Name: water as an amphiprotic substance
Uploaded: 2020-09-24T15:06:51.000Z
Duration: 2 min 30 s
Channel: JoVE

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Water: A Bronsted-Lowry Acid and Base

両性分子は、酸と塩基の両方として作用することができます。水は両性分子の一例です。
アンモニアの水溶液中では、水は塩基であるアンモニアにプロトンを供与することにより酸として機能することができます。対照的に、水は酸からプロトンを受け入れることにより、塩酸の水溶液中で塩基として機能します。
両性であるため、水は自己イオン化を受けることができます。このプロセスでは、1つの水分子が酸として作用し、そのプロトンを受け入れることにより塩基として機能する別の水分子にプロトンを寄付します。
これにより、ヒドロニウムイオンと水酸化物イオンが生成されます。
水の自己イオン化のための平衡定数、KWは、水のイオン積定数とも呼ばれ、1 × 10−14の値を持っています。その平衡表現は、ヒドロニウムイオンの濃度に水酸化物イオンの濃度を掛けたもので、両方の反応物が液体であるため分母はありません。 
25°Cの純水では、ヒドロニウムイオンと水酸化物イオンの濃度は等しく、1×10-7Mです。 
KWは、溶液中のヒドロニウムイオンと水酸化物イオンの濃度を計算し、溶液が酸性か塩基性かを判断するために使用できます。
溶液が酸性の場合、ヒドロニウムイオンの濃度は水酸化物イオンよりも高くなります。対照的に、溶液が塩基性の場合、水酸化物イオンの濃度はヒドロニウムイオンよりも高くなります。
溶液中のヒドロニウムイオンの濃度が5×10−7Mの場合、水酸化物イオン濃度はKWを使用して決定できます。
KWとヒドロニウムイオンの値がわかっているため、水酸化物イオンの濃度は2 × 10-8 Mと計算できます。
水酸化イオン濃度5×10-7Mは水酸化物イオン濃度2×10-8Mよりも高いため、溶液は酸性です。

水：ブレンステッド・ローリーの酸塩基理論

BrøNsted-Lowry酸と水との反応を酸のイオン化といいます。例えば、フッ化水素が水に溶けて電離すると、フッ化水素分子から水分子にプロトンが移動し、ヒドロニウムイオンとフッ化物イオンが生成します。
  <img alt="Image1" src="/files/ftp_upload/11396/11396_Image_1.svg" style="height: 125px;" />
 ある化学種が水分子からプロトンを受け取ると塩基性のイオン化が起こります。下の例では、ピリジン分子（C5NH5）が水に溶けると塩基性のイオン化を起こし、水酸化物イオンとピリジニウムイオンが生成します。
  <img alt="Image2" src="/files/ftp_upload/11396/11396_image 2v2.svg" style="height: 190px;" />
 前述のイオン化反応は、水が塩基（フッ化水素との反応のように）としても酸（アンモニアとの反応のように）としても作用することを示しています。プロトンを供与することも受容することもできる種は、両性物質と呼ばれます。以下の式は、両性物質である炭酸水素イオンと水の2通りの可能な酸-塩基反応を示しています。
  <img alt="Equation1" src="/files/ftp_upload/11396/11396_Equation_1.png" style="height: 95px;" />
 1つ目の式は、酸としての炭酸水素塩と塩基としての水との反応を表し、2つ目の式は、塩基としての炭酸水素塩と酸としての水との反応を表しています。炭酸水素塩を水に加えると、これらの平衡が同時に成立するため、それぞれの平衡を加味して最終的な溶液の組成が決まります。液体状態における両性物質の分子は、以下の式で水について示されているように、互いに反応することができます。
  <img alt="Image3" src="/files/ftp_upload/11396/11396_image_3.svg" style="height: 125px;" />
 同種の分子が反応してイオンを生成する過程を自己解離といいます。液体の水の自己解離はごくわずかで、25 °Cでは10億個の水分子のうち約2個がイオン化しています。水の自己解離の程度は、その平衡定数である水のイオン積KWの値に反映されます。
  <img alt="Equation2" src="/files/ftp_upload/11396/11396_Equation_2.png" style="height: 50px;" />
 純水のわずかな電離は平衡定数の小さな値に反映され、25°CではKWは1.0 × 10−14の値を示します。
  <img alt="Equation3" src="/files/ftp_upload/11396/11396_Equation_3.png" style="height: 50px;" />
 これは吸熱反応であるため、電離の程度とその結果としてのヒドロニウムイオンと水酸化物イオンの濃度は温度とともに増加します。例えば、100 °Cの場合、KWの値は約5.6 × 10−13となり、25 °Cの場合に比べて約50倍になります。
 水の自己解離では、ヒドロニウムイオンと水酸化物イオンが同じ数だけ生成します。したがって、25 °Cの純水では、
  <img alt="Equation4" src="/files/ftp_upload/11396/11396_Equation_4.png" style="height: 50px;" />
 溶液中のこれらのイオンの濃度は、その溶液の性質や他の溶質の化学的挙動を決定する重要な要素であることが多く、これらの濃度を相対的に表現するための語句が定義されています。ヒドロニウムイオンと水酸化物イオンの濃度が同じであれば中性、ヒドロニウムイオンの濃度が水酸化物イオンよりも高ければ酸性、ヒドロニウムイオンの濃度が水酸化物イオンよりも低ければ塩基性となります。
 酸性、塩基性、中性溶液の関係のまとめ
 <table style="margin-left: auto; margin-right: auto;">
 <tbody>
 <tr>
 <td style="color: black; border: 1px solid black; text-align: center;">分類</td>
 <td style="color: black; border: 1px solid black; text-align: center;">相対的なイオン濃度</td>
 <td style="color: black; border: 1px solid black; text-align: center;">25 °CでのpH</td>
 </tr>
 <tr>
 <td style="color: black; border: 1px solid black; text-align: center;">酸性</td>
 <td style="color: black; border: 1px solid black; text-align: center;">[H3O+] > [OH−]</td>
 <td style="color: black; border: 1px solid black; text-align: center;">pH < 7</td>
 </tr>
 <tr>
 <td style="color: black; border: 1px solid black; text-align: center;">中性</td>
 <td style="color: black; border: 1px solid black; text-align: center;">[H3O+] = [OH−]</td>
 <td style="color: black; border: 1px solid black; text-align: center;">pH = 7</td>
 </tr>
 <tr>
 <td style="color: black; border: 1px solid black; text-align: center;">塩基性</td>
 <td style="color: black; border: 1px solid black; text-align: center;">[H3O+] < [OH−]</td>
 <td style="color: black; border: 1px solid black; text-align: center;">pH > 7</td>
 </tr>
 </tbody>
 </table>
 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/14-1-bronsted-lowry-acids-and-bases"> Openstax, Chemistry 2e, Section 14.1: Brønsted-Lowry Acids and Bases</a>, and<a href="https://openstax.org/books/chemistry-2e/pages/14-2-ph-and-poh">Section 14.2: pH and pOH</a>.

ブレンステッド・ローリー酸と水との反応は、酸イオン化と呼ばれます。例えば、フッ化水素が水に溶けてイオン化すると...

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KEY TERMS AND CONCEPTS

主要用語と概念

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 can donate or accept protons is called amphiprotic. The broader term amphoteric is also used for substances that can act as acids or bases under different definitions. Bicarbonate ion is another amphiprotic species. It can react with water as an acid or as a base, and both equilibria can be present at the same time in solution.

Liquid water can also react with itself. This process is called autoionization. It happens only to a very small extent. At 25 °C, about two out of every billion water molecules are ionized. The equilibrium constant for this process is the ion-product constant for water, K_w, and its value is 1.0 × 10^-14 at 25 °C.

Autoionization is endothermic, so more ionization occurs as temperature rises. At 100 °C, K_w is about 5.6 × 10^-13, which is roughly 50 times larger than at 25 °C. Pure water always produces equal amounts of hydronium and hydroxide ions through this process. A solution is neutral when these ion concentrations are equal. It is acidic when hydronium ion concentration is greater than hydroxide ion concentration, and basic when hydroxide ion concentration is greater. At 25 °C, acidic solutions have pH below 7, neutral solutions have pH 7, and basic solutions have pH above 7.

Water as an Amphiprotic Substance

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 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

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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...

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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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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...

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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...

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acid strength and molecular structure

How Bonding Affects Acid Strength

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lewis acids and bases

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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...

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Learn how water can act as both an acid and a base, why it is amphiprotic, and how autoionization affects hydronium, hydroxide, and pH.

Water as an Amphiprotic Substance | JoVE K12

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Acids and Bases: Percent Dissociation and pH

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Water as an Amphiprotic Substance

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