Redox reactions

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

3:1 8:3     2:1 1:1 1:2 3:8 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 trioxide
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–	mmol
mmol of H2C6H6O6	mmol
mass of H2C6H6O6	mg

 

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

 

 

 

There is nothing wrong with this diagram. The electrodes are in the wrong solution. The electrons are traveling the wrong direction down the wire. The salt bridge ions are migrating to the incorrect electrode. The electrons are traveling through the salt bridge. 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⁺ + 2e^1-

 

Correct.

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

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MnO₄^– + 8H⁺ + 5e^–  →  Mn²⁺ +4H₂O

 

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^–

 

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

 

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

 

Correct.