Chapter 19 191

Chapter 19 Electrochemistry

This chapter requires students to use many of the skills they have acquired throughout the year.

They must be able to calculate oxidation states, write half reactions, and balance redox reactions.

To do this, they must apply what they have learned about atomic structure, periodicity, and

balancing equations. Without a solid foundation, this chapter material may be confusing for

students.

This chapter also has many practical applications in batteries and fuel cells that are not tested in

the AP curriculum, but the essentials of electrochemistry and electrolysis will be tested.

Explanation of electrolytic plating, electrolytic corrosion, and the difference between zinc

(galvanizing) and tin coatings on iron, for example, might be required. This chapter includes an

animation of galvanic cells and media player presentations on current generation and voltaic

cells.

Major Concepts to Know

• Students must be able to look at a galvanic cell diagram or equation and determine which

electrode will be the anode and cathode in a spontaneously running cell and know how to

indicate this in a cell diagram. They need to distinguish this from a nonspontaneous

electrolytic cell and understand the differences in effects and terminology. Word associations

can help students remember. A good one is LEO goes GER (loses electrons: oxidation and

gains electrons: reduction). Another is AO goes to CR (anode oxidation electrons go to

cathode reduction). The more active metal (LEO) must be the anode, and electrons must be

freed here to travel to the cathode (GER). Others remember OEL-REG (oil-rig) with similar

meanings.

Chapter 19 192

General Characteristics of Voltaic and Electrolytic Cells

• Students need to know how to read a reduction potentials chart and recognize in a

spontaneous reaction that the anode must be the lower reaction, including using SHE.

Students must be able to calculate standard voltage given the reduction potentials. Often,

students forget to change the sign of the oxidation reaction or mistakenly multiply potentials

by coefficients from the balanced equations.

Chapter 19 193

Determining E0 Half-Cell with the Standard Reference

• Students need to understand the function of a salt bridge. On the AP exam, many students

will show the misconception of electrons moving through the bridge as if it were a wire “to

complete the circuit.”

• Mathematically, students need to be able to relate voltage to K and G and calculate voltages

using the Nernst equation.

Chapter 19 194

The Interrelationship of Delta G0, E0 Cell, and K

• Students should be able to do a variety of problems and know that in aqueous solutions, the

water may be a part of the reaction and the reason (for instance, knowing why H2 will be

produced from H2O before Na+ will reduce to Na). Sometimes on exams students have

appeared to be tempted to just “play with the numbers” to get cell potentials rather than

arguing through the cell set-up. Students need to practice methodically explaining how they

get the answer, not just moving on when they get the right answer by trial and error.

Vocabulary to Know

• Anode

• Cathode

• Cell voltage

• Electrolysis

• Electrolytic cell

• Electromotive force (emf)

• Faraday

• Galvanic cell

• Half-cell reaction

• Nernst equation

• Standard emf

• Standard reduction potential

Chapter 19 195

Math Skills to Know

• Add voltage using E 0cell = E0

cathode - E0

anode

• Calculate new half-cell reactions by combining others (then you don’t multiply by

coefficients since half-cell potentials are “per mol of electrons”)

• Calculate Keq from E 0 using E0cell = [(RT)/(nF)] ln Keq, knowing the correct value of R to

insert

• Use the Nernst equation to calculate voltage at nonstandard conditions

• E = E 0 [(RT)/(nF)] lnQ

• Calculate G0 from E 0 using G

0 = nFE 0 or E using G = nFE

• Calculate quantitative electrolysis problems of every type, including number of grams

produced, number of moles electrons used, number of amperes used, or amount of time to

run

Suggested Problems (* also electronic)

• Galvanic cells and standard emfs: 3, 5, 6, 10, 11*, 12, 13, 16, 17*, 18

• Spontaneity of redox reactions: 19, 21*, 22, 23*, 24*, 26

• Effect of concentration on cell emf: 27, 28, 29*, 30, 31, 32*, 33, 34

• Corrosion: 42

• Electrolysis: 45*, 46, 51

• Additional questions: 67, 72, 81*, 90

Chapter 19 196

Suggested Demonstrations or Labs

Use a battery (9-volt is convenient) and wires dipping into sodium bromide and iodide

solutions with phenolphthalein added. Have students explain the observations (pink and

brown colors and bubbles at different electrodes). Do the same this time with a silver nitrate

solution and observe the differences.

• Melanie M. Cooper, “Project 10: Electrochemistry,” Cooperative Chemistry Lab Manual

(New York, NY: McGraw-Hill, 2006).

• Jeffrey A. Paradis, “Electrochemistry: An Introduction to Voltaic Cells,” Hands On

Chemistry Laboratory Manual (New York, NY: McGraw-Hill, 2006).

Chapter 19 197

Questions

1. What is electrochemistry?

2. What happens in oxidation?

3. What happens in reduction?

4. What are the five steps in the ion-electron method of balancing a redox equation?

5. If a reaction is done in an acidic medium, what should be added to the equation?

6. If a reaction is done in a basic medium, what should be added to the equation?

7. What is a galvanic cell?

8. What is another name for a galvanic cell?

9. Draw the basic set-up of a galvanic cell (page 841).

10. What is the name of the electrode where the half reaction that loses electrons occurs?

Chapter 19 198

11. What is the name of the electrode where the half reaction that gains electrons occurs?

12. What is a salt bridge?

a. What is the function of a salt bridge?

b. Does the type of salt used make a difference?

c. What would happen if the salt bridge was not present?

13. What is measured by a voltmeter, and what is the term for the measurement called?

14. What does emf stand for?

a. What is the symbol for emf?

15. Using an example, explain what a cell diagram denotes.

16. Why is the hydrogen electrode important?

a. What are the standard state conditions?

b. What is the voltage of SHE at these conditions?

c. What is the function of the platinum electrode in SHE?

Chapter 19 199

17. What is the difference between E and E0?

18. How do you calculate voltage?

a. On the emf table, why is one reaction always written backwards for

calculating voltage?

b. What do you do to the voltage if the half reaction is written backwards on

Table 19.1 (page 846)?

c. What do you do if the equation being used to find voltage has coefficients?

Explain why.

19. What type of voltage is required for a spontaneously occurring reaction?

20. How does the standard reductions potential relate to the metal reactivity series from Chapter

4?

a. Identify the strongest oxidizing agent on Table 19.1.

b. Identify the weakest oxidizing agent on Table 19.1.

c. What is the strongest reducing agent on Table 19.1?

d. What is the weakest reducing agent on Table 19.1?

21. What is a faraday?

Chapter 19 200

22. What is voltage at equilibrium?

23. What are the values of K and G at equilibrium?

24. How do positive volts relate to G and K?

a. What equation can calculate G from voltage?

b. What equation can calculate K from voltage?

25. What is the Nernst equation used to calculate?

26. What does Q represent in the Nernst equation?

a. What happens to the voltage if the anode increases in concentration?

b. What happens to the voltage if the anode decreases in concentration?

c. What happens to the voltage if the cathode increases in concentration?

d. What happens to the voltage if the cathode decreases in concentration?

e. These are examples of whose concept of shifting to reach equilibrium?

27. What is a battery?

28. In a car battery, why are the anode, the negative, and the cathode the positive hook-ups?

Chapter 19 201

29. Why can a car battery go dead at colder temperatures?

30. What is corrosion?

a. Why does corrosion of iron in cars occur more when salt is used to keep ice

off of the roads?

b. What is patina?

c. What is a famous “lady” example of copper turning to patina?

d. Why does aluminum not corrode if it is more reactive than iron?

e. What are three ways to protect metals from corrosion?

31. What is electrolysis?

a. What gas is formed at each electrode in the electrolysis of water?

b. In electrolysis, what are the steps needed to calculate grams to be produced,

starting with amps/hour? (see figure 19.20)

Chapter 19 202

Personal Teaching Notes