ph = - log [H 3 O + ]

Transcription

1 E4 Acids, Bases, and Salts March 5-10 Session One of two session lab Complete Parts 1 and 2 in lab. If time allows, start or complete Part 3. 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 X c O a b (X= some element; a, b, and c = some number) Determine if compounds with the formula X c O a b are s or bases by measuring the p p p = - log [ 3 O + ] Example: p 7 = - log [ 1 x 10-7 ] 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 + ] = mole/liter units p values are unitless [ 3 O + ] p 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 Universal indicator p ACID = p < 7 BASE = p > 7 1

2 p measurement p paper p indicator (e.g., bromothymol blue) Acid or Base? p meter Part 1 Data analysis XcOab compounds and Acid-Base formulas Based on the p, indicate if each compound is an or base. LiO 0.01 M p 1 BO C2O24 NO3 ID of (Cl) versus base (N 4O) with: bromothymol blue, phenolphthalein, and p paper. The formulas for s are written in the form bxcoa. Acid/base base 2CO3 (carbonic ) 3PO4 (phosphoric ) Re-write the formula to reflect the or base p XcOab compounds and Acid-Base formulas The formulas for bases are written as Xc(O)n. Rewrite the formulas to reflect if each compound is an or base 0.01 M LiO BO33 C2O24 NO3 NaO Mg(O)2 Ca(O)2 p Acid/base base LiO 3BO3 C23O2 NO3 N4O 2

3 Bronsted Definition of Acids and Bases An is a proton ( + ) donor - forms hydronium ions. Cl- + O + Cl - + O A base is a proton ( + ) acceptor - forms hydroxide ions. O + : N 3 N O - Lewis Definition of Acids and Bases An is an electron pair seeker. Cl- + O A base is an electron pair donor + Cl - + O O + : N 3 N O - Acid-Base Ionization The production of hydronium (or hydroxide ions) is solvent and concentration dependent. + + O anion : Addition of water to C 2 3 O 2 + O Are the p and the production of hydronium (or hydroxide) ions linked to the structure of the (or base)? The number of oxygen and/or hydrogen atoms in X c O a b compounds? The position of element X in the Periodic Table The electronegativity of element X? Example: Are /base characteristics/p dependent on: - the number of atoms? 0.01M NaO 12 Base 1 ClO 4 2 Acid 1 Mg(O) 2 11 Base 2 C 2 3 O 2 Acid 4 Answer: NO. p Acid/Base # atoms [ 3 O + ] p Acid Strength and p Stronger s donate protons/ionize to a greater extent than weak s. Stronger s exhibit a lower p than equimolar concentrations of weaker s. Example M NO 3 = p M COO = p 2.8 Strength: NO 3 > COO 3

4 p Comparisons of Different Acids Based on the p data (below), indicate the strength of boric, acetic, and nitric M Boric, 3BO3 p 5.6 Acetic,C23O2 Nitric,NO3 Nitric Acetic Boric Strength: > > p of 0.10 M acetic, nitric, and boric Acid Strength Strength and Reactivity There is a correlation between (and base) strength and use in everyday life Nitric > Acetic Strength: Rate of reaction of 5 NO3 and 5 M C23O2 with zinc: Zn (s) + 2+ (aq) = Zn 2+ (aq) + 2(g) Boric > Electronegativity and Strength Electronegativity of the elements* VIIIB IA IIA IIIB IVB VB VIB VIIB IB Acid-Base Strength and Structure IIB IIIA IVA VA VIA VIIA Strength: NO3 > C23O2 > 3BO3 Li Be 1. 0 B C N O F Na 0.9 K 0.8 Rb 0.8 Cs 0.7 Fr 0.7 Al Ga In Tl Mg 1.2 Ca 1.0 Sr 1.0 Ba 0.9 Ra 0.9 Sc Y 1.2 La -Lu Ac 1.1 = Metalloids Ti Zr 1.4 f Th V Nb Ta Pa Cr Mo W U Mn Tc Re Np- = Nonmetals Fe Ru Os No Co Rh Ir Ni Pd Pt Cu Ag Au 2.4 Zn Cd g Si Ge Sn Pb P 2.1 As Sb Bi S Se 2.4 Te 2.1 Po Cl 3.0 Br 2.8 I At Example: ClO is bonded to an O atom, which in turn is bonded to the highly electronegative Cl O is a bare proton and an electron pair seeker that readily bonds to a water molecule to form 3O+ ClO + O 3O+ + ClO100% ionization = Metals 4

5 Acid-Base Strength and % Ionization % Ionization Acid-Base Strength and % Ionization % ionization = [ 3O + ] X 100% [] i = 100% I m strong! You are weak! = < 100% % ionization base = [O- ] X 100% [base] i Value in numerator determined from p Q.Your team determines that p M NO 3 = p. What is its % ionization? [ 3 O + ] X 100% [] i = % ionization NO 3 [ 0.01? ] x 100% = % 1000 [0.001] % ionization cannot be greater than 100%; there is likely an error in the p or concentration. [ 3 O + ] p Check p data for errors Q. Your team determines that M NO 3 = p. You correctly respond, your p reading must be wrong! Why? If ionizes completely: NO O 3 O + + NO M M M Answer: [ 3 O + ] cannot be more (more, less) than M and p cannot be less (more, less) than p 3.0. Q. The textbook figure below depicting p and -base strength is misleading. ow? p Acid-base strength Acid-base strength cannot be determined based only on p p AND sample concentration must be known Example 10-4 M ClO 4 (strong ) = p 4 Part 2. Conjugate Acid-Base Pairs Conjugate s and bases differ by one proton. - Conjugate of CO 3 - is 2 CO 3 - Conjugate base of CO 3 - is CO 3 2- Lab Compare the p of conjugate s (0.10 M s) and conjugate bases (0.10 M sodium salts.) 5

6 Conjugate Acid-Base Pairs Conjugate Acid-Base Pairs Exhibit predictable p relationships Dry ada Can p 2CO3(aq) versus CO3- (aq) versus CO32- (aq)? Conjugate Acid-Base Pairs Titration (Parts 3-5) Dry ada Can p 2CO3(aq) versus CO3- (aq) versus CO32- (aq) p increase Base delivered from burette Indicator color change signals the end-point or neutralization Q. ow many ml of 0.10 M NaO will be required to neutralize 5.0 ml of 0.10 M oxalic? Oxalic OOC-COO + 2 NaO 2COONa + 2O 2 mol of NaO is required to neutralize one mol of oxalic : 2 (mmol ) = mmol base 2 (5.0 ml 0.10 M) = x ml 0.10 M base 1 mmol = x ml 0.10 M base = 10 ml of 0.10 M NaO Questions? Contact nkerner@umich.edu 6