Today’s Objectives:

1. Perform calculations to determine quantities of substances involved in REDOX titrations.

2. Identify electron transfer, oxidizing agents, and reducing agents in REDOX reactions.

Section 13.4 (pp. 596-600)

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• There are many industrial and laboratory applications of REDOX stoichiometry:

▫ Mining engineer must know the concentration of iron in a sample of iron ore to decide whether or not a mine would be profitable.

▫ Chemical technicians must monitor the concentration of substances in products

i.e. how much bleach is in a disinfectant

▫ Hospital lab technicians must detect tiny traces of chemicals in human samples.

• How is this different from Chemistry 20 stoichiometry?

▫ We will need to predict the REDOX equation that will occur, then we will use the quantities provided to answer the question.

▫ The math is the same as Chemistry 20, we will just be using our knowledge of REDOX to start the question.

REDOX Stoichiometry

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• Use stoich to predict or analyze the quantity of a chemical involved in a chemical reaction

• For REDOX stoichiometry calculations still assume that all reactions are

▫ spontaneous

▫ fast

▫ stoichiometric

▫ quantitative

• Titration is a common experimental design for quantitative chemical analysis

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REDOX Stoichiometry

• Example #1

▫ A strong acid is painted onto a copper sheet to etch a design. If 500mL of a

0.250 mol/L solution is used, what mass of copper will react?

▫ List entities present, identifying the SOA and SRA: H+(aq) Cu(s) H2O(l)

▫ Write corresponding half reactions. Balance the electrons and combined reactions.

2H+(aq) + 2e-

H2(g)

Cu(s) Cu2+(aq) + 2e-

2H+(aq) + Cu(s) H2(g) + Cu2+

(aq)

▫ List Inventory: 0.250 mol/L m= ?

V= 0.500L

▫ Solve for the unknown quantity:

mCu = 63.55g x 1 mol Cu x 0.25 mol H+ x 0.500 L = 3.97 g mol Cu 2 mol H+ L

SOA SRA

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REDOX Stoichiometry

• Example #2

▫ Nickel metal is oxidized to Ni2+(aq) ions by an acidified potassium dichromate solution.

If 2.50g of metal is oxidized by 50.0mL of solution, what is the concentration of the K2Cr2O7(aq) solution?

▫ List entities present, identifying the SOA and SRA: Ni(s) K+(aq) H

+(aq) Cr2O7

2-(aq) H2O(l)

▫ Write corresponding half reactions. Balance the electrons and combined reactions.

3 [ Ni(s) Ni2+(aq) + 2e- ]

Cr2O72-

(aq) + 14 H+(aq)+ 6 e-

2Cr3+(aq) + 7H2O(l)

3Ni(s) + Cr2O72-

(aq) + 14 H+(aq) 3Ni2+

(aq + 2Cr3+(aq) + 7H2O(l)

▫ List Inventory: 2.50 g o.o500L

[Cr2O72- ] = ?

▫ Solve for the unknown quantity:

[Cr2O72- ] = 1 mol Cr2O7

2- x mol Ni x 2.50 g x __ 1_ = 0.284 mol/L 3 mol Ni 58.69 g 0.0500L

SRASOA

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REDOX Stoichiometry

• A titration is a quantitative laboratory technique used to determine the concentration of an unknown solution

• The titrant is a reagent of known concentration that is slowly added using a burette to a sample solution (aliquot)

▫ Recall that acid-base titrations use a titrant that is usually a strong acid or base solution

• The volume of titrant needed to reach the experimental endpoint is determined by the detection of an abrupt change in a solution property

▫ Ideally this is the same volume as the theoretical equivalence point when equal proportions of reagent have been added

▫ Indicator solutions are commonly used to observe a color change when the titration is complete

Titration Review

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• Always have a titrant that is a strong oxidizing or reducing agent

• Commonly used OA include acidic solutions of MnO4–

(aq) or Cr2O72–

(aq)

▫ Both are strong OA that have an observable color change when they undergo reduction to oxidize a reducing agent in a sample being titrated

no indicator solution is required

• The titrant must be standardized by a titration analysis with a primary standard

▫ Recall that a standard solution of precisely known concentration can be directly prepared from a mass of solid primary standard

▫ Due to the reactive nature of the OA titrant, it often react with itself in the storage container, therefore altering the concentration of the titrant

REDOX Titrations

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REDOX Titrations

– p. 596

Titration Procedure Review

• Record the initial burette volume before any titrant is added to the sample

• Titrate (add titrant) until the reaction is complete

▫ Endpoint reached when the final drop of titrant permanently changes the colour of the sample

▫ It’s important to continuously swirl the solution in the flask

• Record the final burette volume

• Determine the volume of titrant added by calculating the difference between the recorded burette readings

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• Complete several titrations to improve the reliability of the analysis

▫ Require a minimum of three trails that agree within 0.2 mL to improve the

accuracy of the average used for stoichiometric calculations

▫ Overshot trials and outliers that don’t fall in this range are excluded from the average

• Read the titrant volume from the bottom of the meniscus

▫ Interpolate titrant volumes recorded to two decimal places

• Remember that the top of the burette reads 0.00mL

▫ therefore the amount of titrant added equals the initial volume reading subtracted from the final volume reading

Titration Procedure Review

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• Potassium permanganate cannot be directly prepared with a precisely know concentration because the permanganate ion reacts with water and the organic and inorganic impurities in the water, thus potassium permanganate is not used as a primary standard.

▫ Use the following information to determine the standardized concentration of potassium permanganate solution when titrated against samples of acidic tin (II) chloride primary standard.

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Sample Problem 13.14 – p. 597

Titration Data for 10.00mL of acidic 0.0500 mol/L SnCl2(aq) with KMnO4(aq)

Table 1 – p. 597 Trial

Volume (mL) 1 2 3 4

Final 18.4 35.3 17.3 34.1

Initial 1.0 18.4 0.6 17.3

Added 17.4 16.9 16.7 16.8

Endpoint Color dark pink light pink light pink light pink

Overshot Endpoint ⇒ Vadded = (16.9 + 16.7 + 16.8) / 3 = 16.8 mL

• Determine net reaction equation and solve using stoichiometry

K+(aq) MnO4

–(aq) H+

(aq) Sn2+(aq) Cl–

(aq) H2O(l)

2 [ MnO4–

(aq) + 8 H+(aq) + 5 e–

Mn2+(aq) + 4 H2O(l) ]

5 [ Sn2+(aq) Sn4+

(aq) + 2 e– ]

2 MnO4–

(aq) + 16 H+(aq) + 5 Sn2+

(aq) 2Mn2+(aq) + 8 H2O(l) + 5 Sn4+

(aq)

16.8mL 10.00mL

[MnO4–] = ? 0.0500 mol/L

⇒ [MnO4–] = 2 mol MnO4

– x 0.05 mol Sn2+ x 10 mL Sn2+ x 1 = 0.0119 mol/L

5 mol Sn2+ 1 L Sn2+ 16.8 mL MnO4–

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Vadded = 16.8 mL

Sample Problem 13.14 – p. 597

OA

OA

OA OA OA

RARA

RA

RA

SOA

SRA

Homework

• Lab Exercise 13.C – p. 598▫ DUE:

• Practice Qs – p. 598 #1-5

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Today’s Objectives:

1. Perform calculations to determine quantities of substances involved in REDOX titrations.

2. Identify electron transfer, oxidizing agents, and reducing agents in REDOX reactions.

Section 13.4 (pp. 596-600)

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• What is the amount concentration of tin(II) ions in a solution prepared for research on toothpaste?

• The titration evidence collected is below.

• Average volume added = 12.4mL + 12.3mL +12.5mL = 12.4mL KMnO4(aq)3

Titration of 10.00mL of acidic Sn2+(aq) with 0.0832 mol/L KMnO4(aq)

Trial 1 2 3 4

final burette reading (mL) 19.5 15.8 28.1 40.6

initial burette reading (mL) 4.2 3.4 15.8 28.1

Volume of KMnO4(aq) added (mL) 15.3 12.4 12.3 12.5

Endpoint colour Dark pink Light pink Light pink Light pink

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Lab Exercise 13.C – p. 598

▫ What is the concentration of tin(II) ions in a solution given the titration observations?

▫ List entities present, identifying the SOA and SRA: Sn2+(aq) H+

(aq) MnO4-(aq) H2O(l) K

+(aq)

▫ Write corresponding half reactions. Balance the electrons and combined reactions.

▫ Solve for the unknown quantity:

SRA SOA

According to the evidence and the stoichiometric analysis, the amount concentration of tin (II) ions in the solution is 0.258mol/L

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Lab Exercise 13.C – p. 598

Homework

• Lab Exercise 13.D – p. 599▫ DUE:

• Investigation 13.4 – p. 599 (Formal Report)

▫ DUE: Monday, November 9

• Section 13.4 Review – p. 600 #1-7

• Chapter 13 Review – p. 605 #1-33

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