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Oxidation-reduction reaction, also called redox reaction, any chemical reaction in which the oxidation number of a participating chemical species changes. The term covers a large and diverse body of processes. Many oxidation-reduction reactions are as common and familiar as fire, the rusting and dissolution of metals, the browning of fruit, and respiration and photosynthesis—basic life functions.
- Oxidation is the gain of oxygen.
- Reduction is the loss of oxygen.
Most oxidation-reduction (redox) processes involve the transfer of oxygen atoms, hydrogen atoms, or electrons, with all three processes sharing two important characteristics: (1) they are coupled in any oxidation reaction a reciprocal reduction occurs, and (2) they involve a characteristic net chemical change an atom or electron goes from one unit of matter to another. Both reciprocity and net change are illustrated below in examples of the three most common types of oxidation-reduction reactions.
Because both reduction and oxidation are occurring simultaneously, this is known as a redox reaction.
An oxidizing agent is substance which oxidizes something else. In the above example, the iron(III) oxide is the oxidizing agent. A reducing agent reduces something else. In the equation, the carbon monoxide is the reducing agent.
- Oxidizing agents give oxygen to another substance.
- Reducing agents remove oxygen from another substance.
Oxidation and reduction in terms of hydrogen transfer
These are old definitions which are no longer used, except occasionally in organic chemistry.
Oxidation and Reduction with respect to Hydrogen Transfer
- Oxidation is the loss of hydrogen.
- Reduction is the gain of hydrogen.
ore precise definitions of oxidizing and reducing agents are
- Oxidizing agents add oxygen to another substance or remove hydrogen from it.
- Reducing agents remove oxygen from another substance or add hydrogen to it.
Oxidation and reduction in terms of electron transfer
Oxidation and Reduction with respect to Electron Transfer
- Oxidation is loss of electrons
- Reduction is gain of electrons
The following thought pattern can be helpful:
- An oxidizing agent oxidizes something else.
- Oxidation is loss of electrons (OIL RIG).
- Therefore, an oxidizing agent takes electrons from that other substance.
- Therefore, an oxidizing agent must gain electrons.
Here is another mental exercise:
- An oxidizing agent oxidizes something else.
- Therefore, the oxidizing agent must be reduced.
- Reduction is gain of electrons (OIL RIG).
- Therefore, an oxidizing agent must gain electrons.
Assign oxidation numbers to the atoms in each substance.
- Cl2
- GeO2
- Ca(NO3)2
Solution
- Cl2 is the elemental form of chlorine. Rule 1 states that each atom has an oxidation number of 0.
- By rule 3, oxygen is normally assigned an oxidation number of −2. For the sum of the oxidation numbers to equal the charge on the species (zero), the Ge atom is assigned an oxidation number of +4.
- Ca(NO3)2 can be separated into two parts: the Ca2+ ion and the NO3− ion. Considering these separately, the Ca2+ ion has an oxidation number of +2 by rule 2. Now consider the NO3− ion. Oxygen is assigned an oxidation number of −2, and there are three of them. According to rule 4, the sum of the oxidation numbers on all atoms must equal the charge on the species, so we have the simple algebraic equation
x+3(−2)=−1 x+3(−2)=−1
where x is the oxidation number of the N atom and the −1 represents the charge on the species. Evaluating for x,
x+(−6)=−1 x=+5x+(−6)=−1x=+5
Thus the oxidation number on the N atom in the NO3− ion is +5.
-The simplest form of technology is the development and use of basic tools. The prehistoric discovery of how to control fire and the later Neolithic Revolution increased the available sources of food, and the invention of the wheel helped humans to travel in and control their environment. Developments in historic times, including the printing press, the telephone, and the Internet, have lessened physical barriers to communication and allowed humans to interact freely on a global scale.
Technology has many effects. It has helped develop more advanced economies (including today's global economy) and has allowed the rise of a leisure class. Many technological processes produce unwanted by-products known as pollution and deplete natural resources to the detriment of Earth's environment. Innovations have always influenced the values of a society and raised new questions in the ethics of technology. Examples include the rise of the notion of efficiency in terms of human productivity, and the challenges of bioethics. In the past decade and a half, citizens of most industrialized countries have become concerned about the hazards of technology, have created a new set of institutions and activities to control them, and have profoundly changed the ways in which technologies are designed, produced, and used. Over the next decade and a half, more subtle hazards will confront us, strains and contradictions will emerge in the new institutions, and we will still be surprised at the strange ways in which our technologies unintentionally injure us. This shift from better-understood hazards to less understood hazards has placed an enormous burden on science to identify hazards and to assess their risks. Scientists often fluctuate between humility and hubris, and scientific risk assessment has manifested these fluctuations as well. Currently, humility appears to be in ascendance as the limits to knowledge emerge and experts routinely contradict each other in the press or the courtroom. But it should be equally recognized that while the media, the public, or the courts may demand more of science than it can give, some scientists have promised more than they can deliver. Scientists have implied that they know the significant ways in which technologies fail; that they are close to understanding the fundamental causes of cancer and arteriosclerosis; and that hazardous waste can be safely collected, transported, and stored. Thus, while some scientists could limit the burden on science, others continue to extend it, either from hubris, from a desire to reassure the public, or because they relish the challenge and opportunities for further research.
Chemical reaction between a metal and a nonmetal, electrons are transferred from the metal atoms to the nonmetal atoms. For example, when zinc metal is mixed with sulfur and heated, the compound zinc sulfide is produced. Two valence electrons from each zinc atom are transferred to each sulfur atom. Since the zinc is losing electrons in the reaction, it is being oxidized. The sulfur is gaining electrons and is thus being reduced. An oxidation-reduction reaction is a reaction that involves the full or partial transfer of electrons from one reactant to another. Oxidation is the full or partial loss of electrons or the gain of oxygen. Reduction is the full or partial gain of electrons or the loss of oxygen. A redox reaction is another term for an oxidation-reduction reaction.
Each of these processes can be shown in a separate equation called a half-reaction. A half-reaction is an equation that shows either the oxidation or the reduction reaction that occurs during a redox reaction. It is important to remember that the two half-reactions occur simultaneously. The resulting ions that are formed are then attracted to one another in an ionic bond.
Another example of an oxidation-reduction reaction involving electron transfer is the well-known combination of metallic sodium and chlorine gas to form sodium chloride. The rules for assigning oxidation numbers to atoms are as follows:
1. Atoms in their elemental state are assigned an oxidation number of 0.
In H2, both H atoms have an oxidation number of 0.
2. Atoms in monatomic (i.e., one-atom) ions are assigned an oxidation number equal to their charge. Oxidation numbers are usually written with the sign first, then the magnitude, to differentiate them from charges.
In MgCl2, magnesium has an oxidation number of +2, while chlorine has an oxidation number of −1.
3. In compounds, fluorine is assigned a −1 oxidation number; oxygen is usually assigned a −2 oxidation number (except in peroxide compounds [where it is −1] and in binary compounds with fluorine [where it is positive]); and hydrogen is usually assigned a +1 oxidation number [except when it exists as the hydride ion (H−), in which case rule 2 prevails].
In H2O, the H atoms each have an oxidation number of +1, while the O atom has an oxidation number of −2, even though hydrogen and oxygen do not exist as ions in this compound (rule 3). By contrast, by rule 3, each H atom in hydrogen peroxide (H2O2) has an oxidation number of +1, while each O atom has an oxidation number of −1.
4. In compounds, all other atoms are assigned an oxidation number so that the sum of the oxidation numbers on all the atoms in the species equals the charge on the species (which is zero if the species is neutral).
In SO2, each O atom has an oxidation number of −2; for the sum of the oxidation numbers to equal the charge on the species (which is zero), the S atom is assigned an oxidation number of +4. Does this mean that the sulfur atom has a 4+ charge on it? No, it means only that the S atom is assigned a +4 oxidation number by our rules of apportioning electrons among the atoms in a compound.
Determine the Oxidation States of each element in the following reactions:
- Fe(s)+O2(g)→Fe2O3(g)Fe(s)+O2(g)→Fe2O3(g)
- Fe2+Fe2+
- Ag(s)+H2S→Ag2S(g)+H2(g)Ag(s)+H2S→Ag2S(g)+H2(g)
Solutions
- Fe and O2 are free elements; therefore, they each have an oxidation state of 0 according to Rule #1. The product has a total oxidation state equal to 0, and following Rule #6, O has an oxidation state of -2, which means Fe has an oxidation state of +3.
- The oxidation state of Fe corresponds to its charge; therefore, the oxidation state is +2.
- Ag has an oxidation state of 0, H has an oxidation state of +1 according to Rule #5, S has an oxidation state of -2 according to Rule #7, and hence Ag in Ag2S has an oxidation state of +1.
-Because we do not understand the causation or because the effects are still latent. Our concerns have shifted in temporal scale as well. We are less worried about the daily recurrence of commonplace accidents than about confronting the frightening possibility of rare but catastrophic accidents. And in spatial scale we are shifting attention from the local to the regional and global: from local improvement in water or air quality, achieved in almost every industrialized country, to regional frustration in dealing with acid rain, stratospheric ozone depletion, and tropospheric ozone enrichment, and to global uncertainty about carbon dioxide, trace gas enrichment, and nuclear winter. Philosophical debates have arisen over the use of technology, with disagreements over whether technology improves the human condition or worsens it. Similar reactionary movements criticize the pervasiveness of technology, arguing that it harms the environment and alienates people; proponents of ideologies such as transhumanism and techno-progressivism view continued technological progress as beneficial to society and the human condition.
