Name: water as an amphiprotic substance
Uploaded: 2020-09-24T15:06:51.000Z
Duration: 2 min 30 s
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Water: A Bronsted-Lowry Acid and Base

两性分子既能作为 酸又可作为碱。水是两性分子的一个例子。在氨的水溶液中，水可以通过向氨（一种碱）提供 质子而起到酸的作用。相反，在盐酸水溶液中，水通过 从酸中接受质子 而起到碱的作用。因为它是两性的，所以水可以进行自电离。在这个过程中，一个水分子通过将质子提供给 另一个水分子作为酸，而另一个水分子 通过接受该质子作为碱。这会导致水合氢离子 和氢氧根离子的产生。水的自电离平衡常数 Kᴡ，也称为水的离子积常数，可表示为 Kᴡ，其值为 1×10⁻¹⁴。它的平衡表达式书写为水合 氢离子浓度乘以氢氧根离子 浓度，没有分母，因为两种反应物都是液体。在 25 摄氏度的纯水中，水合氢离子的浓度和氢氧根离子的浓度 相等—即 1 10⁻⁷ M。Kw 可 用于计算溶液中水合氢离子 和氢氧根离子的浓度，并确定 溶液是酸性还是碱性。如果溶液是酸性的，水合氢离子的浓度 会比氢氧根离子高。相反，如果溶液是碱性的，氢氧根离子的浓度 会比水合氢离子高。如果溶液中水合氢离子的浓度 为 5 10⁻⁷ M，则氢氧根离子浓度 可以用 Kᴡ 测定。由于 Kw 和水合氢离子的值已知，氢氧根离子的浓度 可计算为 2×10⁻⁸M。由于水合氢离子浓度 5×10⁻⁷M，高于氢氧根离子浓度 2×10⁻⁸M，溶液呈酸性。

水： 布朗斯特-劳里的酸和碱

布朗斯特-劳里 酸与水之间的反应称为酸电离。 例如，当氟化氢溶解在水中并产生电离时，质子会从氟化氢分子转移到水分子，从而产生水合氢离子和氟化物离子： 
  <img alt="Image1" src="/files/ftp_upload/11396/11396_Image_1.svg" style="height: 125px;" />
 当一种物种接受来自水分子的质子时，就会发生碱电离。 在以下示例中，当溶解在水中时，吡啶分子 C5NH5 会发生碱电离，从而产生氢氧化物和吡啶鎓离子： 
  <img alt="Image2" src="/files/ftp_upload/11396/11396_image 2v2.svg" style="height: 190px;" />
 前面的电离反应表明，水可能同时起碱 (如对氟化氢的反应) 和酸 (如对氨的反应) 的作用。 可以捐赠或接受质子的物种被称为两性的，或者更一般地称为苯丙二酸，一个术语可用于除 布朗斯特-劳里以外的酸和碱定义。 下面的方程式显示了两种可能的 酸-碱 反应，分别针对两种两性的物种，二碳酸盐离子和水： 
  <img alt="Equation1" src="/files/ftp_upload/11396/11396_Equation_1.png" style="height: 95px;" />
 第一个方程表示二碳酸作为水的酸反应为碱，第二个表示二碳酸作为水作为酸的碱的反应。 当向水中添加碳酸氢盐时，这两种平衡都是同时建立的，由此产生的溶液的成分可以通过适当的平衡计算来确定。 在液体状态下，两性的物质的分子可以相互反应，如下方程中的水示例所示： 
  <img alt="Image3" src="/files/ftp_upload/11396/11396_image_3.svg" style="height: 125px;" />
 类似分子对产率离子反应的过程称为自电离。 液态水的电离作用非常小；在 25 °C 时，每十亿个水分子中大约有两个电离。 水自电离过程的范围反映在其平衡常数 (水的离子产物常数) 的值中， KW ： 
  <img alt="Equation2" src="/files/ftp_upload/11396/11396_Equation_2.png" style="height: 50px;" />
 纯水的轻微电离反映在平衡常数的小值中；在 25 °C 时， KW 的值为 1.0 × 10−14。 
  <img alt="Equation3" src="/files/ftp_upload/11396/11396_Equation_3.png" style="height: 50px;" />
 这一过程是吸热，因此电离的范围以及由此产生的水合氢离子和氢氧化物离子浓度随温度的升高而增加。 例如，在 100 °C 时， KW 的值约为 5.6 × 10−13 ，大约比 25 °C 时的值大 50 倍 
 水产率的自电离水合氢离子和氢氧化物离子数量相同。 因此，在 25 °C 的纯水中： 
  <img alt="Equation4" src="/files/ftp_upload/11396/11396_Equation_4.png" style="height: 50px;" />
 这些离子在溶液中的浓度通常是该溶液性质及其其他溶质的化学行为的关键决定因素，并已开发出特定词汇来相对描述这些浓度。 如果溶液中的水合氢离子和氢氧化物离子浓度相等，则它是中性的；如果它中的水合氢离子浓度大于氢氧化物离子，则它是酸性的；如果它中的水合氢离子浓度小于氢氧化物离子，则它是碱性的。 
 酸性，基本和中性溶液的关系概要 
 <table style="margin-left: auto; margin-right: auto;">
 <tbody>
 <tr>
 <td style="color: black; border: 1px solid black; text-align: center;">分类 </td>
 <td style="color: black; border: 1px solid black; text-align: center;">相对离子浓度 </td>
 <td style="color: black; border: 1px solid black; text-align: center;">25 °C 时的 pH 值 </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+] > [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;">中立 </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;">基本 </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>
 本文改编自<a href="https://openstax.org/books/chemistry-2e/pages/14-1-bronsted-lowry-acids-and-bases"> Openstax, 化学 2e, 第14.1节： 布朗斯特-劳里的酸和碱</a> ， 和 <a href="https://openstax.org/books/chemistry-2e/pages/14-2-ph-and-poh"> 第14.2节：pH值和pOH</a>。

布朗斯特-劳里酸与水之间的反应称为酸电离。例如，当氟化氢溶于水并发生电离时，……

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KEY TERMS AND CONCEPTS

关键术语与概念

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

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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...

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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...

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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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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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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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Water as an Amphiprotic Substance

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