Acid - Base Neutralization (Parts 3 - 5) E4 Acids, Bases, and Salts Neutralization: HCl(aq) + NH4OH(aq) → NH4Cl(aq) + HOH(l) Session two acid + base → salt + water . Parts 3, 4, and 5 . Complete the team report and discussion presentation. DEMO - proton transfer reaction: HCl(g) + NH3(g) → NH4Cl(s) Carboxylic Acids and Ionizable Protons Carboxylic Acids (Parts 3 - 5) Electronegativity of the elements* 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 VIIIB . Neutralization of carboxylic acids with NaOH IA IIA IIIB IVB VB VIB VIIB IB IIB IIIA IVA VA VIA VIIA H 2.1 Carboxylic acids Li Be B C N O F 1.0 1.5 2.0 2.5 3.0 3.5 4.0 Carboxylic acids contain the group 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 Hf Ta W Re Os Ir Pt Au Hg Tl Pb Bi Po At .0 .T7 h0e. 9e le-cLtur o1n.3e ga1.t5iv i1t.y7 o1f .H9 a2n.2d 2O.2 d i2f.f2e r2 a.4n d1 .t9h e1r.e8f o1r.e8 H1 .i9n t2h.0e 2.2 CFOr ORHa gArco upT ho f Pca rbUo xyNlipc- a Nciod s is io niza ble. 0.7 0.9 1.1 1.3 1.5 1.7 1.3 . The electronegativity of H and C are similar and therefore C-H b ond=s Me atalrelo isdst ab le and= Ntohnmee tpalrs oto n in =t hMeet aCls -H b ond is NO T ioniz abl e 1 Carboxylic acids Acid-Base Reaction Stoichiometry • H O Part 3: Acetic acid . Reaction stoichiometry is dependant on the number of ionizable • | || HC2H3O2 protons. • H− C − C − O − H CH3COOH Acetic acid (Part 3): • | Carboxylic group containing CH3COOH + NaOH → CH3COONa + HOH one ionizable proton • H 1 mol 1 mol • O O Part 4: Oxalic acid Oxalic acid (Part 4): • || || H2C2O4 HOOC-COOH + 2 NaOH → 2COONa + 2HOH 1 mol 2 mol •H − O− C − C − O − H HOOCCOOH Two carboxylic groups each with one ionizable proton Acid-Base Neutralization Equilibria Titration (Parts 3 - 5) • Adding base to the weak (partially ionized) carboxylic . A procedure for determining the concentration acid drives the reaction (donation of proton/s) to completion of a measured volume of acid (or base) by measuring the volume of base (or acid) of known + - • A Ct nHe3uCtrOalOizHat i o n , o n →ly the Hpr odu+c t s C(sHalt3 CanOdO water) concentration required for neutralization are present. + - . A procedure for determining the identity CH3COOH + NaOH O HCH→3C O HOONHa + HOH of a measured mass of acid (or base) by measuring the volume of base (or acid) of known concentration required for neutralization 2 Titration Q. Identify the acid (acetic or oxalic) if 10.0 mL of 0.10 M NaOH neutralizes 10.0 mL of 0.10 M acid. Acetic acid CH3COOH + NaOH → CH3COONa + HOH Oxalic acid HOOC-COOH + 2 NaOH → 2COONa + 2HOH Answer. Acetic acid. mmol acid = mmol base 10.0 mL • 0.10 M = 10.0 mL • 0.10 M note: 5.0 mL of 0.10 M oxalic acid neutralizes 10.0 mL of 0.10 M NaOH: Base delivered Indicator color change 2 (mmol acid) = mmol base from burette signals the end-point 2 (5.0 mL • 0.10 M) = 10.0 mL • 0.10 M DEMO or neutralization Part 3 Acid-Base Neutralizations and Indicators Indicator End Point pH . Compare the use of three indicators for monitoring the neutralization of acetic acid with sodium hydroxide . The pH interval at which the indicator changes color • Bromothymol blue and bromophenol blue Acids are colorless Bases are pink Acids turn yellow Bases turn blue Phenolphthalein: pH 8.2 - pH 10.1 ↑ • Phenolphthalein pH 9.1 Acids are colorless Bases turn pink . The indicator color change is observable at the mid-point of the end point pH range. 3 Indicator pH End Points Equivalence Point pH . The pH of the salt and water products of the acid-base neutralization reaction = the equivalence point pH. CH3COOH + NaOH → CH3COONa + HOH Equivalence point pH ≈ pH 8 “Ideally, the indicator end point pH equals the equivalence point pH = pH 8. “ DEMO . Indicators have different end-point pH color changes . For information on lab indicators, see the lab manual, p. 183 Titration and Indicator pH End Points Titration of Oxalic Acid (Part 4) Q. You titrate 0.4502 g of oxalic acid with 1.0 M NaOH. How many mL of 1.0 M NaOH are required for neutralization? Oxalic acid: MW = 90.04 HOOC-COOH + 2 NaOH → 2COONa + 2HOH 0.5 mol 1 mol 45.02 g 1 mol (or 1000 mmol) Answer. Fact: 0.4502 g acid will neutralize 10 mmol of NaOH. X mL • 1.0 M NaOH = 10 mmol NaOH X = 10 mL of 1.0 M NaOH 4 See Table, p.110 Part 5. Identification of an Unknown Acid Name Formula MW EW MP 2-hydroxybenzoic 180 180 135 • Identify a carboxylic acid from melting point and COOH titration data acid acetate OCOCH3 • The melting point range will be determined using a melting point apparatus trans-cinnamic acid 148 148 135- CH CH COOH • The equivalent weight will be determined by titrating a 136 known mass of the acid with NaOH 2-chlorobenzoic acid Cl 157 157 140 COOH COOH cis-butenedioic HOOC 116 58 139- H (maleic) acid H 140 Equivalent Weight (EW) Unknowns and Carboxylic Acid Structure Theoretical • Benzoic acid • COOH . EW = Molecular Wt/# ionizable protons • • • . One ionizable proton. Examples: H SO = molecular weight of 98 2 4 Benzoic acid = C6 H5COOH H2SO4 = equivalent weight of 49 HCl = molecular weight of 36.5 MW = EW HCl = equivalent weight of 36.5 . No ionizable protons. Not an acid. 5 Q. An acid has an equivalent weight (EW) of 88.92. The EW = molecular wt/#ionizable protons. List the number of any compound from the table that may be the acid. • O H H H H O Adipic acid • | | | | | | | | HOOC(CH2 )4COOH H - O − C − C − C − C − C − C − O − H Compound Formula MW • | | | | 1. Ethanoic acid CH3COOH 60 • H H H H .Two ionizable protons. 2. Butanoic acid CH3CH2CH2COOH 88 3. Oxalic acid HOOC-COOH 90 Molecular Weight = 146 4. 1-methyl, 1,2 benzene CH3C6H3(COOH)2 180 Equivalent weight = ______7_3______? dicarboxylic acid Answer. Compounds # 2 and # 4. A teammate titrates a carboxylic acid and calculates the EW from Experiment Data (Part 5) EW of the acid to be 72.50. The result is correct within 2%. The MW of the acid is determined to be 1.5 x 102 . Determine the equivalent weight of an unidentified A. How many ionizable protons/mol are in the acid?that carboxylic acid by determining the volume of base of • known concentration required to neutralize a measured Let X = # ionizable protons. mass of the unknown acid EW = 72.50 = MW/X = 150/X X= ~ 2 B. Circle each structure that is consistent with the data. • EW = Mass (g) of acid neutralizing 1 mol OH-. Adipic Acid, MW = 146: Trans-cinnamic acid, MW=148 HO2CCH2CH2CH2CH2CO2H -CH=CH-COOH EW = mass acid (g) moles of OH- neutralized Citric Acid, MW = 192: Oxalic acid, MW = 90 HO2C-CH2-C(OH)-CH2-CO2H HO2C—CO2H CO2H 6 Q. You titrate 0.175 g of an acid. 30.00 mL of 0.10 M EW of Oxalic Acid (Part 4) NaOH neutralizes the sample. 1) What is the acid’s EW based on the data? Q. 10.0 mL of 1.0 M NaOH neutralizes 0.4502 grams of 2) Identify the acid from the list below. oxalic acid, what is the equivalent weight of the acid? Name Formula MW MP Oxalic acid: MW = 90.04 2-hydroxybenzoic acid acetate COOH 180 135 HOOC-COOH + 2 NaOH → 2COONa + 2HOH OCOCH3 0.5 mol 1 mol trans-cinnamic acid 148 135-136 CH CH COOH 45.02 g 1 mol (or 1000 mmol) 2-chlorobenzoic acid Cl 157 140 COOH Answer. 0.4502 g neutralized 10.0 mmol of NaOH. cis-butenedioic (maleic) acid H H 116 139-140 The mass of acid needed to neutralize 1 mol = 45.02g = EW HOOC COOH Equivalent weight determination. Equivalent Weight determination. 1) Determine mol of NaOH the acid has neutralized. 2) Determine the mass of acid that would Known: The acid mass neutralized 30.00 mL of 0.10 M NaOH neutralize one mol of hydroxide ions. - mol OH = V(L) x M NaOH - Known: 0.175 g = 0.0030 mol OH - X g acid 1 mol OH = 0.0300L x 0.10 mol L X = 58 g = 3.0 x 10-3 mol 7 Q. What is the identity of the acid? Name Formula MW MP 2-hydroxybenzoic acid acetate COOH 180 135 OCOCH3 trans-cinnamic acid 148 135-136 CH CH COOH 2-chlorobenzoic acid Cl 157 140 COOH Questions? cis-butenedioic (maleic) acid √ H H 116 139-140 Contact [email protected] HOOC COOH 8.