14 Principles of Neutralization Titrations

Transcription

1 14 Principles of Neutralization Titrations 14A. Solutions and Indicators for Acids/Base Titrations 14A1 Standard Solution Strong acids: Hl, HlO 4, H 2 SO 4. Strong bases: NaOH, OH. 14A2 Acid/Base Indicator HO O O OH O O O H 3 O OH O colorless red Fig. 141 olor change for phenolphthalein. HIn H 2 O In H 3 O acid color base color In H 2 O InH OH base color acid color [H O ][In ] 3 a [HIn] [HIn] [H3 O ] a [In ] [HIn] [HIn] acid color, [H 3 O ] a [In ] 1 [HIn] 1 [In ] base color, [H 3 O ] 0.1a [In ] ph ph(acid color) log(a) pa 1 ph(basic color) log(0.1a) pa 1 indicator ph range pa ± 1 Fig. 142 Indicator color as a function of ph (p a 5.0). *Variables That Influence the Behavior of Indicator temperature, ionic strength, and the presence of organic solvents and colloidal particles *The ommon Acid/Base Indicator: Table

2 Table 141 Some Important Acid/Base Indicators ommon Name Transition olor Indicator pa* Range, ph hange Type# Thymol blue R Y Y B ethyl yellow R Y 2 ethyl orange R O 2 Bromocresol green Y B 1 ethyl red R Y 2 Bromocresol purple Y P 1 Bromothymol blue Y B 1 Phenol red Y R 1 resol purple Y P 1 Phenolphthalein R 1 Thymolphthalein B 1 Alizarin yellow GG 12 Y 2 * At ionic strength of 0.1 B blue; colorless; O orange; P purple; R red; Y yellow. # (1) Acid type: HIn H 2 O H 3 O In # (2) Base Type: In H 2 O InH For the reaction HIn H 2 O H 3 O In B. Titration of Strong Acids and Strong Bases H 3 O in an aqueous solution of a strong acid have two sources: (1) the reaction of the acid with water (2) the dissociation of water 14B1 Titration of a Strong Acid with a Strong base (1) preequivalence: [unreact acid] (2) equivalence: ph 7.00 (3) postequivalence: [excess base]: ph pw poh poh Ex. 141 Derive a titration curve for the titration of ml of Hl with 0.00 NaOH. Initial point: ph log [H 3 O ] log After addition of.00 ml of reagent (50.0mL ) (.00mL 0.00) Hl (50.0.0) ml ( 2.500mmol 1.000mmol) mL [H 3 O ] and ph log

3 Equivalence point [H 3 O ] w ph log After addition of 25. ml of reagent 1.00 ( ) ( ) NaOH [OH ] NaOH , poh log ph Table 142 hanges in ph during the titration of a strong acid with a strong base ph Volume of NaOH, ml ml of Hl with 0.00 NaOH ml of Hl with NaOH *The Effect of oncentration: * hoosing an Indicator: Table 142 and Fig. 143 Fig. 144 Titration curves for Hl with NaOH. urve A ( ): ml of Hl with 0.00 NaOH. urve B (): ml of Hl with NaOH. Volume of NaOH, ml 87

4 14B2. Titrating a Strong Base with a Strong Acid 142 alculate the ph during the titration of ml of NaOH with 0.00 Hl after the addition of the following volumes of reagent: (a) ml, (b) ml, (c) ml. (a)at ml added ( ) ( ) [OH ] NaOH ( ) poh log( ) 3.17, ph (b) Equivalence point: [H 3 O ][OH ] 6.71 [H 3 O 14 7 ] w ; ph 7.00 (c) At ml added [H ( ) ( ) ph log ( ) O ] Hl 6.62 Volume of Hl, ml 14. Titration urves for Weak Acids Fig Titration curves for NaOH with Hl. urve A (): ml of NaOH with 0.00 Hl. urve B ( ): ml of NaOH with Hl. Ex. 143 Generate a curve for the titration of ml of 0.00 acetic acid (a ) with 0.00 NaOH. Initial ph [H 3 O ] a HOAc ph log ( ) ph After Addition of 5.00 ml of reagent buffer solution consisting of NaOAc and HOAc

5 HOAc NaOAc /55.00 [H3O ] ph /55.00 Equivalence Point ph OAc H 2 O HOAc OH [OH ] [HOAc], [OAc ] [OH ] [ OH ] 1.00 w a 1.75 [OH ] ph (log ) 8.73 ph After Addition of 50. ml of Base [OH ] NaOH ph (log ) 9.00 Table 143 hanges in ph during the titration of a weak acid with a strong base Volume of NaOH, ml ml of 0.00 HOAc with 0.00 NaOH The Effect of oncentration: Fig. 145 ph 2. The Effect of Reaction ompleteness: Fig ml of HOAc with NaOH

6 3. hoosing an indicator; the Feasibility of Titration: Fig. 145 & 6. Transition range (TR) Phph. TR BTB TR BG TR Volume of NaOH, ml Fig. 145 urve for the titration of acetic acid with NaOH. urve A: 0.00 acidbase. urve B: acidbase. Volume of NaOH, ml Fig. 146 The effect of acid strength (dissociation constant, a ) on titration curves. Each curve represents the titration of ml of 0.00 acidbase. 14D. Titration urves for Weak Bases Ex. 144 A 50.00mL aliquot of NaN is titrated with 0.00 Hl. The reaction is N H 3 O HN H 2 O alculate the ph after the addition of (a) 0.00, (b).00, (c) and (d) ml of acid. (a) Initial ph N H 2 O HN OH 14 [OH ][HN] 1.00 b w 1.61 [N ] 6.2 [OH ] [HN], [OH ] b NaN a [N ] NaN [OH ] ph (log ).95 (b).00 ml of reagent NaN HN

7 1.000/60.00 [H3O ] /60.00 ph log (4.13 ) 9.38 (c) ml of reagent HN [H 3 O ] 6 a HN ph log ( ) 5.34 (d) ml of reagent [H3O ] Hl ph log ( ) 2.88 Transition range (TR) Ph ph. TR BTB TR BG TR Fig. 147 The effect of base strength ( b ) on titration curves. Each curve represents the titration of ml of 0.00 base with 0.00 Hl. Volume of Hl, ml 14E The omposition of Solutions During Acid/Base Titration Fig. 148 Plots of relative amount of HOAc and OAc during a titration. The straight lines show the change in relative amounts of HOAc (α 0 ) and OAc (α 1 ) during the titration of ml of 0.00 HOAc. The curved line is the titration curve for the system. 91