HCl(aq) + NH4OH(aq) NH4Cl(aq) + HOH(l)

Transcription

1 Part 3. Neutralization and Indicators. E4 Acids, Bases, and Salts Acid two Session Study the use of indicators as monitors for an acid-base neutralization reaction. Neutralization: HCl(aq) + NH4OH(aq) NH4Cl(aq) + HOH(l) + base + water acid salt Parts 3, 4, and 5 Complete the team report and discussion presentation. DEMO - proton transfer reaction: HCl(g) + NH3(g) NH4Cl(s) Carboxylic Acids (Parts 3-5) Monitor the neutralization of carboxylic acids with NaOH Carboxylic acids Carboxylic acids contain the group Carboxylic Acids and Ionizable Protons Electronegativity of the elements* IA IIA IIIB IVB VB VIB VIIB 9 VIIIB IB IIB IIIA IVA VA VIA VIIA H 2.1 Li Be B C N O F Na Mg Al Si P S Cl K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Rb Sr Y Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn Sb Te I Cs Ba La Hf Ta W Re Os Ir Pt Au Hg Tl Pb Bi Po At 0.7 The0.9electronegativity H and O differ and1.9 therefore in the -Lu of H carboxylic Fr Ra group Ac Thof Pa U Np-acids N o is ionizable of H and 1.3 C are similar and therefore C-H 0.7 The0.9 electronegativity = Metalloids Metals bonds are stable and= Nonmetals the proton in =the C-H bond is NOT ionizable 1

2 Carboxylic acids (Parts 3 and 4) H O Part 3: Acetic acid HC 2 H 3 O 2 H C C O H CH 3 H Carboxylic group containing one ionizable proton O O Part 4: Oxalic acid H 2 C 2 O 4 H O C C O H HOOC Two carboxylic groups each with one ionizable proton Acid-Base Neutralization Equilibria Adding base to the weak (partially ionized) carboxylic acid drives the reaction (donation of proton/s) to completion At CH neutralization, 3 only the H products + + CH (salt 3 COO and water) - are present. + CH OH NaOH CH 3 COONa HOH + HOH Acid-Base Reaction Stoichiometry Reaction stoichiometry is dependant on the number of ionizable protons. Acetic acid (Part 3): CH 3 + NaOH CH 3 COONa + HOH 1 mol 1 mol Oxalic acid (Part 4): HOOC- + 2 NaOH 2COONa + 2HOH 1 mol 2 mol Titration (Parts 3-5) A procedure for determining the concentration of a measured volume of acid (or base) by measuring the volume of base (or acid) of known concentration required for neutralization A procedure for determining the identity of a measured mass of acid (or base) by measuring the volume of base (or acid) of known concentration required for neutralization 2

3 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 CH 3 + NaOH CH 3 COONa + HOH Oxalic acid HOOC- + 2 NaOH 2COONa Answer. Acetic acid. + 2HOH mmol acid = mmol base 10.0 ml 0.10 M = 10.0 ml 0.10 M DEMO Base delivered from burette Indicator color change signals the end-point or neutralization note: 5.0 ml of 0.10 M oxalic acid neutralizes 10.0 ml of 0.10 M NaOH: 2 (mmol acid) = mmol base 2 (5.0 ml 0.10 M) = 10.0 ml 0.10 M Bromothymol blue and bromophenol blue Acids turn yellow Phenolphthalein Acids are colorless Part 3 Indicators Compare the use of three indicators for monitoring the neutralization of acetic acid with sodium hydroxide Bases turn blue Bases turn pink Indicator End Point ph The ph interval at which the indicator changes color Acids are colorless Bases are pink Phenolphthalein: ph ph 10.1 ph 9.1 The indicator color change is observable at the mid-point of the end point ph range. 3

4 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. CH 3 + NaOH CH 3COONa + HOH Equivalence point ph ph 8 Ideally, the indicator end point ph equals the equivalence point ph = ph 8. Indicators have different end-point ph color changes DEMO 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 g of oxalic acid with 1.0 M NaOH. How many ml of 1.0 M NaOH are required for neutralization? Oxalic acid: MW = HOOC- + 2 NaOH 2COONa + 2HOH 0.5 mol 1 mol g 1 mol (or 1000 mmol) Answer. Fact: 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

5 Part 5. Identification of an Unknown Acid Identify a carboxylic acid from melting point and titration data See Table, p.110 Name 2-hydroxybenzoic acid acetate Formula OCOCH 3 MW 180 EW 180 MP 135 The melting point range will be determined using a melting point apparatus The equivalent weight will be determined by titrating a known mass of the unknown acid with NaOH trans-cinnamic acid CH CH chlorobenzoic acid Cl cis-butenedioic (maleic) acid H HOOC H Equivalent Weight (EW) Theoretical EW = Molecular Wt/# ionizable protons Examples: H 2 SO 4 = molecular weight of 98 H 2 SO 4 = equivalent weight of 49 HCl = molecular weight of 36.5 HCl = equivalent weight of 36.5 Unknowns and Carboxylic Acid Structure Benzoic acid One ionizable proton. No ionizable protons. Not an acid. Benzoic acid = C 6 H 5 MW = = EW 5

6 O H H H H O Adipic acid HOOC(CH 2 ) 4 H - O C C C C C C O H H H H H Two ionizable protons. Molecular Weight = 146 Equivalent weight =? 73 Q. An acid has an equivalent weight (EW) of The EW = molecular wt/#ionizable protons. List the number of any compound from the table that may be the acid. Compound Formula MW 1. Ethanoic acid CH Butanoic acid CH3CH2CH Oxalic acid HOOC methyl, 1,2 benzene dicarboxylic acid CH3C 6 H3()2 180 Answer. Compounds # 2 and # 4. A teammate titrates a carboxylic acid and calculates the EW of the acid to be The result is correct within 2%. The MW of the acid is determined to be 1.5 x 10 2 A. How many ionizable protons/mol are in the acid?that Let X = # ionizable protons. EW = = MW/X = 150/X X= ~ 2 B. Circle each structure that is consistent with the data. EW from Experiment Data (Part 5) Determine the equivalent weight of an unidentified carboxylic acid by determining the volume of base of known concentration required to neutralize a measured mass of the unknown acid EW = Mass (g) of acid neutralizing 1 mol OH -. Adipic Acid, MW = 146: HO 2 CCH 2 CH 2 CH 2 CH 2 CO 2 H Trans-cinnamic acid, MW=148 -CH=CH- Citric Acid, MW = 192: Oxalic acid, MW = 90 HO 2 C-CH 2 -C(OH)-CH 2 -CO 2 H HO 2 C CO 2 H CO 2 H EW = mass acid (g) moles of OH - neutralized 6

7 EW of Oxalic Acid (Part 4) Q ml of 1.0 M NaOH neutralizes grams of oxalic acid, what is the equivalent weight of the acid? Oxalic acid: MW = HOOC- + 2 NaOH 2COONa + 2HOH 0.5 mol 1 mol g 1 mol (or 1000 mmol) Answer g neutralized 10.0 mmol of NaOH. The mass of acid needed to neutralize 1 mol = 45.02g = EW Q. You titrate g of an acid ml of 0.10 M NaOH neutralizes the sample. 1) What is the acid s EW based on the data? 2) Identify the acid from the list below. Name Formula MW MP 2-hydroxybenzoic acid acetate OCOCH 3 trans-cinnamic acid chlorobenzoic acid CH cis-butenedioic (maleic) acid H H HOOC Cl CH Equivalent weight determination. 1) Determine mol of NaOH the acid has neutralized. Known: The acid mass neutralized ml of 0.10 M NaOH mol OH - = V(L) x M NaOH = L x 0.10 mol L = 3.0 x 10-3 mol Equivalent Weight determination. 2) Determine the mass of acid that would neutralize one mol of hydroxide ions. Known: g = mol OH - X g acid 1 mol OH - X = 58 g 7

8 Q. What is the identity of the acid? Name Formula MW MP 2-hydroxybenzoic acid acetate OCOCH 3 trans-cinnamic acid CH CH 2-chlorobenzoic acid Cl cis-butenedioic (maleic) acid H H HOOC Questions? Contact nkerner@umich.edu 8