Name: redox reactions and oxidation numbers
Uploaded: 2020-09-03T13:38:26.000Z
Duration: 3 min 11 s
Channel: JoVE

K12 Standards

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NC K-12 Science Standard Course of Study

9-12

Physical Science

Matter and its Interactions

PS.PSc.2.4

Oxidation-Reduction Reactions

Certain processes that are vital to life, including photosynthesis, combustion, and corrosion, fall into the class of reactions called oxidation&#8211;reduction, or redox, reactions. 

Redox reactions consist of two simultaneous processes: oxidation and reduction. 

The term oxidation means an increase in oxidation number, which corresponds to the loss of electrons, while reduction means a decrease in oxidation number, which corresponds to the gain of electrons. To remember the role of electrons, use the acronym OIL RIG, which stands for: &#8220;oxidation is losing, reduction is gaining.&#8221; 

Oxidation and reduction are complementary processes. In a redox reaction between two reactants, one reactant loses electrons and is oxidized, while the other reactant gains electrons and is reduced.

Consider the oxidation&#8211;reduction reaction between potassium &#8212; an alkali metal &#8212; and chlorine, a nonmetal. 

The neutral potassium atom loses an electron to become a potassium ion. Potassium is oxidized, and its charge increases from zero in the neutral atom to one-plus in the cation.

The neutral chlorine atom gains an electron and becomes a chloride ion. Chlorine is reduced, and its charge decreases from zero in the neu

oxidation-reduction reactions

<h4>Oxidation&#8211;Reduction Reactions</h4>

Earth&#8217;s atmosphere contains about 20% molecular oxygen, O2, a chemically reactive gas that plays an essential role in the metabolism of aerobic organisms and in many environmental processes that shape the world. The term oxidation was originally used to describe chemical reactions involving O2, but its meaning has evolved to refer to a broad and important reaction class known as oxidation&#8211;reduction (redox) reactions. 

Some redox reactions involve the transfer of electrons between reactant species to yield ionic products, such as the reaction between sodium and chlorine to yield sodium chloride:

<img alt="Chemical reaction equation, 2Na(s) + Cl&#8322;(g) &#8594; 2NaCl(s), synthesis of sodium chloride." src="/files/core/CDNSource/lessons/1030/1030_Equations-01.png" style="height: 40px;" data-altupdatedat="1761830439000" data-alt="Eq1" />

It is helpful to view the process with regard to each individual reactant, that is, to represent the fate of each reactant in the form of an equation called a half-reaction:

<img alt="Sodium metal oxidation equation, 2Na(s) &#8594; 2Na&#8314;(s) + 2e&#8315;, chemical reaction formula." src="/files/core/CDNSource/lessons/1030/1030_Equations-02.png" style="height: 40px;" data-altupdatedat="1761830439000" data-alt="Eq2" />

<img alt="Chlorine reduction reaction formula: Cl&#8322;(g) + 2e&#8315; &#8594; Cl&#8315;(s). Chemical equation diagram." src="/files/core/CDNSource/lessons/1030/1030_Equations-03.png" style="height: 40px;" data-altupdatedat="1761830439000" data-alt="Eq3" />

These equations show that Na atoms lose electrons while Cl atoms (in the Cl2 molecule) gain electrons, the &#8220;s&#8221; subscripts for the resulting ions signifying they are present in the form of a solid ionic compound. For redox reactions of this sort, the loss and gain of electrons define the complementary processes that occur:

oxidation = loss of electrons

reduction = gain of electrons

In this reaction, sodium is oxidized and chlorine undergoes reduction. Viewed from a more active perspective, sodium functions as a reducing agent (reductant), since it provides electrons to (or reduces) chlorine. Likewise, chlorine functions as an oxidizing agent (oxidant), as it effectively removes electrons from (oxidizes) sodium.

reducing agent = species that is oxidized

oxidizing agent = species that is reduced

In general, an oxidizing agent gains an electron from the reducing agent, and itself gets reduced. The charge of an oxidizing agent becomes more negative. Similarly, a reducing agent loses an electron to the oxidizing agent, and itself gets oxidized. The charge of a reducing agent becomes more positive.

Some redox processes, however, do not involve the transfer of electrons. Consider, for example, a reaction similar to the one yielding NaCl:

<img alt="Chemical reaction equation: H&#60;sub&#62;2&#60;/sub&#62;(g) + Cl&#60;sub&#62;2&#60;/sub&#62;(g) &#8594; HCl(g)." src="/files/core/CDNSource/lessons/1030/1030_Equations-04.png" style="height: 40px;" data-altupdatedat="1761830439000" data-alt="Eq4" />

The product of this reaction is a covalent compound, so the transfer of electrons in the explicit sense is not involved. To clarify the similarity of this reaction to the previous one and permit an unambiguous definition of redox reactions, a property called oxidation number has been defined. 

This text is adapted from <a href="https://openstax.org/books/chemistry-2e/pages/4-2-classifying-chemical-reactions">Openstax, Chemistry 2e, Section 4.2: Classifying Chemical Reactions.</a>

Oxidation&ndash;Reduction Reactions

Earth&rsquo;s atmosphere contains about 20% molecular oxygen, O2, a chemically reactive gas that plays an essential ...

Education

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Chemistry

Chemical Quantities and Aqueous Reactions

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KEY TERMS AND CONCEPTS

redox reactions and oxidation numbers

Redox reactions are chemical changes in which electrons move between reactants, and oxidation numbers help track that change. The term oxidation once referred only to reactions with molecular oxygen, but it now covers a wider reaction class called oxidation-reduction, or redox, reactions.

Some redox reactions form ionic products. A simple example is the reaction between sodium and chlorine that produces sodium chloride. To follow what happens to each reactant, chemists often write half-reactions. These show sodium atoms losing electrons and chlorine atoms in Cl2 gaining electrons. The solid product is shown with an s subscript because it is part of a solid ionic compound.

In this kind of reaction, oxidation means loss of electrons, and reduction means gain of electrons. Sodium is oxidized, while chlorine is reduced. Sodium also acts as the reducing agent because it gives electrons to chlorine. Chlorine acts as the oxidizing agent because it takes electrons from sodium. A reducing agent is the species that is oxidized, and an oxidizing agent is the species that is reduced.

An oxidizing agent gains an electron from the reducing agent, so its charge becomes more negative. A reducing agent loses an electron to the oxidizing agent, so its charge becomes more positive. Some redox reactions do not involve obvious electron transfer. In a similar reaction that forms a covalent compound instead of sodium chloride, the electron transfer is not written in the same direct way. That is why oxidation numbers were defined. They make redox reactions easier to identify and compare.

Redox Reactions and Oxidation Numbers

reaction stoichiometry

Using Balanced Equations to Predict Amounts

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limiting reactant

Limiting Reactant in Water Formation

Limiting reactants control how much product forms in a chemical reaction. In a balanced equation, the listed amounts of reactants and products are called stoichiometric amounts. In real reactions, the reactants are not always present in those exact proportions. The limiting reactant is the reactant that gets used up first. Once it is gone, the reaction stops making more product. The other reactant is called the excess reactant. Chemists often use extra amounts of one reactant to make sure the...

reaction yield

Calculating Percent Yield in Reactions

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general properties of solutions

Solvents, Solutes, and Solution Types

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solution concentration and dilution

Molarity and Dilution in Solutions

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solubility of ionic compounds

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chemical reactions in aqueous solutions

Aqueous Reactions and Ionic Equations

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precipitation reactions

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Redox reactions are chemical changes in which electrons move between reactants, and oxidation numbers help track that change. The term oxidation once referred only to reactions with molecular oxygen, but it now covers a wider reaction class called oxidation-reduction, or redox, reactions. Some redox reactions form ionic products. A simple example is the reaction between sodium and chlorine that produces sodium chloride. To follow what happens to each reactant, chemists often write...

oxidation numbers

Assigning Oxidation States in Redox Reactions

Oxidation states help track electron transfer in redox reactions. An oxidation number, also called an oxidation state, is a hypothetical number that shows the effective charge of an atom or element. Chemists assign it by using a set of rules. In an ionic compound, oxidation numbers are based on how many electrons move between reacting species. For example, when calcium chloride forms, calcium loses two valence electrons and the two chlorine atoms each gain one electron. In CaCl2, calcium has...

acids, bases and neutralization reactions

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synthesis and decomposition reactions

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key terms and concepts

Learn how redox reactions involve electron transfer, identify oxidation and reduction, and use oxidation numbers to track reactions that do not show direct transfer.

Redox Reactions and Oxidation Numbers | JoVE K12

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Chemical Reactions

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Matter And Its Interactions

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