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

Il modello di Brønsted-Lowry definisce gli acidi e le basi in termini di protoni, dove gli acidi sono donatori di protoni, e le basi sono accettori di protoni. D'altro canto, il modello di Lewis definisce acidi e basi in termini di coppie di elettroni, dove gli acidi di Lewis sono accettori di coppie di elettroni, e le basi di Lewis sono donatrici di coppie di elettroni. In un acido di Brønsted, come l'acido acetico, l'idrogeno può anche agire come un acido di Lewis perché ha un orbitale libero per accettare gli elettroni donati da una base, come l'acqua, che agisce come una base di Lewis.

Il vantaggio del modello di Lewis è che consente agli scienziati di classificare un numero maggiore di composti come acidi compresi quelli che non hanno protoni ionizzabili. Per esempio, il trifluoruro di boro non può essere classificato come acido dal modello Brønsted-Lowry perché non contiene idrogeno. Tuttavia, il trifluoruro di boro possiede un ottetto incompleto con un orbitale vuoto, che può accettare una coppia di elettroni da una base di Lewis, come l'ammoniaca, e, quindi, può agire come un acido di Lewis.

Il prodotto risultante formato da tali reazioni acido-base di Lewis è detto addotto acido-base di Lewis. Alcune molecole, come l'anidride carbonica, possono riorganizzare i loro elettroni per agire come un acido di Lewis. Per esempio, nella reazione fra acqua e anidride carbonica, una coppia di elettroni si sposta dal legame pi carbonio-ossigeno all'ossigeno terminale dell'anidride carbonica.

Il risultante orbitale vuoto sull'atomo di carbonio gli consente di accettare la coppia di elettroni da una molecola d'acqua e di agire come un acido di Lewis. Quando la molecola d'acqua dona la coppia di elettroni, agisce come una base di Lewis. In un ulteriore riarrangiamento, un protone viene trasferito dall'ossigeno dell'acqua all'ossigeno terminale dell'anidride carbonica, determinando la formazione dell'addotto dell'acido carbonico.

Piccoli cationi metallici, come Al possono riacquisire coppie di elettroni e agire come acidi di Lewis. Per esempio, Al accetta coppie solitarie di elettroni dall'acqua e forma ioni esaaquaalluminio. Qui, le molecole d'acqua donano coppie di elettroni e agiscono come una base di Lewis.

acidi e basi di lewis

Nel 1923, G. N. Lewis propose una definizione generalizzata del comportamento acido-base in cui gli acidi e le basi sono identificati dalla loro capacità di accettare o donare una coppia di elettroni e formare un legame covalente coordinato.

Un legame covalente coordinato (o legame dativo) si verifica quando uno degli atomi nel legame fornisce entrambi gli elettroni di legame. Ad esempio, un legame covalente coordinato si verifica quando una molecola d'acqua si combina con uno ione idrogeno per formare uno ione idronio. Un legame covalente coordinato si ottiene anche quando una molecola di ammoniaca si combina con uno ione idrogeno per formare uno ione ammonio. Entrambe queste equazioni sono mostrate qui.

<img alt="Image1" src="/files/ftp_upload/11407/11407_Image_1.svg" style="height: 175px;" />

Le reazioni che comportano la formazione di legami covalenti coordinati sono classificate come chimica acido-base di Lewis. La specie che dona la coppia di elettroni che compone il legame è una base di Lewis, la specie che accetta la coppia di elettroni è un acido di Lewis e il prodotto della reazione è un addotto acido-base di Lewis. Come illustrano i due esempi sopra, le reazioni acido-base di Brønsted-Lowry rappresentano una sottocategoria delle reazioni acide di Lewis, in particolare quelle in cui la specie acida è H^+. Di seguito sono descritti alcuni esempi che coinvolgono altri acidi e basi di Lewis.

L'atomo di boro nel trifluoruro di boro, BF_3, ha solo sei elettroni nel suo guscio di valenza. Essendo inferiore all'ottetto preferito, BF_3 è un ottimo acido di Lewis e reagisce con molte basi di Lewis; uno ione fluoruro è la base di Lewis in questa reazione, donando una delle sue coppie solitarie:

<img alt="Image2" src="/files/ftp_upload/11407/11407_Image_2.svg" style="height: 150px;" />

Nella seguente reazione, ciascuna delle due molecole di ammoniaca, le basi di Lewis, dona una coppia di elettroni a uno ione d'argento, l'acido di Lewis:

<img alt="Image3" src="/files/ftp_upload/11407/11407_Image_3.svg" style="height: 150px;" />

Gli ossidi non metallici agiscono come acidi di Lewis e reagiscono con gli ioni ossido, basi di Lewis, per formare ossianioni:

<img alt="Image4" src="/files/ftp_upload/11407/11407_Image_4.svg" style="height: 150px;" />

Molte reazioni acido-base di Lewis sono reazioni di spostamento in cui una base di Lewis sposta un'altra base di Lewis da un addotto acido-base, o in cui un acido di Lewis sposta un altro acido di Lewis:

<img alt="Image5" src="/files/ftp_upload/11407/11407_Image_5.svg" style="height: 350px;" />

Un altro tipo di chimica acido-base di Lewis prevede la formazione di uno ione complesso (o un complesso di coordinazione) comprendente un atomo centrale, tipicamente un catione di metallo di transizione, circondato da ioni o molecole chiamate ligandi. Questi ligandi possono essere molecole neutre, come H_2O o NH_3, o ioni, come CN^– o OH^–. Spesso i ligandi agiscono come basi di Lewis, donando una coppia di elettroni all'atomo centrale.

Questo testo è adattato da<a href="https://openstax.org/books/chemistry-2e/pages/15-2-lewis-acids-and-bases"> Openstax, Chemistry 2e, Section 15.2: Lewis Acids and Bases</a>.

Acidi e basi di Lewis

Nel 1923, G. N. Lewis propose una definizione generalizzata del comportamento acido-base in cui acidi e basi sono identificati dalla loro capacità di accettare o di ...

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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...

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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...

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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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