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. Reminder: Prelab report, page 112, due at start of lab. Acids and Bases Q. Are acidbase properties of substances predictable based on their chemical composition? Common consumer products ousehold acids Part 1. Structure and AcidBase Properties Information Acids and Bases usually contain O,, and another element (X). Teams will be provided with compounds labeled with the formula XcOab (X= some element; a, b, and c = some number) ousehold bases XcOab compounds and AcidBase formulas The formulas for acids are written in the form bxcoa. Lab Determine if compounds labeled with the formula XcOab are acids or bases by measuring the p 2CO3 (carbonic acid) XcOab compounds and AcidBase formulas XcOab compounds and AcidBase formulas The formulas for bases are written in the form Xc(O)n. 3PO4 (phosphoric acid) Q. Acid and base compounds typically contain oxygen, hydrogen, and another element (X). Based on formulas of common acids and bases, what distinguishes acids from bases with regard to element X? Common acids: 2CO3, 3PO4, ClO, NO 3 Common bases: NaO, Mg(O)2, Ca(O) 2 X = nonmetal in acids; X = metal in bases. Answer: NaO Mg(O)2 Ca(O)2 N4O 1
Part 1 Objectives p measurement Based on p data and the chemical composition of tested compounds, determine if acidbase properties can be predicted from: the number of oxygen and/or hydrogen atoms in XcOab compounds? the electronegativity of element X? Disccussion questions 13, page 125 p paper p p meter DEMO Universal Indicator Demo 1 2 3 ACID = p < 7 Dry ice is added to water that contains a few drops of universal indicator to produce carbonic acid 4 5 6 7 Dry ice = CO 2(s) 8 2O (l) Water 9 10 Universal indicator BASE = p > 7 14 p scale? Why? On average, at 25oC, only 1 in 10,000,000 2O molecules ionize: + Universal indicator CO 2 (s) dry ice 2CO3 (aq) carbonic acid Universal indicator changes colors when p changes; as the solution becomes more acidic, the p drops p p = log [3O+] Example: p 7 = log [ 1 x 107] 10,000,000 = 1 1 x 10 7 M [3O+] = mole/liter units p values are unitless 2
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 1013 10 2 2 10 12 10 3 3 1011 10 4 4 10 10 10 5 5 109 10 6 6 10 8 10 7 7 10 7 10 8 8 10 6 10 9 9 105 10 10 10 10 4 10 14 14 10 0 K w = [ 3 O + ] [O ] = 1.0 x 10 14 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 Acids and Bases An acid is a proton ( + ) donor forms hydronium ions. Cl + Acid O Base + Cl + O A base is a proton ( + ) acceptor forms hydroxide ions. O + : N 3 N + 4 + O Acid Base 3
AcidBase Ionization Acidbase 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 AcidBase Strength and % Ionization % Ionization AcidBase Strength and % Ionization = 100% I m strong! You are weak! = < 100% % ionization acid = [ 3O + ] i X 100% [acid] i % ionization base = [O ] i X 100% [base] i Value in numerator determined from p 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. % ionization ClO 4 % ionization 2 CO 3 [0.001] x 100% [0.001] = 100% [0.001] x 100% [0.010] = 10% Q. The textbook figure below depicting p and acidbase strength is misleading. ow? p Acidbase strength Acidbase 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 Strength: ClO 4 is a strong acid; 2 CO 3 is a weak acid. 4
AcidBase Strength and Structure Acidbase 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 is a bare proton when bonded to an O atom, which in turn is attached to highly electronegative Cl in ClO Bond polarity is a dominant factor in determining relative strength of the oxyacids. 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 BrO versus IO. Li Be B C N O F 1.0 1.5 Answer: BrO > IO 2.0 2.5 3.0 3.5 4.0 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 No 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) Acid Strength YIKES! This metal pan is reacting! Maybe something is in the tomato sauce? Rate: > NO 3 C 2 3 O 2 I m dissolving! DEMO Part 2. Conjugate AcidBase Pairs Lab Determine and compare the p of conjugate acids and bases. Information Conjugate acids and bases differ by one proton. Conjugate acid of CO 3 is 2 CO 3 Conjugate base of CO 3 is CO 2 3 Teams will be provided with 0.10 M acids; conjugate bases will be provided as 0.10 M sodium salts Conjugate AcidBase Pairs Exhibit predictable p relationships p 2 CO 3 (aq) versus CO 3 (aq) versus CO 3 2 (aq)? DEMO Canada Dry Indicator p scale 5
Conjugate AcidBase Pairs ourly I Exam (Tuesday, November 6) Canada Dry p 2 CO 3 (aq) versus CO 3 (aq) versus CO 3 2 (aq) Review Sessions Saturday, November 3, 11am 1pm, 1210 chem. Monday, November 5, 6 8 pm, Angell all, Auditorium B I m dissolving! p increase Any Questions? Contact nkerner@umich.edu 6