Name: electron-pair reactions in acid-base chemistry
Uploaded: 2020-09-24T15:06:51.000Z
Duration: 2 min 33 s
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Chemistry

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PS.Chm.6.3

Lewis Acids and Bases

Brønsted-Lowryモデルは、酸と塩基をプロトンで定義し、酸はプロトン供与体、塩基はプロトン受容体です。対照的に、ルイスモデルは、ルイス酸が電子対受容体であり、ルイス塩基が電子対ドナーである電子対に関して酸と塩基を定義します。 
ブレンステッド酸では、酢酸と同様に、水素は、水のように塩基から提供された電子を受け入れる空の軌道を持ち、ルイス塩基として機能するため、ルイス酸としても機能します。
ルイスモデルの利点は、イオン化可能なプロトンを持たない化合物を含む、より多くの化合物を酸として分類できることです。たとえば、三フッ化ホウ素は水素を含まないため、Brønsted-Lowryモデルでは酸として分類できません。
しかし、三フッ化ホウ素は、アンモニアなどのルイス塩基からの電子対を受け入れることができる空の軌道を持つ不完全なオクテットを持っているため、ルイス酸として作用することができます。
このようなルイス酸-塩基反応によって形成される生成物は、ルイス酸-塩基付加物と呼ばれます。
二酸化炭素などの一部の分子は、電子を再配置してルイス酸として機能することができます。
例えば、水と二酸化炭素の反応では、電子対が炭素-酸素π結合から二酸化炭素の末端酸素に移動します。
結果として生じる炭素原子上の空の軌道により、炭素原子は水分子からの電子対を受け入れ、ルイス酸として機能することができます。水分子が電子対を供与すると、ルイス塩基として機能します。
さらに転位すると、プロトンが水の酸素から二酸化炭素の末端酸素に移動し、炭酸付加体が形成されます。
Al(III)のような小さな金属カチオンは、電子対を再獲得し、ルイス酸として作用することができます。例えば、Al(III)は水から孤立電子対を受け入れ、ヘキサアクアアルミニウムイオンを形成します。ここでは、水分子が電子対を供与し、ルイス塩基として機能します。

ルイス酸及びルイス塩基

1923年、G. N. Lewisは、酸と塩基の一般的な定義を提案しました。酸と塩基は、一対の電子を受け入れたり、提供したりして、配位共有結合を形成する能力によって識別されるというものです。
 配位共有結合とは、結合する原子の1つが両方の結合電子を提供する場合に生じます。例えば、水分子が水素イオンと結合してヒドロニウムイオンになると、配位共有結合が生じます。また、アンモニア分子が水素イオンと結合してアンモニウムイオンになると、配位共有結合が生じます。以下に、この2つの式を示します。
 <img alt="Image1" src="/files/ftp_upload/11407/11407_Image_1.svg" style="height: 175px;" />
 配位共有結合の形成を伴う反応は、ルイス酸-塩基化学に分類されます。結合を構成する電子対を供与する種はルイス塩基、電子対を受容する種はルイス酸、そして反応の生成物はルイス酸-塩基付加物です。上の2つの例が示すように、Brønsted-Lowry酸塩基反応は、ルイス酸反応の一種であり、具体的には、酸種がH+であるものが該当します。他のルイス酸や塩基を含むいくつかの例を以下に説明します。
 三フッ化ホウ素のホウ素原子は価電子が6個しかないため、非常に優れたルイス酸であり、多くのルイス塩基と反応します。この反応ではフッ化物イオンが孤立電子対の1つを供与してルイス塩基となります。
 <img alt="Image2" src="/files/ftp_upload/11407/11407_Image_2.svg" style="height: 150px;" />
 次の反応では、ルイス塩基である2つのアンモニア分子のそれぞれが、ルイス酸である銀イオンに1対の電子を供与しています。
 
 <img alt="Image3" src="/files/ftp_upload/11407/11407_Image_3.svg" style="height: 150px;" />
 非金属の酸化物はルイス酸として働き、ルイス塩基である酸化物イオンと反応してオキシ陰イオンを形成します。
 <img alt="Image4" src="/files/ftp_upload/11407/11407_Image_4.svg" style="height: 150px;" />
 ルイス酸-塩基反応の多くは、あるルイス塩基が別のルイス塩基を酸-塩基付加物から追い出す置換反応、あるいはあるルイス酸が別のルイス酸を追い出す置換反応です。
 <img alt="Image5" src="/files/ftp_upload/11407/11407_Image_5.svg" style="height: 350px;" />
 ルイス酸塩基化学のもう一つのタイプは、中心原子（典型的には遷移金属陽イオン）が配位子と呼ばれるイオンや分子に取り囲まれた複合イオン（または配位錯体）の形成です。これらの配位子は、H2OやNH3のような中性分子であったり、CN–やOH–のようなイオンの場合もあります。多くの場合、配位子はルイス塩基として働き、中心原子に一対の電子を供与します。
 上記の文章は以下から引用しました。<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>.

1923年、G.N.ルイスは、酸と塩基が受け入れる能力または...

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Acids and Bases

酸と塩基

KEY TERMS AND CONCEPTS

主要用語と概念

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...

ph scale

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

relative-strengths-of-conjugate-acid-base-pairs

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

acidbase-strengths-and-dissociation-constants

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

chemistry

JoVE Core 化学は、簡潔で理解しやすいアニメーションビデオレッスンを使用して、一般的な化学の概念を生き生きとしたものにします。さらに、活動する科学者のビデオでは、現在世界中の実験室で行われている古典的そして独創的な研究・実験をお見せします。

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Electron-Pair Reactions in Acid-Base Chemistry

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