Redox
Sodium gives one outer electron to fluorine, bonding them to form sodium fluoride. The sodium atom is oxidized, and fluorine is reduced. This simple exchange illustrates the core of redox chemistry: a type of chemical reaction in which the oxidation states of the reactants change. Oxidation is defined as the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or a decrease in the oxidation state. These two processes occur simultaneously within every redox reaction. They cannot happen independently. A substance that loses electrons acts as a reducing agent, while the substance gaining electrons serves as the oxidizing agent. Scientists classify these reactions into two main classes based on what moves between atoms. Electron-transfer reactions involve only one electron flowing from the species being oxidized to the species being reduced. Atom transfer reactions move entire atoms from one substrate to another. For example, rusting iron involves oxygen accepting electrons released by iron atoms.
The term redox first appeared in a 1928 article written by Leonor Michaelis and Louis B. Flexner. Before this portmanteau existed, scientists used separate words for oxidation and reduction without linking them conceptually. Oxidation originally implied a reaction with oxygen to form an oxide. Later, the definition expanded to encompass substances that accomplished chemical reactions similar to those of oxygen. Ultimately, the meaning generalized to include all processes involving the loss of electrons or an increase in the oxidation state of a chemical species. The word reduction originally referred to the loss in weight upon heating a metallic ore such as a metal oxide to extract the metal. Antoine Lavoisier demonstrated that this loss of weight was due to the loss of oxygen as a gas. In 1928, Michaelis and Flexner combined the terms to describe the simultaneous nature of these opposing changes. Electrochemists later proposed alternative vocabulary like electronation and de-electronation to describe these processes at electrodes. IUPAC has recognized these terms in its Compendium of Chemical Terminology, also known as the Gold Book.
Potassium permanganate acts as a strong oxidizing agent when mixed with glycerol, creating a violent redox reaction accompanied by self-ignition. Substances that have the ability to oxidize other substances are said to be oxidative or oxidizing agents. These agents remove electrons from another substance and are thus themselves reduced. Because they accept electrons, the oxidizing agent is also called an electron acceptor. Oxidants usually contain elements in high oxidation states or highly electronegative elements like fluorine, chlorine, bromine, or iodine. Nitric acid serves as a strong oxidizer often used in industrial contexts. Conversely, reducing agents transfer electrons to another substance and are thus themselves oxidized. The word reduction originally referred to the loss in weight upon heating a metallic ore such as a metal oxide to extract the metal. Electropositive elemental metals like lithium, sodium, magnesium, iron, zinc, and aluminium act as good reducing agents. They donate electrons relatively readily. Hydride transfer reagents such as NaBH4 and LiAlH4 reduce by atom transfer. These reagents transfer the equivalent of hydride or H minus ions to carbonyl compounds.
A Galvanic cell uses a zinc electrode in a ZnSO4 solution connected with a wire and a porous disk to a copper electrode in a CuSO4 solution. Each half-reaction has a standard electrode potential equal to the potential difference or voltage at equilibrium under standard conditions. The reduction potential measures the tendency of the oxidizing agent to be reduced. Its value is zero for hydrogen ions gaining an electron to form hydrogen gas by definition. Positive values indicate oxidizing agents stronger than hydrogen ions, while negative values indicate weaker ones. For instance, fluorine has a potential of plus 2 point 866 volts, whereas zinc ions have a potential of minus 0 point 763 volts. Electron transfer reactions occur by two distinct pathways: inner sphere electron transfer and outer sphere electron transfer. Redox reactions can occur slowly, as in the formation of rust, or rapidly, as in the case of burning fuel. Electron transfer reactions are generally fast, occurring within the time of mixing. Analysis of bond energies and ionization energies in water allows calculation of the thermodynamic aspects of redox reactions.
Blast furnaces combine iron oxides and coke to produce molten iron through high-temperature reduction processes. Rusting forms when oxygen combines with other elements to create hydrated iron oxides like Fe2O3 dot nH2O. This electrochemical oxidation of metals occurs in reaction with an oxidant such as oxygen. Cathodic protection controls corrosion by making a metal surface the cathode of an electrochemical cell. A simple method connects protected metal to a more easily corroded sacrificial anode to act as the anode. The sacrificial metal corrodes instead of the protected metal. Electroplating uses redox reactions to coat objects with a thin layer of material. Chrome-plated automotive parts and silver plating cutlery rely on these principles. Galvanization protects steel structures from environmental damage. Metal ores often contain metals in oxidized states such as oxides or sulfides. Pure metals are extracted by smelting at high temperatures in the presence of a reducing agent. Oxidation is used in production of cleaning products and for oxidizing ammonia to produce nitric acid.
Aerobic cellular respiration involves the oxidation of substrates like glucose C6H12O6 and the reduction of oxygen to water. Photosynthesis involves the reduction of carbon dioxide into sugars and the oxidation of water into molecular oxygen. Biological energy is frequently stored and released using redox reactions. In animal cells, mitochondria perform similar functions to those found in plant chloroplasts. The reduced carbon compounds reduce nicotinamide adenine dinucleotide NAD plus to NADH. This molecule contributes to creation of a proton gradient that drives synthesis of adenosine triphosphate ATP. Redox state describes the balance of GSH over GSSG, NAD plus over NADH, and NADP plus over NADPH in a biological system. Enzymatic browning takes place in most fruits and vegetables as an example of a redox reaction. Wide varieties of aromatic compounds are enzymatically reduced to form free radicals containing one more electron than their parent compounds. These flavoenzymes regenerate unchanged parent compounds while reducing molecular oxygen to superoxide.
Iron is mined as ores such as magnetite Fe3O4 and hematite Fe2O3. Titanium is mined as its dioxide usually in the form of rutile TiO2. These oxides must be reduced to obtain corresponding metals often achieved by heating these oxides with carbon or carbon monoxide. Electron transfer reactions are central to myriad processes and properties in soils. Redox potential quantified as Eh platinum electrode potential relative to standard hydrogen electrode serves as a master variable controlling chemical reactions. Early theoretical research applied to flooded soils and paddy rice production laid groundwork for subsequent work on thermodynamic aspects of redox. Later work expanded understanding of redox reactions related to heavy metal oxidation state changes. Pedogenesis and morphology depend heavily on soil redox status. Organic compound degradation and formation occur through complex redox pathways. Wetland delineation relies on measuring redox potential to identify specific environmental conditions. Soil remediation techniques utilize redox chemistry to neutralize contaminants. Free radical chemistry plays a role in natural soil processes.
Continue Browsing
Common questions
What is the definition of redox chemistry?
Redox chemistry is a type of chemical reaction in which the oxidation states of the reactants change. This process involves the simultaneous occurrence of oxidation and reduction where electrons are transferred between species.
Who coined the term redox and when did it appear?
The term redox first appeared in a 1928 article written by Leonor Michaelis and Louis B. Flexner. They combined separate words for oxidation and reduction to describe the simultaneous nature of these opposing changes.
How do oxidizing agents differ from reducing agents?
Oxidizing agents accept electrons and are themselves reduced, while reducing agents donate electrons and are themselves oxidized. Substances that lose electrons act as reducing agents and substances gaining electrons serve as oxidizing agents.
When was the word reduction originally used to describe weight loss?
Antoine Lavoisier demonstrated that the original meaning of reduction referred to the loss in weight upon heating a metallic ore such as a metal oxide to extract the metal. This weight loss occurred due to the loss of oxygen as a gas.
Why does rusting occur on iron surfaces?
Rusting forms when oxygen combines with other elements to create hydrated iron oxides like Fe2O3 dot nH2O. This electrochemical oxidation of metals occurs in reaction with an oxidant such as oxygen.