Name: oxidation states in redox reactions
Uploaded: 2020-03-26T07:22:24.000Z
Duration: 4 min 56 s
Channel: JoVE

K12 Standards

Texas Essential Knowledge and Skills

High School

Chemistry

CHEM.9

CHEM.9.B

Redox Reactions

L’oxydoréduction, abréviation de réduction-oxydation, est un type de réaction chimique classé par le transfert net d’électrons. Dans cette réaction, une molécule perd des électrons, appelée oxydation, et l’autre molécule gagne des électrons, appelée réduction.
Pour vous aider à faire la différence entre les deux, rappelez-vous l'expression « OIL-RIG », qui signifie « l'oxydation est perdante, la réduction est gagnante ». La molécule oxydée est appelée agent réducteur car elle réduit l’autre réactif. De même, la molécule qui est réduite est appelée agent oxydant car elle oxyde l’autre molécule.
Maintenant que nous avons réglé la terminologie, examinons un exemple de réaction d'oxydoréduction, la formation de l'oxyde de magnésium minéral. Au cours de la réaction, chaque atome de magnésium perd deux électrons. Ainsi, le magnésium est oxydé. Chaque atome d’oxygène gagne deux électrons ; Ainsi, l’oxygène est réduit.
Cependant, toutes les réactions ne sont pas des réactions redox. Par exemple, la réaction de l’oxyde de calcium avec le dioxyde de carbone pour former du carbonate de calcium n’est pas une réaction d’oxydoréduction. Alors, comment pouvons-nous identifier une réaction redox ?
Pour ce faire, nous suivons le nombre d’oxydation de chaque élément au fur et à mesure qu’il passe du réactif au produit. Le nombre d’oxydation est la charge hypothétique qu’un atome aurait si ses liaisons à différents éléments étaient ioniques, ce qui signifie que les électrons sont attribués à l’atome le plus électronégatif. La somme des nombres d’oxydation d’une molécule est égale à sa charge globale.
Revenons à l'oxyde de magnésium. Il s’agit d’un composé neutre, de sorte que la somme des nombres d’oxydation du magnésium et de l’oxygène est égale à zéro. Le magnésium peut donner deux électrons, son nombre d’oxydation est donc de plus deux. L’oxygène peut accepter deux électrons, son nombre d’oxydation est donc inférieur à deux.
Qu’en est-il de la réaction ? Les composés élémentaires neutres purs ont un nombre d’oxydation de zéro. Ainsi, le magnésium et l’oxygène commencent tous deux avec des nombres d’oxydation de zéro. Les nombres d'oxydation du magnésium et de l'oxygène ont changé au cours de la réaction, il s'agit donc d'une réaction d'oxydoréduction.
Regardons maintenant la réaction du carbonate de calcium que nous avons vue plus tôt. Les deux réactifs sont neutres, de sorte que la somme des nombres d’oxydation pour les deux composés est nulle. Comme nous l’avons vu avec l’oxyde de magnésium, le calcium a un nombre d’oxydation de plus deux et d’oxygène moins deux. Ensuite, le carbone dans la molécule de dioxyde de carbone a un nombre d’oxydation de plus quatre et chaque oxygène moins deux.
Qu’en est-il du produit ? Le calcium est plus deux et le carbone plus quatre, tout comme dans les réactifs. Chaque oxygène est égal à moins deux, soit un total de moins six, avec un nombre d’oxydation nette de zéro. Étant donné qu’aucun des nombres d’oxydation n’a changé, il ne s’agit pas d’une réaction d’oxydoréduction.
Présentons maintenant les quatre types de réactions redox. La première est une réaction de déplacement unique, où un atome en déplace un autre. Vous le verrez dans une réaction de thermite, où un métal est réduit et l'autre métal est oxydé.
Le type suivant est une réaction de combustion, qui se produit entre un combustible et un oxydant pour former des produits oxydés et de la chaleur. Vous le voyez en laboratoire lors de la combustion du méthane avec de l’oxygène lors de l’utilisation d’un bec Bunsen.
La troisième est une réaction de synthèse, où deux réactifs se combinent pour former un seul produit, comme dans la synthèse de l’ammoniac, où l’azote est combiné à l’hydrogène pour former de l’ammoniac.
Enfin, le quatrième type est une réaction de décomposition, où un réactif absorbe suffisamment d’énergie pour rompre ses liaisons et former des composés plus petits. C’est ce qui se passe avec les feux d’artifice, où le chlorate de potassium se décompose en chlorure de potassium et en oxygène après chauffage.
Dans cet atelier, vous allez effectuer et identifier différents types de réactions d'oxydoréduction qui transforment le cuivre solide en oxyde de cuivre, puis de nouveau en cuivre solide.

réactions redox

<h4>Oxydation et réduction</h4>
Certaines réactions chimiques peuvent être classées comme des réactions de réduction-oxydation, ou réactions d’oxydoréduction. L’oxydation est le processus par lequel la matière, comme un atome ou une molécule ionique, perd un ou plusieurs électrons, et la réduction est le processus par lequel la matière gagne un ou plusieurs électrons.
<h4>États d’oxydation</h4>
Chaque atome d’une molécule a son propre état d’oxydation ou nombre d’oxydation. L’état d’oxydation décrit le degré d’oxydation d’une molécule par rapport à sa forme élémentaire libre. L’état d’oxydation s’exprime comme la charge qu’un atome aurait si chacune de ses liaisons avec d’autres éléments était purement ionique. Cela signifie que les électrons de la liaison sont attribués à l’atome le plus électronégatif. L’état d’oxydation d’un atome sous sa forme élémentaire libre est défini par 0.
Il y a quelques règles qui sont suivies pour déterminer l’état d’oxydation. Les éléments des groupes I et II ont généralement des états d’oxydation de +1 et +2, respectivement. L’hydrogène et l’oxygène ont généralement des états d’oxydation de +1 et -2, respectivement, et les halogènes ont généralement un état d’oxydation de -1. De plus, les états d’oxydation des atomes d’une molécule s’additionnent toujours à la charge sur la molécule. Ainsi, l’état d’oxydation d’un atome non répertorié ci-dessus peut souvent être déduit. Prenons l’exemple du dioxyde de carbone (CO2), qui est une molécule neutre. Si chacune des deux molécules d’oxygène contribue à -2, l’état d’oxydation du carbone doit être de +4 pour annuler le -4 des oxygènes.
Pour une approche plus générale, dessinez la structure de Lewis de la molécule, identifiez les liaisons entre les différents atomes et attribuez chaque liaison à l’atome le plus électronégatif. Ensuite, comptez le nombre d’électrons sur chaque atome, chaque liaison apportant deux électrons. Soustrayez le nombre d’électrons qui se trouvent actuellement sur l’atome du nombre standard d’électrons de valence pour cet atome afin d’obtenir le nombre d’oxydation.
Considérez à nouveau le dioxyde de carbone. Chaque oxygène a deux doublets non liants d’électrons et est relié au carbone central par une double liaison. L’oxygène est plus électronégatif que le carbone, de sorte que chaque liaison C=O, qui représente quatre électrons, est attribuée à son oxygène. Ainsi, chaque oxygène se voit attribuer un total de huit électrons (quatre à partir des doublets non liants et quatre à partir de la double liaison), et le carbone n’en reçoit aucun. Le nombre par défaut d’électrons de valence pour l’oxygène est de six, donc le nombre d’oxydation pour chaque oxygène est de 6 – 8 = -2. Le nombre par défaut d’électrons de valence pour le carbone est de quatre, donc le nombre d’oxydation pour le carbone est de 4 – 0 = +4.
<h4>Réactions redox</h4>
Toutes les réactions chimiques ne sont pas classées comme des réactions d’oxydoréduction. Une réaction redox est une réaction au cours de laquelle il y a un changement dans l'état d'oxydation d'un atome. Ainsi, pour vérifier si une réaction est une réaction d’oxydoréduction, déterminez les états d’oxydation de chaque atome dans les réactifs et les produits et recherchez tout changement.
De nombreuses réactions d’oxydoréduction impliquent un transfert d’électrons directement d’une molécule ou d’un atome à un autre. Dans ces réactions, si une molécule, ou un atome, gagne un électron, une autre molécule, ou atome, doit perdre un électron. Une façon simple de se souvenir des définitions de l’oxydation et de la réduction est d’utiliser l’expression OIL-RIG, qui signifie : Oxidation Is Losing – Reduction Is Gaining.
La molécule qui gagne un électron est réduite, mais on l’appelle un oxydant ou un agent oxydant car elle oxyde l’autre molécule. De même, la molécule qui perd un électron est oxydée, mais on l’appelle réducteur ou agent réducteur car elle réduit l’autre molécule.
Il existe quatre principaux types de réactions qui impliquent généralement des processus d’oxydoréduction.
<ol>
	<li style="font-size :17px">Réaction à déplacement unique : Un atome déplace un autre atome qui fait partie d’un composé, et le remplace.</li>
	<li style="font-size :17px">Réaction de combustion : Un composé est réduit par un oxydant puissant, généralement de l’oxygène gazeux. Les réactions de combustion qui se produisent entre les hydrocarbures et les composés organiques produisent généralement du dioxyde de carbone et de l’eau.</li>
	<li style="font-size :17px">Réaction de synthèse : Deux réactifs forment un seul produit.</li>
	<li style="font-size :17px">Réaction de décomposition : Un seul réactif se brise en deux produits ou plus.</li>
</ol>
<h4>Références</h4>
1. Harris, D. C. (2015). Analyse chimique quantitative. New York, NY : W. H. Freeman et compagnie.

Réactions redox

Oxydation et réduction
Certaines réactions chimiques peuvent être classées comme des réactions de réduction-oxydation, ou réactions d’oxydoréduction. L’oxydation est le processus de ...

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oxidation states in redox reactions

Oxidation states help identify redox reactions. In a redox reaction, oxidation is the loss of one or more electrons, and reduction is the gain of one or more electrons. These reactions are often tracked by changes in oxidation state, also called oxidation number.

Each atom in a molecule has an oxidation state. This number shows how oxidized an atom is compared with its free elemental form. It is written as the charge an atom would have if every bond were purely ionic. In that model, bonding electrons are assigned to the more electronegative atom. An atom in its elemental form has an oxidation state of 0.

Several rules help determine oxidation states. Group I elements usually have +1, and Group II elements usually have +2. Hydrogen is typically +1, oxygen is typically -2, and halogens usually are -1. The oxidation states in a molecule or ion must add up to the total charge, so unknown values can often be found from the rest.

Carbon dioxide is a clear example. CO2 is neutral, so its total oxidation states must add to zero. Each oxygen is usually -2, so carbon must be +4 to balance the two oxygens. Another way to find oxidation numbers is to draw the Lewis structure, assign each bond to the more electronegative atom, and count electrons on each atom.

In CO2, oxygen is more electronegative than carbon, so each C=O bond is assigned to oxygen. Each oxygen gets eight electrons total, including lone pairs and bonding electrons. Oxygen has six valence electrons by default, so its oxidation number is 6 - 8 = -2. Carbon has four valence electrons by default and is assigned none, so its oxidation number is 4 - 0 = +4.

A reaction is a redox reaction when an atom’s oxidation state changes from reactants to products. To check for redox, compare oxidation states on both sides of the reaction. Many redox reactions also involve direct electron transfer between atoms or molecules. If one species gains an electron, another must lose one.

The phrase OIL-RIG helps remember the process: oxidation is losing, and reduction is gaining. The species that gains electrons is reduced, but it is also called the oxidizing agent or oxidant because it oxidizes the other species. The species that loses electrons is oxidized, but it is also called the reducing agent or reductant because it reduces the other species.

Four major reaction types often involve redox processes. In single displacement reactions, one atom replaces another atom in a compound. In combustion reactions, a compound is reduced by a strong oxidant, usually oxygen gas, and hydrocarbons or other organic compounds often produce carbon dioxide and water. In synthesis reactions, two reactants form one product. In decomposition reactions, one reactant breaks into two or more products.

Oxidation States in Redox Reactions

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The elemental makeup of a compound defines its chemical identity, and chemical formulas are the most concise way of representing this elemental makeup. ...

Chemical Equations

Chemical equations represent the identities and relative quantities of substances involved in a chemical reaction. The substances undergoing reaction are ...

CHEM.9.A

Chemical Reactions in Aqueous Solutions

Chemical substances interact in many different ways. Certain chemical reactions exhibit common patterns of reactivity. Due to the vast number of chemical ...

Precipitation Reactions

In a precipitation reaction, aqueous solutions of soluble salts react to give an insoluble ionic compound &#x2013; the precipitate. The reaction occurs ...

Oxidation-Reduction Reactions

Oxidation&ndash;Reduction Reactions

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

Oxidation Numbers

In redox reactions, the transfer of electrons occurs between reacting species. Electron transfer is described by a hypothetical number called the ...

Oxidation and Reduction
Some chemical reactions can be classified as reduction-oxidation reactions, or redox reactions. Oxidation is the process of ...

Acids, Bases and Neutralization Reactions

An acid-base reaction is one in which a hydrogen ion, H+, is transferred from one chemical species to another. Such reactions are of central importance to ...

Reaction Stoichiometry

A balanced chemical equation provides a great deal of information in a very succinct format. Chemical formulas provide the identities of the reactants and ...

Limiting Reactant

The relative amounts of reactants and products represented in a balanced chemical equation are often referred to as stoichiometric amounts. However, in ...

Reaction Yield

The theoretical yield of a reaction is the amount of product estimated to form based on the stoichiometry of the balanced chemical equation. The ...

Stoichiometry, Product Yield, and Limiting Reactants

Chemical equations represent how a chemical reaction proceeds from reactants to products through physical or chemical change using chemical formulas.
...

CHEM.9.C

Science concepts. The student understands how matter is accounted for in chemical reactions. The student is expected to:

Oxidation States in Redox Reactions

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