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Chemistry LibreTexts

Galvanic Cells

Skills to Develop

  • Explain: the chemistry in a battery operation;
    • oxidation and reduction reactions;
    • the technique used to represent a battery;
    • the various parts of a battery.
  • Classify chemical reactions as oxidation, reduction or other types.

Chemistry of Batteries

Chemistry is the driving force behind the magic of batteries. A battery is a package of one or more galvanic cells used for the production and storage of electric energy by chemical means. A galvanic cell consists of at least two half cells, a reduction cell and an oxidation cell. Chemical reactions in the two half cells provide the energy for the galvanic cell operations.

Each half cell consists of an electrode and an electrolyte solution. Usually the solution contains ions derived from the electrode by oxidation or reduction reaction.

A galvanic cell is also called a voltaic cell. The spontaneous reactions in it provide the electric energy or current.

Two half cells can be put together to form an electrolytic cell, which is used for electrolysis. In this case, electric energy is used to force nonspontaneous chemical reactions.

Oxidation Reduction Reactions

Many definitions can be given to oxidation and reduction reactions. In terms of electrochemistry, the following definition is most appropriate, because it let's us see how the electrons perform their roles in the chemistry of batteries.

Note

Loss of electrons is oxidation (LEO), and gain of electrons is reduction (GER).

Oxidation and reduction reactions cannot be carried out separately. They have to appear together in a chemical reaction. Thus oxidation and reduction reactions are often called redox reactions. In terms of redox reactions, a reducing agent and an oxidizing agent form a redox couple as they undergo the reaction:

\(\ce{Oxidant + n\: e^- \rightarrow Reductant}\)
\(\ce{Reducant \rightarrow Oxidant + n\: e^-}\)

An oxidant is an oxidizing reagent, and a reductant is a reducing agent. The

reductant | oxidant or oxidant | reductant

Two members of the couple are the same element or compound, but of different oxidation state.

Copper-Zinc Voltaic Cells

battery.gifAs an introduction to electrochemistry let us take a look at a simple voltaic cell or a galvanic cell.

When a stick of zinc (\(\ce{Zn}\)) is inserted in a salt solution, there is a tendency for \(\ce{Zn}\) to lose electrons according to the reaction,

\(\ce{Zn \rightarrow Zn^2+ + 2 e^-}\).

The arrangement of a \(\ce{Zn}\) electrode in a solution containing \(\ce{Zn^2+}\) ions is a half cell, which is usually represented by the notation:

\(\ce{Zn | Zn^2+}\),

Zinc metal and \(\ce{Zn^2+}\) ion form a redox couple, \(\ce{Zn^2+}\) being the oxidant, and \(\ce{Zn}\) the reductant. The same notation was used to designate a redox couple earlier.

Similarly, when a stick of copper (\(\ce{Cu}\)) is inserted in a copper salt solution, there is also a tendency for \(\ce{Cu}\) to lose electrons according to the reaction,

\(\ce{Cu \rightarrow Cu^2+ + 2 e^-}\).

This is another half cell or redox couple: \(\ce{Cu | Cu^2+}\).

However, the tendency for \(\ce{Zn}\) to lose electrons is stronger than that for copper. When the two cells are connected by a salt bridge and an electric conductor as shown to form a closed circuit for electrons and ions to flow, copper ions (\(\ce{Cu^2+}\)) actually gain electrons to become copper metal. The reaction and the redox couple are respectively represented below,

\(\ce{Cu^2+ + 2 e^- \rightarrow Cu}\),       \(\ce{Cu^2+ | Cu}\).

This arrangement is called a galvanic cell or battery as shown here. In a text form, this battery is represented by,

\(\ce{Zn | Zn^2+ || Cu^2+ | Cu}\),

in which the two vertical lines ( || ) represent a salt bridge, and a single vertical line ( | ) represents the boundary between the two phases (metal and solution). Electrons flow through the electric conductors connecting the electrodes, and ions flow through the salt bridge. When

\(\ce{[Zn^2+]} = \ce{[Cu^2+]} = \textrm{1.0 M}\),

the voltage between the two terminals has been measured to be 1.100 V for this battery.

A battery is a package of one or more galvanic cells used for the production and storage of electric energy. The simplest battery consists of two half cells, a reduction half cell and an oxidation half cell.

Oxidation and Reduction Reactions - A Review

The overall reaction of the galvanic cell is

\(\ce{Zn + Cu^2+ \rightarrow Zn^2+ + Cu}\)

Note that this redox reaction does not involve oxygen at all. For a review, note the following:

  • \(\ce{Oxidant + n\: e^- \rightarrow Reductant}\)
  • Example: \(\ce{Cu^2+ + 2 e^- \rightarrow Cu}\)
  • \(\ce{Cu^2+}\) is the oxidizing agent and \(\ce{Cu}\) the reducing agent.

 

  • \(\ce{Reductant \rightarrow Oxidant + n\: e^-}\)
  • Example: \(\ce{Zn \rightarrow Zn^2+ + 2 e^-}\).
  • \(\ce{Zn}\) is the reducing agent, and \(\ce{Zn^2+}\) the oxidizing agent.

Theoretically, any redox couple may form a half cell, and any two half cells may combine to give a battery, but we have considerable technical difficulty in making some couples into a half cell.

Questions

  1. In the reaction:
    \(\ce{Zn \rightarrow Zn^2+ + 2 e^-}\)
    Is \(\ce{Zn}\) oxidized or reduced?
  2. In the reaction:
    \(\ce{Zn + Cu^2+ \rightarrow Zn^2+ + Cu}\)
    Which is reduced, \(\ce{Cu^2+}\),  \(\ce{Cu}\),  \(\ce{Zn^2+}\), or \(\ce{Zn}\)?
  3. Which one has a stronger tendency to lose electrons when they are in contact with an electrolyte, \(\ce{Cu^2+}\),  \(\ce{Cu}\),  \(\ce{Zn^2+}\), or \(\ce{Zn}\)?
  4. What is responsible for the conduction of electricity in the solution?
  5. The salt-bridge is the path for what to move?

Solutions

  1. Zinc is oxidized.

    Skill -
    Identify and explain redox reactions. Loss of electrons by \(\ce{Zn}\) means \(\ce{Zn}\) is oxidized, LEO.

  2. Copper ion, not the metal is reduced.

    Skill -
    Identify and explain oxidized and reduced species. The \(\ce{Cu^2+}\) ions gain electrons to become \(\ce{Cu}\) (metal) atoms. Thus \(\ce{Cu^2+}\) is reduced.

  3. Zinc has a strong tendency to lose electrons.

    Discussion -
    The \(\ce{Zn}\) metal is more reactive than copper. In an acidic solution, \(\ce{Zn}\) atoms lose electrons to \(\ce{H+}\) ions, but copper atoms will not. The tendency is measured in terms of standard reduction potential.

  4. Ions of the electrolyte in the solution are responsible.

    Skill -
    Explain conductance of solution. The positive and negative ions move in opposite directions in a solution leading to conduction of electricity.

  5. The salt bridge is the path for the ions in the solution to move.

Next page: Oxidation States

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