Redox Reactions

Foundation for Advancement in Conservation


© 2008

Introduction

Oxidation and reduction reactions contribute to the corrosion of iron and degradation of other materials that conservators wish to preserve. An understanding of the chemistry of these reactions is key to undertaking effective preservation techniques.

Before reviewing this tutorial refresh your memory on atomic structure and the periodic table in any basic chemistry book, such as Chemistry for Dummies by John T. Moore.

Learning Objectives

After completing this tutorial, you will be able to:

Definition

Redox reactions are the transfer of the control of electrons from one reactant to another. The mnemonic for remembering these reactions is "OIL RIG".

O Oxidation
I Is
L Loss
R Reduction
I Is
G Gain

Electron Control

An element is:

Image showing that when an element is reduced it gains electrons and hydrogen and loses oxygen, and when it is oxidized it loses electrons and hydrogen and gains oxygen

Redox Reaction

If an element is reduced in a chemical reaction, then something else must be oxidized. You can't have one without the other.

Fe (s) + CuSO4 (aq) ⟶ FeSO4 (aq) + Cu (s)

Iron is oxidized and is the reducing agent

Copper is reduced and is the oxidizing agent.

Detailed View of the Reaction

Iron the metallic state reacts with copper sulfate in solution to produce the ferrous sulfate and metallic copper.

Fe (s) + CuSO4 (aq) ⟶ FeSO4 (aq) + Cu (s)

Here is the equation without the sulfate, which is unchanged on either side of the equation.

Fe (s) + Cu2+ (aq) ⟶ Fe2+ (aq) + Cu (s)

This is easier to see if we split the equation up.

Fe ⟶ Fe2+ + 2 electrons

Cu2+ + 2 electrons ⟶ Cu

Relationship between Oxidizing and Reducing Agent

A reducing agent causes something else to be reduced and is oxidized in the process.

An oxidizing agent causes something else to be oxidized and is reduced in the process.

Diagram showing the relationship between the reducing agent, which loses electrons and is oxidized, and the oxidizing agent which gains electrons and is reduced

Oxidation numbers

This brings us to oxidation numbers. These can be very useful if you remember the rules.

  1. The oxidation numbers must add up to the charge on the molecule or ion.
  2. Any atom in its elemental form has an oxidation number 0.
  3. In compounds:
    • Group 1 elements have an oxidation number +1
    • Group 2 elements have an oxidation number +2
  4. The halogens usually have an oxidation number -1.
  5. Hydrogen usually has an oxidation number +1.
  6. Oxygen has an oxidation number -2,
    • except O—O bonds where it is 0
    • and in peroxides where it is -1.

Copper Sulfate Example

Let's take a look at copper sulfate (CuSO4) again.

We can write this Cu2+ SO42-.

The oxygen has an oxidation number -2 (rule 6): 4 × -2 = -8
However, there is a 2- charge on the ion, so sulfur must have an oxidation number of +6 (rule 1).

Oxidation numbers of the elements in CuSO4:
Cu = +2
S = +6
O = -2

The molecule does not have a charge so
Cu + S = +8
O × 4 = -8

If, at the end of a reaction, the oxidation number is less then the element has been reduced. If it is more, then the element has been oxidized.

Determining what is Oxidized and Reduced

Oxidation numbers can be used to determine whether an element has been oxidized or reduced in a reaction.

Let us examine the role of sulfur in the following reaction.

(Note: Do not try this. Hydrogen sulfide smells strongly of rotten eggs and is toxic.) Chemical formula with oxidation numbers, showing sulfur dioxide reacting with hydrogen sulfide to form sulfur and water

Determining what is Oxidized and Reduced Continued

Chemical formula with oxidation numbers, showing sulfur dioxide reacting with hydrogen sulfide to form sulfur and water

In order to produce elemental sulfur with an oxidation number of 0, the sulfur in SO2 has gained 4 electrons (4 × e-) and has been reduced
(+4 → 0) and the two sulfur atoms in 2H2S have both lost two electrons
(-2 → 0) and has been oxidized.

Activity Series

Activity Series

Strongest Reducing Agent
Weakest Oxidizing Agent
Li  Lithium
Ca  Calcium
K   Potassium
Na  Sodium
Mg  Magnesium
Al  Aluminum
Mn  Manganese
Zn  Zinc
Fe  Iron
Ni  Nickel
Sn  Tin
Pb  Lead
Cu  Copper
Ag  Silver
Pt  Platinum
Au  Gold
Weakest Reducing Agent
Strongest Oxidizing Agent

Each element will reduce the ions of the elements beneath it. That is, each element will give up electrons to the elements beneath it in the series.

Each element will be oxidized by the ions of the elements beneath it.

Zn + Fe2+ ⟶ Zn2+ + Fe

Fe + Zn2+ ↛ Fe2+ + Zn

Using the Activity Series to make Predictions

Activity Series

Strongest Reducing Agent
Li  Lithium
Ca  Calcium
K   Potassium
Na  Sodium
Mg  Magnesium
Al  Aluminum
Mn  Manganese
Zn  Zinc
Fe  Iron
Ni  Nickel
Sn  Tin
Pb  Lead
Cu  Copper
Ag  Silver
Pt  Platinum
Au  Gold
Weakest Reducing Agent

Would it be possible to store a silver nitrate solution in a copper container? That is, will the following reaction occur:
Cu (s) + 2AgNO3(aq) ⟶ Cu(NO3)2 (aq) + 2 Ag (s)

Would it be possible to store a silver spoon in a zinc nitrate solution? That is, will the following reaction occur?
Ag (s) + Zn(NO3)2 (aq) ⟶ 2 AgNO3 (aq) + (aq) + Zn (s)



Activity Series

Strongest Reducing Agent
Li  Lithium
Ca  Calcium
K   Potassium
Na  Sodium
Mg  Magnesium
Al  Aluminum
Mn  Manganese
Zn  Zinc
Fe  Iron
Ni  Nickel
Sn  Tin
Pb  Lead
Cu  Copper
Ag  Silver
Pt  Platinum
Au  Gold
Weakest Reducing Agent

Summary

In this tutorial you learned:

  • How to define oxidation and reduction in terms of control of the electrons within a chemical reaction
  • How to use the mnemonic OIL RIG to recall the meaning of reduction and oxidation.
  • The relationship between an oxidizing agent and a reducing agent.
  • How the activity series of elements can be used to predict what materials will be oxidized or reduced when left in contact with certain other materials over time.

References

These articles on adhesives can be found at JAIC Online.

Chen, Runying; Jakes, Kathryn A.; "Cellulytic biodegredation of cotton fibers from a deep-ocean environment"; JAIC 2001, 40 (2). 1–13.

Di Pietro, Giovanna Di Giovanna; Ligternik, Frank; "Silver-mirroring edge patterns: Diffusion-reaction models for the formation of silver mirroring on silver gelatin glass plates"; JAIC 2002, 41 (2). 111–126.

Drayman-Weisser, Terry; "A perspective on the history of the conservation of archaeological copper alloys in the United States"; JAIC 1994, 33 (2). 141–152.

Credits

Researched and written by Sheila Fairbrass Siegler

Instructional Design by Cyrelle Gerson of Webucate Us

Project Management by Eric Pourchot

Special thanks to members of the Association of North American Graduate Programs in Conservation (ANAGPIC) and the AIC Board of Directors for reviewing these materials.

This project was conceived at a Directors Retreat organized by the Getty Conservation Institute and was developed with grant funding from the Getty Foundation.

Converted to HTML5 by Avery Bazemore, 2021

© 2008 Foundation for Advancement in Conservation