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: Pre-lab report, page 112, due at start of lab. Acids and Bases Q. Are acid-base properties of substances predictable based on their chemical composition? Common consumer products ousehold acids ousehold bases Part 1. Structure and Acid-Base 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) Lab Determine if compounds labeled with the formula XcOab are acids or bases by measuring the p 1
XcOab compounds and Acid-Base formulas The formulas for acids are written in the form bxcoa. 2CO3 (carbonic acid) 3PO4 (phosphoric acid) XcOab compounds and Acid-Base formulas The formulas for bases are written in the form Xc(O)n. NaO Mg(O)2 Ca(O)2 N4O XcOab compounds and Acid-Base formulas 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, N4O Answer: 2
Part 1 Objectives Based on p data and the chemical composition of tested compounds, determine if acid-base properties can be predicted from: - the number of oxygen and/or hydrogen atoms in X c O a b compounds? - the electronegativity of element X? Disccussion questions 1-3, page 125 p measurement p paper DEMO p meter Universal indicator p 1 2 3 4 5 6 7 8 9 10 14 ACID = p < 7 BASE = p > 7 3
Universal Indicator Demo Dry ice is added to water that contains a few drops of universal indicator to produce carbonic acid 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: 1 10,000,000 = 1 x 10-7 M p p = - log [ 3 O + ] Example: p 7 = - log [ 1 x 10-7 ] [ 3 O + ] = mole/liter units p values are unitless 4
p and [ 3 O + ] 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 [ 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 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: [ 3 O + ] 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 5
[ [O - 3 O + ] p ] 10-1 1 10-13 10-2 2 10-12 10-3 3 10-11 10-4 4 10-10 10-5 5 10-9 10-6 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 Q. A sample of 0.1 M N 3 has a p = 11.0. What is [O - ]? Answer: 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 6
Acid-Base Ionization and Concentration 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 Acid-Base Strength and % Ionization % Ionization = 100% = < 100% I m strong! You are weak! 7
Acid-Base Strength and % Ionization % 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 Strength: Q. The textbook figure below depicting p and acid-base strength is misleading. ow? p Acid-base strength 8
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 is a bare proton when bonded to an O atom, which in turn is attached to highly electronegative Cl in ClO O Bond polarity is a dominant factor in determining relative strength of the oxyacids. 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: > 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) Rate: > DEMO 9
Acid Strength YIKES! This metal pan is reacting! Maybe something is in the tomato sauce? I m dissolving! Part 2. Conjugate Acid-Base 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 Acid-Base Pairs Exhibit predictable p relationships Canada Dry p 2 CO 3 (aq) versus CO 3 - (aq) versus CO 3 2- (aq)? DEMO Indicator p scale 10
Conjugate Acid-Base Pairs Canada Dry p 2 CO 3 (aq) versus CO 3 - (aq) versus CO 3 2- (aq) p (increase or decrease)? Any Questions? Contact nkerner@umich.edu 11