O F 1.0 1.5 Answer: NO 2.0 2.5 3.0 3.5 4.0 3 > C 2 3 O 2 Na Mg Al Si P S Cl 0.9 1.2 1.5 1.8 2.1 2.5 3.0 K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br 0.8 1.0 1.3 1.5 1.6 1.6 1.5 1.8 1.8 1.8 1.9 1.6 1.6 1.8 2.0 2.4 2.8 Rb Sr Y Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn Sb Te I 0.8 1.0 1.2 1.4 1.6 1.8 1.9 2.2 2.2 2.2 1.9 1.7 1.7 1.8 1.9 2.1 2.5 Cs Ba La f Ta W Re Os Ir Pt Au g Tl Pb Bi Po At 0.7 0.9 -Lu 1.3 1.5 1.7 1.9 2.2 2.2 2.2 2.4 1.9 1.8 1.8 1.9 2.0 2.2 Fr Ra Ac Th Pa U Np- N o 0.7 0.9 1.1 1.3 1.5 1.7 1.3 = Metalloids = Nonmetals = Metals Acid strength and structure Q. The electronegativity of N = 3.0 and C = 2.5. Predict the comparative rate of reaction of 5 M NO 3 and 5 M C 2 3 O 2 with zinc: Zn (s) + 2 + (aq) = Zn 2+ (aq) + 2(g) Rate: > NO 3 C 2 3 O 2 Acid-Base Strength and % Ionization % ionization acid = [ 3 O + ] i X 100% [acid] i % ionization base = [O - ] i X 100% [base] i Value in numerator determined from p DEMO 6
Acid-Base Strength and % Ionization % Ionization Q. 0.001 M ClO 4 = p 3.0; 0.010 M 2 CO 3 = p 3.0 1. Calculate the % ionization of ClO 4 and 2 CO 3. 2. Indicate if the acids are strong or weak. = 100% I m strong! You are weak! = < 100% % ionization ClO 4 % ionization 2 CO 3 [0.001] x 100% [0.001] = 100% [0.001] x 100% [0.010] = 10% Strength: ClO 4 is a strong acid; 2 CO 3 is a weak acid. Q. The textbook figure below depicting p and acid-base strength is misleading. ow? Part 2. Conjugate Acid-Base Pairs p Acid-base strength Acid-base strength cannot be determined based only on p p AND sample concentration must be known to determine strength Example 10-4 M ClO 4 (strong acid) = p 4 Conjugate acids and bases differ by one proton. - Conjugate acid of CO 3 - is 2 CO 3 - Conjugate base of CO 3 - is CO 3 2- Lab Compare the p of conjugate acids and bases. Teams will be provided with 0.10 M acids; conjugate bases will be provided as 0.10 M sodium salts 7
Conjugate Acid-Base Pairs Conjugate Acid-Base Pairs Exhibit predictable p relationships Canada Dry Canada Dry p 2 CO 3 (aq) versus CO 3 - (aq) versus CO 3 2- (aq)? p 2 CO 3 (aq) versus CO 3 - (aq) versus CO 3 2- (aq) DEMO Indicator p scale p increase ourly I Exam (Tuesday, March 11) Review Sessions Saturday, March 8, 11am - 1pm, 1210 chem. Monday, March 10, 6-8 pm, 1210 chemistry I m dissolving! Any Questions? Contact nkerner@umich.edu 8