In Lab 9, students performed acid-base titrations. Redox reactions can also be used in titrations. An example is the titration of ascorbic acid (H2C6H6O6) in lemon juice using triiodide (I3–). A starch indicator will turn the solution blue-black at the endpoint. The half-reactions involved are shown below.

C6H6O6 + 2 H+ + 2 e–

 → 

H2C6H6O6

    +0.06 V

I3– + 2 e–

 → 

3 I–

    +0.53 V

 

(a) What is the net redox reaction that occurs? (Use the lowest possible coefficients. Omit states-of-matter from your answer.)

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[removed]

 

(b) What is the stoichiometry of H2C6H6O6 to I3–?

[removed]3:1 [removed]8:3     [removed]2:1 [removed]1:1 [removed]1:2 [removed]3:8 [removed]1:3

(c) Use the data given below to determine the amount of ascorbic acid in lemon juice. (Note: The recommended daily allowance of ascorbic acid (Vitamin C) is 90 mg.)

Data Table P6: Titration of ascorbic acid in lemon juice with triiodide

concentration of I3–

0.0210 M 

volume lemon juice

83.44 mL 

mass lemon juice

84.94 g 

equivalence volume of I3–

14.93 mL 

mmol of I3–

[removed]mmol 

mmol of H2C6H6O6

[removed]mmol 

mass of H2C6H6O6

[removed]mg   

           

 

Determine the errors (if any) with each galvanic cell set-up when the anode is on the left. (Select all that apply.)

 

[removed]There is nothing wrong with this diagram. [removed]The electrodes are in the wrong solution. [removed]The electrons are traveling the wrong direction down the wire. [removed]The salt bridge ions are migrating to the incorrect electrode. [removed]The electrons are traveling through the salt bridge. [removed]The electrodes and solutions are in the wrong compartment.

 

 

[removed]There is nothing wrong with this diagram. [removed]The electrodes are in the wrong solution. [removed]The electrons are traveling the wrong direction down the wire. [removed]The salt bridge ions are migrating to the incorrect electrode. [removed]The electrons are traveling through the salt bridge. [removed]The electrodes and solutions are in the wrong compartment.

 

 

[removed]There is nothing wrong with this diagram. [removed]The electrodes are in the wrong solution. [removed]The electrons are traveling the wrong direction down the wire. [removed]The salt bridge ions are migrating to the incorrect electrode. [removed]The electrons are traveling through the salt bridge. [removed]The electrodes and solutions are in the wrong compartment.

 

 

[removed]There is nothing wrong with this diagram. [removed]The electrodes are in the wrong solution. [removed]The electrons are traveling the wrong direction down the wire. [removed]The salt bridge ions are migrating to the incorrect electrode. [removed]The electrons are traveling through the salt bridge. [removed]The electrodes and solutions are in the wrong compartment.

 

 

[removed]There is nothing wrong with this diagram. [removed]The electrodes are in the wrong solution. [removed]The electrons are traveling the wrong direction down the wire. [removed]The salt bridge ions are migrating to the incorrect electrode. [removed]The electrons are traveling through the salt bridge. [removed]The electrodes and solutions are in the wrong compartment.

Consider your experimental results from part A of this lab. Suppose your strongest reducing agent were added to your strongest oxidizing agent. (Use the lowest possible coefficients. Omit states-of-matter from your answers.)

(a) Write the half-reaction for your strongest reducing agent.

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Mg  →  Mg2+ + 2e1-

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Correct.

(b) Write the half-reaction for your strongest oxidizing agent.

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MnO4- + 8H+ + 5e-  →  Mn2+ +4H2O

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Correct.

(c) Note the number of electrons in each half reaction.

In order to balance the number of electrons lost and gained, the oxidation half-reaction must be multiplied by and the reduction half-reaction must be multiplied by

(d) Write the net redox reaction.

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 Assemble a battery, represented by the diagram below with the cathode in compartment A, with Sn2+/Sn and Cu2+/Cu couples in which the voltage reads positive. (Use the . Use the lowest possible coefficients. Omit states-of-matter from your answer.)

 

(a) What half-reaction occurs in compartment A?

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Sn  →  Sn2+ +2e-

[removed]

Your answer contains an ambiguous or incomplete reaction equation. Check all the components on the reactant-side of the equation. Check all the components on the product-side of the equation.

(b) What half-reaction occurs in compartment B?

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Cu2+ +2e-  →  Cu

[removed]

Your answer contains an ambiguous or incomplete reaction equation. Check all the components on the reactant-side of the equation. Check all the components on the product-side of the equation.

(c) Write the net redox reaction.

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Sn + Cu2+  →  Sn2+ + Cu

[removed]

Correct.

 

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