Chapter 15:Chapter 15:Acid / Base ChemistryAcid / Base Chemistry

15.3 pH Changes in Acid-Base 15.3 pH Changes in Acid-Base Reaction SystemsReaction Systems

pH curvespH curves A pH curve is a graph showing the continuous change A pH curve is a graph showing the continuous change

in pH during a titration (an acid-base reaction)in pH during a titration (an acid-base reaction) Inv. 15.3 Demonstration – pH curvesInv. 15.3 Demonstration – pH curves

Titration of Titration of NaOHNaOH and and NaNa22COCO33 with HCl with HCl

1414

pHpH

77

00

vol. of titrant (HCl) added (mL)vol. of titrant (HCl) added (mL)

Titration Terminology Titration Terminology (Gr.11)(Gr.11)

TitrationTitration – the progressive transfer of a solution – the progressive transfer of a solution from a buret (called the from a buret (called the titranttitrant) into a measured ) into a measured volume of another solution (called the volume of another solution (called the samplesample).).

Equivalence pointEquivalence point – the volume of titrant required – the volume of titrant required to neutralize the sample (# mol acid = # mol to neutralize the sample (# mol acid = # mol base).base).

EndpointEndpoint – the pH at the equivalence point of a – the pH at the equivalence point of a titration.titration.

Indicator Indicator – a chemical which is added to the sample – a chemical which is added to the sample that changes colour at the equivalence point of a that changes colour at the equivalence point of a titration.titration.

Buffering regionBuffering region – a horizontal region of the pH – a horizontal region of the pH curve where pH is not changing significantly.curve where pH is not changing significantly.

Interpreting pH curvesInterpreting pH curves Titration of NaOH with HClTitration of NaOH with HCl 1414 buffer region titrantbuffer region titrant

pH endpoint equivalence pointpH endpoint equivalence point 7 X7 X

equivalence point volume equivalence point volume buffer regionbuffer region 0 0 0 10 20 30 400 10 20 30 40 volume of titrant added (mL)volume of titrant added (mL)

Which is the titrant?Which is the titrant? Take 2 titrations involving NaOH and HCl Take 2 titrations involving NaOH and HCl

pHpH pHpH

vol. of titrant vol. of titrantvol. of titrant vol. of titrant HCl is titrant NaOH is titrantHCl is titrant NaOH is titrant (pH high to low) (pH low to high)(pH high to low) (pH low to high)

Choosing an IndicatorChoosing an Indicator pH curves are used to determine which indicator(s) are suitable pH curves are used to determine which indicator(s) are suitable

for a particular titration for a particular titration The goal is to get an indicator that changes colour in the same The goal is to get an indicator that changes colour in the same

region as the endpoint of your titrationregion as the endpoint of your titration

Titration of NaOH with HClTitration of NaOH with HCl

14 unsuitable: alizarin yellow (too early)14 unsuitable: alizarin yellow (too early)

pH good indicators: bromothymol pH good indicators: bromothymol blue, blue,

7 litmus and phenol red 7 litmus and phenol red

unsuitable: orange IV (too late)unsuitable: orange IV (too late)

00

vol. of titrant (HCl) added (mL)vol. of titrant (HCl) added (mL)

Multiple EndpointsMultiple Endpoints Titration of NaTitration of Na22COCO33 with HCl with HCl 1414 endpoint 1endpoint 1

8 X8 XpH pH endpoint 2endpoint 2 equivalence point volume 1equivalence point volume 1

4 X 4 X equivalence point volume 2equivalence point volume 2 0 0 0 10 20 30 400 10 20 30 40 volume of titrant added (mL)volume of titrant added (mL)

Choosing Indicators for Choosing Indicators for Multiple EndpointsMultiple Endpoints

Titration of NaTitration of Na22COCO33 with HCl with HCl 1414 endpoint 1 - metacresol purple (7.4-9.0)endpoint 1 - metacresol purple (7.4-9.0)

8 X8 XpH pH endpoint 2 – methyl orange (3.2-4.4)endpoint 2 – methyl orange (3.2-4.4)

4 X4 X 0 0 0 10 20 30 400 10 20 30 40 volume of titrant added (mL)volume of titrant added (mL)

Interpreting EndpointsInterpreting Endpoints

The Bronsted-Lowry 5-step method can be used The Bronsted-Lowry 5-step method can be used to write proton-transfer reactions that explain the to write proton-transfer reactions that explain the endpoints on a pH curveendpoints on a pH curve

In general, we only observe distinct endpoints on In general, we only observe distinct endpoints on a pH curve when a proton has been a pH curve when a proton has been quantitativelyquantitatively transferred from an acid to a base; incomplete transferred from an acid to a base; incomplete reactions are not observedreactions are not observed

The number of endpoints that are observed The number of endpoints that are observed represent the number of proton transfer reactions represent the number of proton transfer reactions between B-L acids and B-L bases in a titrationbetween B-L acids and B-L bases in a titration

Using the B-L 5-Step Using the B-L 5-Step Method Method

to Explain Endpointsto Explain Endpoints Titration of NaOH with HClTitration of NaOH with HCl 1414

pHpH 7 X we can write a proton transfer 7 X we can write a proton transfer

reaction to explain this endpointreaction to explain this endpoint 0 0 0 10 20 30 400 10 20 30 40 volume of titrant added (mL)volume of titrant added (mL)

Writing B-L Equations Writing B-L Equations to Explain Endpointsto Explain Endpoints

To explain the endpoint observed for To explain the endpoint observed for the titration of NaOH with HCl, we the titration of NaOH with HCl, we use the B-L 5-step method:use the B-L 5-step method:

SA ASA A Na Na ++ OH OH -- H H33O O ++ Cl Cl -- H H22O O SB B BSB B B H H ++

Endpoint: HEndpoint: H33O O ++ + OH + OH -- 2 H 2 H22O O

Using the B-L 5-Step Using the B-L 5-Step Method Method

to Explain Endpointsto Explain Endpoints Titration of NaOH with HClTitration of NaOH with HCl 1414

pHpH 7 X H7 X H33O O ++ + OH + OH -- 2 H 2 H22OO 0 0 0 10 20 30 400 10 20 30 40 volume of titrant added (mL)volume of titrant added (mL)

Polyprotic Acids and BasesPolyprotic Acids and Bases Polyprotic acids are those which can donate Polyprotic acids are those which can donate

more than one proton; in most cases, it is more than one proton; in most cases, it is equal to the number of hydrogens in the equal to the number of hydrogens in the chemical formulachemical formula– E.g. HE.g. H22SOSO4 4 : can donate 2 protons: can donate 2 protons– E.g. HE.g. H33POPO4 4 : can donate 3 protons: can donate 3 protons

Polyprotic bases are those which can accept Polyprotic bases are those which can accept more than one proton; in most cases it is more than one proton; in most cases it is equal to the magnitude of the charge on equal to the magnitude of the charge on the anionthe anion– E.g. COE.g. CO3 3 2-2- : can accept 2 protons : can accept 2 protons– E.g. POE.g. PO4 4 3-3- : can accept 3 protons : can accept 3 protons

Using the B-L 5-Step Using the B-L 5-Step Method Method

to Explain Polyprotic to Explain Polyprotic EndpointsEndpoints Titration of NaTitration of Na22COCO33 with HCl with HCl

1414 8 X 8 X endpoint 1endpoint 1

pHpH

4 X 4 X endpoint 2endpoint 2 0 0 0 10 20 30 400 10 20 30 40 volume of titrant added (mL)volume of titrant added (mL)

Writing B-L Equations Writing B-L Equations to Explain Polyprotic to Explain Polyprotic

EndpointsEndpoints To explain the first endpoint observed To explain the first endpoint observed

for the titration of Nafor the titration of Na22COCO33 with HCl, we with HCl, we will use the B-L 5-step method:will use the B-L 5-step method:

SA ASA A Na Na ++ COCO33 2-2- H H33O O ++ Cl Cl -- H H22O O SB B B SB B B

H H ++

Endpoint 1:Endpoint 1: H H33O O ++ + CO + CO33 2-2- H H22OO + HCO+ HCO33 - -

Writing B-L Equations Writing B-L Equations to Explain Polyprotic to Explain Polyprotic

EndpointsEndpoints To explain the To explain the secondsecond endpoint, we must consider endpoint, we must consider

not just the original entities, but those entities not just the original entities, but those entities that were that were consumedconsumed in the first reaction and in the first reaction and entities that were entities that were createdcreated in the first reactionin the first reaction

SA SA A AA A Na Na ++ COCO33 2-2- H H33O O ++ Cl Cl -- H H22O O HCOHCO33 --

B BB B SBSB

H H ++

Endpoint 2:Endpoint 2: H H33O O ++ + HCO + HCO33 -- H H22OO + H+ H22COCO33

Using the B-L 5-Step Using the B-L 5-Step Method Method

to Explain Polyprotic to Explain Polyprotic EndpointsEndpoints Titration of NaTitration of Na22COCO33 with HCl with HCl

1414

8 X 8 X HH33OO++ + CO + CO33 2-2- H H22OO + HCO+ HCO33 - - pHpH

4 X 4 X HH33OO+ + + HCO+ HCO33 -- H H22O + O + HH22COCO33

0 0 0 10 20 30 400 10 20 30 40 volume of titrant added (mL)volume of titrant added (mL)

Other Polyprotic SystemsOther Polyprotic Systems

Consider the titration of phosphoric acid, Consider the titration of phosphoric acid, HH33POPO4,4, with sodium hydroxide, NaOH: with sodium hydroxide, NaOH:

1111 X X endpoint 2endpoint 2

pHpH

X X endpoint 1endpoint 1

00

Vol. of NaOH (mL)Vol. of NaOH (mL)

Other Polyprotic SystemsOther Polyprotic Systems

To explain the first endpoint observed To explain the first endpoint observed for the titration of Hfor the titration of H33POPO44 with NaOH, with NaOH, we use the B-L 5-step method:we use the B-L 5-step method:

SA ASA A HH33POPO44 Na Na ++ OH OH -- H H22O O SB B SB B H H ++

Endpoint 1:Endpoint 1: H H33POPO44 + OH + OH -- H H22OO + H+ H22POPO44 --

Other Polyprotic SystemsOther Polyprotic Systems

To explain the To explain the secondsecond endpoint, we must consider endpoint, we must consider not just the original entities, but those entities that not just the original entities, but those entities that were were consumedconsumed in the first reaction and entities in the first reaction and entities that were that were createdcreated in the first reactionin the first reaction

A SAA SA HH33POPO44 Na Na ++ OH OH -- H H22O O HH22POPO44 --

SB BSB B BB

H H ++

Endpoint 2:Endpoint 2: H H22POPO44 -- + OH + OH -- H H22OO + HPO+ HPO4 4 2-2-

Other Polyprotic SystemsOther Polyprotic Systems The B-L method suggests there is the possibility of a The B-L method suggests there is the possibility of a thirdthird

endpoint:endpoint:

A SAA SA HH33POPO44 Na Na ++ OH OH -- H H22O O HH22POPO44 -- HPOHPO44 2-2-

SB BSB B BB BB

However, because it was not observed in the pH curve, it must However, because it was not observed in the pH curve, it must mean the reaction was not quantitative, therefore we must mean the reaction was not quantitative, therefore we must write it with a double arrowwrite it with a double arrow

H H + + > 50%> 50%

Endpoint 3:Endpoint 3: HPO HPO44 2-2- + OH + OH -- H H22OO + PO+ PO4 4 3-3-

(not observed)(not observed)

Other Polyprotic SystemsOther Polyprotic Systems Consider the titration of phosphoric acid, HConsider the titration of phosphoric acid, H33POPO4,4,

with sodium hydroxide, NaOH:with sodium hydroxide, NaOH:

1111 HPOHPO442-2- + OH + OH -- H H22OO + PO+ PO443- 3- (not observed)(not observed)

HH22POPO44-- + OH + OH-- H H22OO + HPO+ HPO442- 2- X XpHpH

X X HH33POPO44 + OH + OH -- H H22OO + H+ H22POPO44 --

00 Vol. of NaOH (mL)Vol. of NaOH (mL)

Homework:Homework:

Answer Q. 24-29 p.539-540.Answer Q. 24-29 p.539-540.