Module 3

Module 3—Electrochemical Reactions

Explore

Corrosion is not the only problem iron presents. Iron is one of the most plentiful elements in Earth’s crust; as such, iron can be present in groundwater. In Alberta and elsewhere, iron can be a problem in well water. Iron(II) ions give well water a pale yellow-brown colour, and iron(III) ions can create insoluble precipitates. Iron in water also gives water a metallic taste and can stain dishes and laundry.

Many people using well water install water treatment systems to remove iron from their water. A popular method to remove iron from water involves treating iron with potassium permanganate, KMnO4(s). How might this system be able to remove iron(II) ions from well water?

You may recall that permanganate ion is a strong oxidizing agent. When potassium permanganate is pumped into the filter, it brings about the oxidation of iron(II) ions to iron(III) ions. Iron(III) ions are poorly soluble in water and precipitate. The precipitate is then removed by the filter.

Self-Check

SC 1.  Write the half-reactions and balanced net ionic equation for the reaction between potassium permanganate and iron(II) ions that occurs in the iron filter system. Assume acidic conditions.

SC 2.  Is the reaction spontaneous?

SC 3. Potassium permanganate in an aqueous solution at low concentrations has an intense purple colour. How would a consumer know if the quantity of potassium permanganate used in the iron filter was sufficient?

Read

The iron filtration system demonstrates the application of a redox reaction in a water treatment system. It also demonstrates how sensitive some processes are to the quantities of substances used.

In previous chemistry courses you studied the stoichiometric method and performed a titration experiment. Can you think of how this water treatment system is similar to a titration and a stoichiometric relationship?

Read “Titration” on page 804, and read all of page 596 of the textbook.

Self-Check

SC 4. In previous chemistry courses you performed titrations in acid-base systems. You used pH indicators to detect the endpoint of the titration.

Explain the following aspects of a titration:

1. What unique glassware is used in a titration and what is the function of each piece?
2. What is an endpoint?
3. What property of a potassium permanganate solution could be used as an endpoint for a titration?

Read

You will recall from your work in previous science courses that a titration involves adding a solution with a known concentration to a test solution with an unknown concentration. When you performed a titration experiment previously, you made careful observations and you recorded the volume of each solution involved in the reaction. Using the information about the volumes in addition to the molar concentration of the titrant allows for calculation of the chemical quantities involved. You will also recall that knowledge of the balanced chemical equation for the reaction occurring in the titration is critical to performing any titration analysis.

Given the importance of writing redox reactions to performing quantitative analysis, you may understand why so much emphasis was placed on learning how to write balanced chemical equations for redox reactions earlier in this module.

To complete your work in this lesson you will need to join the following two sets of skills:

• writing redox reactions
• performing stoichiometric calculations using titration data

Carefully read and work through “Sample problem 13.14” on page 597 of the textbook to learn how to join these skills.

Self-Check

SC 5. Complete “Practice” questions 3 and 5 on page 598 of the textbook.

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