Chapter 15: Acid / Base Chemistry 15.3 pH Changes in Acid-Base Reaction Systems

pH curves A pH curve is a graph showing the continuous change in pH during a titration (an acid-base reaction) Inv. 15.3 Demonstration – pH curves Titration of NaOH and Na2CO3 with HCl 14 pH 7 vol. of titrant (HCl) added (mL)

Titration Terminology (Gr.11) Titration – the progressive transfer of a solution from a buret (called the titrant) into a measured volume of another solution (called the sample). Equivalence point – the volume of titrant required to neutralize the sample (# mol acid = # mol base). Endpoint – the pH at the equivalence point of a titration. Indicator – a chemical which is added to the sample that changes colour at the equivalence point of a titration. Buffering region – a horizontal region of the pH curve where pH is not changing significantly.

Interpreting pH curves Titration of NaOH with HCl 14 buffer region titrant pH endpoint equivalence point 7 X equivalence point volume buffer region 0 10 20 30 40 volume of titrant added (mL)

Which is the titrant? pH pH Take 2 titrations involving NaOH and HCl vol. of titrant vol. of titrant HCl is titrant NaOH is titrant (pH high to low) (pH low to high)

Choosing an Indicator pH curves are used to determine which indicator(s) are suitable for a particular titration The goal is to get an indicator that changes colour in the same region as the endpoint of your titration Titration of NaOH with HCl 14 unsuitable: alizarin yellow (too early) pH good indicators: bromothymol blue, 7 litmus and phenol red unsuitable: orange IV (too late) vol. of titrant (HCl) added (mL)

Multiple Endpoints Titration of Na2CO3 with HCl 14 endpoint 1 8 X pH endpoint 2 equivalence point volume 1 4 X equivalence point volume 2 0 10 20 30 40 volume of titrant added (mL)

Choosing Indicators for Multiple Endpoints Titration of Na2CO3 with HCl 14 endpoint 1 - metacresol purple (7.4-9.0) 8 X pH endpoint 2 – methyl orange (3.2-4.4) 4 X 0 10 20 30 40 volume of titrant added (mL)

Interpreting Endpoints The Bronsted-Lowry 5-step method can be used to write proton-transfer reactions that explain the endpoints on a pH curve In general, we only observe distinct endpoints on a pH curve when a proton has been quantitatively transferred from an acid to a base; incomplete reactions are not observed The number of endpoints that are observed represent the number of proton transfer reactions between B-L acids and B-L bases in a titration

Using the B-L 5-Step Method to Explain Endpoints Titration of NaOH with HCl 14 pH 7 X we can write a proton transfer reaction to explain this endpoint 0 10 20 30 40 volume of titrant added (mL)

Writing B-L Equations to Explain Endpoints To explain the endpoint observed for the titration of NaOH with HCl, we use the B-L 5-step method: SA A Na + OH - H3O + Cl - H2O SB B B H + Endpoint: H3O + + OH - 2 H2O

Using the B-L 5-Step Method to Explain Endpoints Titration of NaOH with HCl 14 pH 7 X H3O + + OH - 2 H2O 0 10 20 30 40 volume of titrant added (mL)

Polyprotic Acids and Bases Polyprotic acids are those which can donate more than one proton; in most cases, it is equal to the number of hydrogens in the chemical formula E.g. H2SO4 : can donate 2 protons E.g. H3PO4 : can donate 3 protons Polyprotic bases are those which can accept more than one proton; in most cases it is equal to the magnitude of the charge on the anion E.g. CO3 2- : can accept 2 protons E.g. PO4 3- : can accept 3 protons

Using the B-L 5-Step Method to Explain Polyprotic Endpoints Titration of Na2CO3 with HCl 14 8 X endpoint 1 pH 4 X endpoint 2 0 10 20 30 40 volume of titrant added (mL)

Writing B-L Equations to Explain Polyprotic Endpoints To explain the first endpoint observed for the titration of Na2CO3 with HCl, we will use the B-L 5-step method: SA A Na + CO3 2- H3O + Cl - H2O SB B B H + Endpoint 1: H3O + + CO3 2- H2O + HCO3 -

Writing B-L Equations to Explain Polyprotic Endpoints To explain the second endpoint, we must consider not just the original entities, but those entities that were consumed in the first reaction and entities that were created in the first reaction SA A A Na + CO3 2- H3O + Cl - H2O HCO3 - B B SB H + Endpoint 2: H3O + + HCO3 - H2O + H2CO3

Using the B-L 5-Step Method to Explain Polyprotic Endpoints Titration of Na2CO3 with HCl 14 8 X H3O+ + CO3 2- H2O + HCO3 - pH 4 X H3O+ + HCO3 - H2O + H2CO3 0 10 20 30 40 volume of titrant added (mL)

Other Polyprotic Systems Consider the titration of phosphoric acid, H3PO4, with sodium hydroxide, NaOH: 11 X endpoint 2 pH X endpoint 1 Vol. of NaOH (mL)

Other Polyprotic Systems To explain the first endpoint observed for the titration of H3PO4 with NaOH, we use the B-L 5-step method: SA A H3PO4 Na + OH - H2O SB B H + Endpoint 1: H3PO4 + OH - H2O + H2PO4 -

Other Polyprotic Systems To explain the second endpoint, we must consider not just the original entities, but those entities that were consumed in the first reaction and entities that were created in the first reaction A SA H3PO4 Na + OH - H2O H2PO4 - SB B B H + Endpoint 2: H2PO4 - + OH - H2O + HPO4 2-

Other Polyprotic Systems The B-L method suggests there is the possibility of a third endpoint: A SA H3PO4 Na + OH - H2O H2PO4 - HPO4 2- SB B B B However, because it was not observed in the pH curve, it must mean the reaction was not quantitative, therefore we must write it with a double arrow H + > 50% Endpoint 3: HPO4 2- + OH - H2O + PO4 3- (not observed)

Other Polyprotic Systems Consider the titration of phosphoric acid, H3PO4, with sodium hydroxide, NaOH: 11 HPO42- + OH - H2O + PO43- (not observed) H2PO4- + OH- H2O + HPO42- X pH X H3PO4 + OH - H2O + H2PO4 - Vol. of NaOH (mL)

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