Voltaic Cells

M-19-Lesson 1

Focus Question

How do voltaic cells harness energy from chemical reactions?

New Vocabulary

salt bridge

electrochemical cell

voltaic cell

half-cell

anode

cathode

reduction potential

standard hydrogen electrode

Review Vocabulary

oxidation: the complete or partial loss of electrons from a reacting substance; increases an atom’s or ion’s oxidation number

reduction: the complete or partial gain of electrons by the atoms of a substance; decreases the atom’s or ion’s oxidation number

Redox in Electrochemistry

• A salt bridge is a pathway to allow the passage of ions from one side to another, so that ions do not build up around the electrodes.
• An electrochemical cell is an apparatus that uses a redox reaction to produce electrical energy or uses electrical energy to cause a chemical reaction.

Redox in Electrochemistry

• A voltaic cell is a type of electrochemical cell that converts chemical energy to electrical energy by a spontaneous redox reaction.

Redox in Electrochemistry

• An electrochemical cell consists of two parts called half-cells, in which the separate oxidation and reduction reactions take place.
• The electrode where oxidation takes place is called the anode.
• The cathode is the electrode where reduction occurs.
• Electric potential energy is a measure of the amount of current that can be generated from a voltaic cell to do work.

Chemistry of Voltaic Cells

• Electric charge can flow between two points only when a difference in electric potential energy exists between the two points.
• A volt is a unit used to measure cell potential—the force from the difference in electric potential energy between two electrodes.

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Chemistry of Voltaic Cells

• The tendency of a substance to gain electrons is its reduction potential.
• When two half-reactions are coupled, the voltage �generated corresponds �to the difference in �potential between the half-reactions.

Calculating Electrochemical Cell Potentials

• The standard hydrogen electrode consists of a small sheet of platinum immersed in a hydrochloric acid solution that has a hydrogen ion concentration of 1 M. Hydrogen gas (H₂), at a pressure of 1 atm, is bubbled in and the temperature is maintained at 25°C.
• The standard hydrogen electrode is the standard against which all other reduction potentials are measured.

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Calculating Electrochemical Cell Potentials

SHE

Calculating Electrochemical Cell Potentials

• Formula for Cell Potential

• The standard potential of a cell is the standard potential of the reduction half-cell minus the standard potential of the oxidation half-cell

Use with Example Problem 1.

Problem

The following reduction half-reactions represent the half-cells of a voltaic cell.

I₂(s) + 2e^– → 2I^–(aq)

Fe²⁺(aq) + 2e^– → Fe(s)

Determine the overall cell reaction and the standard cell potential. Describe the cell using cell notation.

Response

ANALYZE THE PROBLEM

You are given the half-cell equations and can find standard reduction potentials in Table 1. The half-reaction with the lower reduction potential will be an oxidation. With this information, you can write the overall cell reaction, calculate the standard cell potential, and describe the cell in cell notation.

Calculate a Cell Potential

EVALUATE THE ANSWER

The calculated potential is reasonable given the potentials of the half-cells. E⁰ is reported to the correct number of significant figures.

Calculate a Cell Potential

• Cell potentials can be used to determine if a proposed reaction under standard conditions will be spontaneous.
• If the calculated potential is positive, the reaction is spontaneous.
• If the calculated potential is negative, the reaction is not spontaneous.

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Use Standard Reduction Potentials

Quiz

Which is a pathway to allow the passage of ions from one side of a redox process to the other?

1.

CORRECT

Quiz

Which is an apparatus that uses a redox reaction to produce electrical energy or uses electrical energy to cause a chemical reaction?

2.

CORRECT

Quiz

Which is a type of electrochemical cell that converts chemical energy to electrical energy by a spontaneous redox reaction?

3.

CORRECT

Quiz

Which is the name of the two parts of an electrochemical cell, in which the separate oxidation and reduction reactions take place?

4.

CORRECT

Quiz

Which is the tendency of a substance to gain electrons?

5.

CORRECT