Chemistry of Batteries

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 magics 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.

We will make this introduction using a typical setup as depicted here. The picture shows a copper zinc galvanic cell (battery).

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 nonsponaneous chemical reactions.

Oxidation Reduction Reactions

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

Loss of electrons is oxidation, and gain of electrons is reduction.

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:

Oxidant + n e^- ® Reductant
Reducant ® 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

As an introduction to electrochemistry let us take a look of a simple Voltaic cell or a galvanic cell.

When a stick of zinc (Zn) is inserted in a salt solution, there is a tendency for Zn to loose electron according to the reaction,

Zn = Zn²⁺ + 2 e^-. The arrangement of a Zn electrode in a solution containing Zn²⁺ ions is a half cell, which is usually represented by the notation: Zn | Zn²⁺, Zinc metal and Zn²⁺ ion form a redox couple, Zn²⁺ being the oxidant, and Zn the reductant. The same notation was used to designate a redox couple earlier.

Similarly, when a stick of copper (Cu) is inserted in a copper salt solution, there is also a tendency for Cu to loose electron according to the reaction,

Cu = Cu²⁺ + 2 e^-. This is another half cell or redox couple: Cu | Cu²⁺.

However, the tendency for Zn to loose electron 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 (Cu²⁺) actually gains electron to become copper metal. The reaction and the redox couple are respectively represented below,

Cu²⁺ + 2 e^- = Cu, Cu²⁺ | Cu. This arrangement is called a galvanic cell or battery as shown here. In a text form, this battery is represented by, Zn | Zn²⁺ || Cu²⁺ | 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 [Zn²⁺] = [Cu²⁺] = 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

Zn + Cu²⁺ = Zn²⁺ + Cu Note that this redox reaction does not involve oxygen at all. For a review, note the following: Oxidant + n e^- = Reductant
Example: Cu²⁺ + 2 e = Cu
Cu²⁺ is the oxidizing agent and Cu the reducing agent.

Reductant = n e^- + Oxidant
Example: Zn = Zn²⁺ + 2 e^-.
Zn is the reducing agent, and Zn²⁺ 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.

Links to Some Battry Companies

Batteries play an important role in our lives. These links are provided so that you can visit some of the battery companies on the internet. Because these are links, and they are subject to change over time.

Links to Fuel Cell Companies

A fuel cell differs from a battery in that the fuel is continuously supplied. To find out more about fuel cells, here are some links to fuel cell companies.

Fuel Cell World An overall view of fuel cell technology with many links to other sources.
Ballard Power Systems, Inc. A very informative page about fuel cell technology. Nice diagrams and photographs are available from the website. Ballard Power Systems, Inc. is located in Vancouver, Canada.
Solid Oxide Fuel Cell of Global Thermoelectric of Calgary, Canada
Fuelcell Technologies, Kingston, Canada
The Hydrogen and Fuel Cell Letter A publication related to fuel cell development.
Solid oxide fuel cell
Fuelcell energy
Epyx: the future of energy

Confidence Building Questions

In the reaction:
Zn -> Zn²⁺ + 2 e^-
Is Zn oxidized or reduced?

Skill -
Identify and explain redox reactions. Loss of electrons by Zn means Zn is oxidized, Leo.
In the reaction:
Zn + Cu²⁺ = Zn²⁺ + Cu
Which is reduced, Cu²⁺, Cu, Zn²⁺, or Zn?

Skill -
Identify and explain oxidized and reduced species. The Cu²⁺ ions gain electrons to become Cu (metal) atoms. Thus Cu²⁺ is reduced.
Which one has a stronger tendency to loose electrons when they are in contact with an electrolyte, Cu²⁺, Cu, Zn²⁺, or Zn?

Discussion -
The Zn metal is more reactive than copper. In an acidic solution, Zn atoms loose electron to H⁺ ions, but copper atoms will not. The tendency is measured in terms of standard reduction potential.
What is responsible for the conduction of electricity in the solution?

Skill -
Explain conductance of solution. The positive and negative ions move in opposite directions in a solution leading to conduction of electricity.
The salt-bridge is the path for what to move?

Skill -

Next page: Oxidation states