Name: electron-pair reactions in acid-base chemistry
Uploaded: 2020-09-24T15:06:51.000Z
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Lewis Acids and Bases

Le modèle de Brønsted-Lowry définit les acides et les bases en termes de protons, où les acides sont des donneurs de protons, et les bases sont des accepteurs de protons. En revanche, le modèle de Lewis définit les acides et les bases en termes de paires d'électrons, où les acides de Lewis sont accepteurs de paires d'électrons et bases de Lewis sont des donneurs de paires d'électrons. Dans un acide Brønsted, comme l'acide acétique, l'hydrogène peut également agir comme un acide de Lewis car il a une orbitale vide pour accepter les électrons donnés d'une base, comme l'eau, agissant comme une base de Lewis.L'avantage du modèle de Lewis est qu'il permet aux scientifiques de classer un plus grand nombre de composés comme acides y compris ceux qui n'ont pas de protons ionisables. Par exemple, le trifluorure de bore ne peut pas être classé comme un acide par le modèle de Brønsted-Lowry car il ne contient pas d'hydrogène. Cependant, le trifluorure de bore possède un octet incomplet avec une orbitale vide qui peut accepter une paire électron à partir d'une base de Lewis, telle que l'ammoniac, Par conséquent, il peut agir comme un acide de Lewis.Le produit résultant formé par ces réactions acide-base de Lewis est appelé un adduit acide-base de Lewis. Certaines molécules, comme le dioxyde de carbone, peuvent déplacer leurs électrons pour agir comme un acide de Lewis. Par exemple, dans la réaction entre l'eau et le dioxyde de carbon, une paire d'électrons se déplace de la liaison pi carbone-oxygène à l'oxygène terminal du dioxyde de carbone.L'orbite vide résultante sur l'atome de carbone lui permet d'accepter la paire d'électrons d'une molécule d'eau et agissent comme un acide de Lewis. Comme la molécule d'eau donne la paire d'électrons, il agit comme une base de Lewis. Dans un réarrangement ultérieur, un proton est transféré de l'oxygène de l'eau à l'oxygène terminal du dioxyde de carbone, résultant en la formation du produit d'adduit de l'acide carbonique.Des petits cations métalliques, comme Al peuvent réacquérir des paires d'électrons et agir comme des acides de Lewis. Par exemple, Al accepte les paires isolées d'électrons de l'eau et forme des ions hexaaquaaluminum. Ici, les molécules d'eau donnent des paires d'électrons et agissent comme une base de Lewis.

acides et bases de lewis

En 1923, G. N. Lewis a proposé une définition généralisée du comportement acido-basique dans lequel les acides et les bases sont identifiés par leur capacité à accepter ou à donner un doublet d'électrons et à former une liaison covalente de coordination. 
 Une liaison covalente de coordination (ou liaison dative) se produit lorsque l'un des atomes de la liaison fournit les deux électrons liants. Par exemple, une liaison covalente de coordination se produit lorsqu'une molécule d'eau se combine à un ion hydrogène pour former un ion hydronium. Une liaison covalente de coordination résulte également lorsqu'une molécule d'ammoniac se combine à un ion hydrogène pour former un ion ammonium. Ces deux équations sont illustrées ici. 
 <img alt="Image1" src="/files/ftp_upload/11407/11407_Image_1.svg" style="height: 175px;" /> 
 Les réactions impliquant la formation de liaisons covalentes de coordination sont classées comme la chimie acide-base de Lewis. L'espèce qui fait don du doublet électronique qui compose la liaison est une base de Lewis, l'espèce qui accepte le doublet électronique est un acide de Lewis, et le produit de la réaction est un adduit acide-base de Lewis. Comme l'illustrent les deux exemples ci-dessus, les réactions acide-base de Brønsted-Lowry représentent une sous-catégorie des réactions d'acides de Lewis, plus particulièrement celles dans lesquelles l'espèce acide est H+. Quelques exemples impliquant d'autres acides et bases de Lewis sont décrits ci-dessous. 
 L'atome de bore dans le trifluorure de bore, BF3, n'a que six électrons sur sa couche de valence. Étant insuffisant pour l'octet préféré, BF3 est un très bon acide de Lewis et réagit avec de nombreuses bases de Lewis ; un ion fluorure est la base de Lewis dans cette réaction, donnant un de ses doublets libres : 
 <img alt="Image2" src="/files/ftp_upload/11407/11407_Image_2.svg" style="height: 150px;" /> 
 Dans la réaction suivante, chacune des deux molécules d'ammoniac, les bases de Lewis, donne un doublet d'électrons à un ion argent, l'acide de Lewis : 
 
 <img alt="Image3" src="/files/ftp_upload/11407/11407_Image_3.svg" style="height: 150px;" /> 
 Les oxydes non métalliques agissent comme des acides de Lewis et réagissent avec les ions oxyde, bases de Lewis, pour former des oxyanions : 
 <img alt="Image4" src="/files/ftp_upload/11407/11407_Image_4.svg" style="height: 150px;" /> 
 De nombreuses réactions acide-base de Lewis sont des réactions de déplacement dans lesquelles une base de Lewis déplace une autre base de Lewis d'un adduit acide-base, ou dans laquelle un acide de Lewis déplace un autre acide de Lewis : 
 <img alt="Image5" src="/files/ftp_upload/11407/11407_Image_5.svg" style="height: 350px;" /> 
 Un autre type de chimie acide-base de Lewis implique la formation d'un ion complexe (ou d'un complexe de coordination) comprenant un atome central, généralement le cation d'un métal de transition, entouré d'ions ou de molécules appelés ligands. Ces ligands peuvent être des molécules neutres, comme H2O ou NH3, ou des ions, comme CN– ou OH&#8211. Souvent, les ligands agissent comme des bases de Lewis, donnant un doublet d'électrons à l'atome central. 
 Ce texte est adapté de<a href="https://openstax.org/books/chemistry-2e/pages/15-2-lewis-acids-and-bases"> Openstax, Chimie 2e, Section 15.2 : Acides et bases de Lewis</a>.

Acides et bases de Lewis

En 1923, G. N. Lewis a proposé une définition généralisée du comportement acide-base dans laquelle les acides et les bases sont identifiés par leur capacité à accepter ou à ...

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electron-pair reactions in acid-base chemistry

Electron-pair reactions explain Lewis acid-base chemistry. In 1923, G. N. Lewis described acids and bases by how they accept or donate a pair of electrons. When that electron pair forms a coordinate covalent bond, one atom supplies both bonding electrons. This bond is also called a dative bond.

A simple example is the reaction of water with a hydrogen ion to make a hydronium ion. Ammonia can also combine with a hydrogen ion to form an ammonium ion. In both cases, the reaction creates a Lewis acid-base adduct. The electron-pair donor is the Lewis base, and the electron-pair acceptor is the Lewis acid.

Brønsted-Lowry acid-base reactions fit inside Lewis acid chemistry. In those reactions, the acid species is H+. Other Lewis acids show the same electron-pair pattern in different ways. Boron trifluoride, BF3, is a strong Lewis acid because boron has only six electrons in its valence shell and still needs an octet. A fluoride ion can donate a lone pair to BF3 and form an adduct.

Lewis acids can also bind more than one base. In one example, two ammonia molecules each donate a pair of electrons to a silver ion, which acts as the Lewis acid. Nonmetal oxides also behave as Lewis acids and react with oxide ions, which are Lewis bases, to form oxyanions. Many Lewis acid-base reactions are displacement reactions. In these reactions, one Lewis base can replace another base in an adduct, or one Lewis acid can replace another acid.

Lewis acid-base chemistry also helps form complex ions, or coordination complexes. These complexes have a central atom, often a transition metal cation, surrounded by ligands. Ligands can be neutral molecules such as H2O or NH3, or ions such as CN- or OH-. In many cases, the ligands act as Lewis bases and donate electron pairs to the central atom.

Electron-Pair Reactions in Acid-Base Chemistry

electron-pair-reactions-in-acid-base-chemistry

bronsted-lowry acids and bases

Proton Transfer in Acids and Bases

Acids and bases can be understood through proton transfer. Early scientists described acids by their sour taste, their effect on dyes, and their reaction with limestone, which releases carbon dioxide. Robert Boyle also noted that acid solutions can lose these properties after contact with alkali, or base, solutions. The modern view developed step by step. Humphry Davy showed that hydrogen is the essential part of acids. Joseph Louis Gay-Lussac then described acids as substances that neutralize...

water: a bronsted-lowry acid and base

Water as an Amphiprotic Substance

Water can act as both a Brønsted-Lowry acid and a Brønsted-Lowry base. When hydrogen fluoride dissolves in water, it donates protons to water. This is acid ionization. The products are hydronium ions and fluoride ions. Water can also accept protons from another substance. When pyridine dissolves in water, it accepts a proton from water molecules. This is base ionization. The products are hydroxide ions and pyridinium ions. These reactions show that water can play either role. A substance that...

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pH and pOH in Water

pH and pOH show how hydronium and hydroxide ions behave in water and other aqueous solutions. These ion concentrations are linked by the ion product of water, K w , so when one goes up, the other goes down. Because ion concentrations can differ by huge amounts, a logarithmic scale is used to describe them. pH is defined from the hydronium ion concentration, [H3O+], and pOH is the matching p-function for hydroxide ion concentration. These two values are also connected through the K w...

relative strengths of conjugate acid-base pairs

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How Ka and Kb Show Acid-Base Strength

Brønsted-Lowry acid-base chemistry is based on proton transfer. That idea helps show how the strength of an acid is linked to the strength of its conjugate base, and vice versa. Acid and base strength is measured with ionization constants, Ka and Kb, which tell how far an ionization reaction goes. For the conjugate pair HA and A−, the acid ionizes in water and the base reaction can be written for the partner species. When the two reactions are added, they give the autoionization of water. The...

strong acid and base solutions

Strong Acids, Strong Bases, and pH

Strong acids and strong bases change the pH of a solution by dissociating completely in water. A strong acid produces hydronium ions, which are H3O+ ions, and a strong base produces hydroxide ions, OH-. Because the dissociation is complete, the ion concentration can be found from the starting concentration and the molar ratio. For example, 0.20 M hydrobromic acid dissociates fully in water. It produces 0.20 M hydronium ions and 0.20 M bromide ions. A 0.015 M KOH solution also dissociates...

weak acid solutions

Weak Acid Equilibrium in Water

Weak acid equilibrium in water explains why many acids only partially react when dissolved. Most weak acid molecules stay in their original form, and only a small amount of hydronium ions forms. Weak acids are common in nature. They help create the tangy taste of citrus fruits, the stinging sensation from insect bites, and the smells linked to body odor. Acetic acid is a familiar weak acid. It is the main ingredient in vinegar. Its equation uses a double arrow to show that the reaction with...

weak base solutions

Weak Base pH and Kb Calculations

Weak base pH and Kb calculations show how partially reacting bases change water chemistry. Weak bases produce hydroxide ions when they dissolve by reacting with water molecules. Because the reaction only happens partly, these compounds are classified as weak bases. Weak bases are common in nature and in technology. Ammonia is one major example. Global production of ammonia is typically well over 100 metric tons each year. It is used as an agricultural fertilizer, a starting material for...

mixtures of acids

pH of Acid Mixtures

The pH of acid mixtures depends on the acids present, their dissociation constants, and their starting concentrations. When a solution contains two different acids, the pH can be found by choosing the method that fits their relative strengths. This makes acid mixtures a useful case for applying acid-base rules in chemistry. In a mixture of a strong acid and a weak acid, the strong acid dissociates completely and supplies almost all of the hydronium ions in the solution. The weak acid...

ions as acids and bases

Salt Hydrolysis and pH of Solutions

Salt hydrolysis explains why some salt solutions are acidic, basic, or neutral. Salts are ionic compounds made of cations and anions. In water, either ion may react with water through an acid or base ionization reaction. The pH of the solution depends on the relative acid-base strength of the ions in the salt. Ammonium chloride is a good example of an acidic salt. When it dissolves, it separates into ammonium ions and chloride ions. The ammonium ion is the conjugate acid of ammonia, so it can...

determining the ph of salt solutions

Salt Hydrolysis and pH in Solutions

Salt hydrolysis and pH in solutions depend on the ions released when a salt dissolves. Some ions react with water, while others do not. The balance between the cation and anion helps show whether the solution becomes acidic, basic, or neutral. Salts with pH-neutral anions and cations that produce hydronium ions give acidic solutions with pH below 7. Ammonium nitrate is one example. In NH4NO3, NO3− does not react with water, but NH4+ forms hydronium ions, so the solution is acidic. Other salts...

acid/base strengths and dissociation constants

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Acid and Base Strength in Water

Acid and base strength in water depends on how much a substance ionizes after it dissolves. If ionization is nearly complete, the substance is called strong. If only a small amount ionizes, it is called weak. Weak acids and weak bases are much more common than strong ones. The most common strong acids are HClO4, HCl, HBr, HI, HNO3, and H2SO4. The most common strong bases are LiOH, NaOH, KOH, Ca(OH)2, Sr(OH)2, and Ba(OH)2. These strong acids and bases ionize essentially completely in water. <br...

polyprotic acids

Stepwise Ionization of Polyprotic Acids

Polyprotic acids lose hydrogen ions in steps. The number of ionizable protons in a molecule determines whether an acid is monoprotic, diprotic, or triprotic. This video also shows that polyprotic bases can accept more than one proton. Monoprotic acids have one ionizable hydrogen atom per molecule. Examples include HCl, HNO3, and HCN. Their reactions with water form hydronium ions and the matching anion. Acetic acid, CH3CO2H, is also monoprotic because only the hydrogen in the carboxyl group...

acid strength and molecular structure

How Bonding Affects Acid Strength

Acid strength depends on bonding, electronegativity, and oxidation number. These factors help explain why some hydrogen compounds act as strong acids, while others behave as weak acids or bases. For binary acids, which are hydrogen compounds with one other nonmetal, acidity increases as the H-A bond gets weaker down a group. That is why the order for group 17 is HF &lt; HCl &lt; HBr &lt; HI, and for group 16 it is H2O &lt; H2S &lt; H2Se &lt; H2Te. Across a period, acid strength increases as...

lewis acids and bases

Electron-pair reactions explain Lewis acid-base chemistry. In 1923, G. N. Lewis described acids and bases by how they accept or donate a pair of electrons. When that electron pair forms a coordinate covalent bond, one atom supplies both bonding electrons. This bond is also called a dative bond. A simple example is the reaction of water with a hydrogen ion to make a hydronium ion. Ammonia can also combine with a hydrogen ion to form an ammonium ion. In both cases, the reaction creates a Lewis...

key terms and concepts

Learn how Lewis acids and bases use electron pairs to form coordinate covalent bonds, adducts, complex ions, and examples like BF3, Ag+, and hydronium.

Electron-Pair Reactions in Acid-Base Chemistry | JoVE K12

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Electron-Pair Reactions in Acid-Base Chemistry

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