Name: acid and base strength in water
Uploaded: 2025-10-30T13:58:47.000Z
Duration: 3 min 2 s
Channel: JoVE

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Acid/Base Strengths and Dissociation Constants

酸和碱可以根据它们是 强酸、强碱、弱酸 还是弱碱来分类。强酸和强碱很少，所以大部分的酸和碱都很弱。强酸，如盐酸，在水中溶解时完全分解成氢离子 和氯离子。强碱，如氢氧化钠，完全分解成钠离子和氢氧根离子。弱酸和弱碱部分解离，同时以电离和非电离的形式存在。例如，醋酸和它的弱共轭碱，醋酸盐，都存在于水溶液中。弱酸或弱碱的离解程度，可以用它的平衡常数来测量。弱酸的平衡常数 有一个特殊的名字，酸离解常数，或 Ka。对于一个一般的弱酸 HA，给定温度下的 Ka 可以通过平衡方程进行计算：将产物 A 离子和氢离子的浓度 除以反应物 HA 和水的浓度。由于水是液体，其浓度 在反应过程中几乎保持不变，因此不包括在方程式中。Ka 值越高，酸越强。亚硝酸比醋酸强，因为亚硝酸的 Ka 比 醋酸的 Ka 大。弱碱的平衡常数，碱 离解常数，或 Kb，作用方式与 Ka 类似。对于一个一般的弱碱 B，给定温度下的 Kb 可以通过将产物 BH 离子和氢氧根离子的浓度 除以反应物 B 的浓度来 确定。与酸一样，碱的强度 也与 Kb 成正比。例如，乙胺比尿素相对较强，因为乙胺的 Kb 大于尿素的 Kb。酸的强度也可以用电离 百分数来表示。酸的电离百分数可以 用平衡时的氢离子浓度 除以初始酸浓度，再乘以一百来计算。类似地，碱的电离百分数 可以用平衡时的氢氧根离子浓度 除以碱的初始浓度，再乘以一百来计算。电离百分数越高，酸或碱越强。

酸碱强度和解离常数

当溶解在水中时，酸或碱的相对强度是其电离的程度。 如果电离反应基本上是完整的，则该酸或碱被称为强；如果电离相对较少，则该酸或碱弱。 相比强的来说，有更多的弱酸和碱。 下面列出了最常见的强酸和碱： 
 <table style="margin-left: auto; margin-right: auto;">
 <tbody>
 <tr>
 <td style="color: black; border: 1px solid black; text-align: center;">强酸 </td>
 <td style="color: black; border: 1px solid black; text-align: center;">强碱 </td>
 </tr>
 <tr>
 <td style="color: black; border: 1px solid black; text-align: center;">HClO4</td>
 <td style="color: black; border: 1px solid black; text-align: center;">LiOH </td>
 </tr>
 <tr>
 <td style="color: black; border: 1px solid black; text-align: center;">HCl </td>
 <td style="color: black; border: 1px solid black; text-align: center;">NaOH </td>
 </tr>
 <tr>
 <td style="color: black; border: 1px solid black; text-align: center;">HBr </td>
 <td style="color: black; border: 1px solid black; text-align: center;">KOH</td>
 </tr>
 <tr>
 HI <td style="color: black; border: 1px solid black; text-align: center;"></td>
 <td style="color: black; border: 1px solid black; text-align: center;">Ca(OH) 2</td>
 </tr>
 <tr>
 <td style="color: black; border: 1px solid black; text-align: center;">HNO3</td>
 <td style="color: black; border: 1px solid black; text-align: center;">SR (OH) 2 </td>
 </tr>
 <tr>
 <td style="color: black; border: 1px solid black; text-align: center;">H2SO4. </td>
 <td style="color: black; border: 1px solid black; text-align: center;">BA (OH) 2</td>
 </tr>
 </tbody>
 </table>
 酸的相对强度可以通过在水溶液中测量其平衡常数来量化。 在同一浓度的溶液中，酸离子的电离强度越强，因此产率的水合氢离子浓度比弱酸高。 一种酸的平衡常数称为酸离子常数 Ka。 对于酸 HA 的反应：   
 <img alt="Eq1" src="/files/ftp_upload/11395/11395eq1.png" style="text-align: center; height: 50px;" />
 酸离子常数的写为  
 <img alt="Eq2" src="/files/ftp_upload/11395/11395eq2.png" style="text-align: center; height: 90px;" />
 其中浓度是均衡的。 虽然水是反应中的反应物，但也是溶剂，因此我们不在方程中包括 [H2O]。 酸的Ka越大，H3O+ 和 A−的浓度相对于平衡混合物中的非电离酸浓度越大，酸越强。 当≈∞完全电离时，一种酸被分类为“强”，在这种情况下， HA 的浓度为零，酸电离常数为不可测量的大 (Ka )。 部分电离的酸被称为“弱”，其酸性电离常数可以通过实验测定。 
 为了说明这一想法，下面显示了三个酸电离方程和kA值。 电离常数从所列方程式的第一个增加到最后一个，表示相对酸强按 CH3CO2H < HNO2 < HSO4− 的顺序增加。
 <img alt="Eq3-5" src="/files/ftp_upload/11395/11395eq3-5.png" style="text-align: center; height: 135px;" />
 另一种衡量酸强度的方法是其电离百分比。 弱酸的电离百分比是根据平衡混合物的成分定义的：  
 <img alt="Eq6" src="/files/ftp_upload/11395/11395eq6.png" style="text-align: center; height: 90px;" />
 其中分子相当于酸偶联物碱的浓度 (根据化学计量法， [a− ] = [H3O+])。 与 Ka 值不同，弱酸的电离百分比随酸的初始浓度而变化，通常随着浓度的增加而下降。 
 就像酸一样，碱的相对强度反映在水溶液中其基电离常数 (KB) 的幅度中。 在同一浓度的溶液中，碱离子化程度越高，因此产率比弱碱的氢氧化物离子浓度更高。 强度较高的碱的电离常数大于强度较低的碱。 对于碱， B 的反应：  
 <img alt="Eq7" src="/files/ftp_upload/11395/11395eq7.png" style="text-align: center; height: 50px;" />
 电离常数被写入为  
 <img alt="Eq8" src="/files/ftp_upload/11395/11395eq8.png" style="text-align: center; height: 90px;" />
 下面对三种弱碱的数据进行的检查显示，碱强度的增加顺序为 NO2− < CH2 CO2− < NH3。 
 <img alt="Eq9-11" src="/files/ftp_upload/11395/11395eq9-11.png" style="text-align: center; height: 135px;" />
 对于酸，碱的相对强度也反映在其百分比电离中，计算为 
 <img alt="Eq12" src="/files/ftp_upload/11395/11395eq12.png" style="text-align: center; height: 90px;" />
 但会因碱电离常数和溶液的初始浓度而异。 
 本文改编自<a href="https://openstax.org/books/chemistry-2e/pages/14-3-relative-strengths-of-acids-and-bases"> Openstax, 化学 2e, 第14.3节：酸和碱的相对强度。 </a>

酸或碱的相对强度是指其在水中溶解时发生电离的程度。如果电离反应基本完全，……

Education

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JoVE Core

Chemistry

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Acids and Bases

酸与碱

KEY TERMS AND CONCEPTS

关键术语与概念

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.

Acid strength can be measured with the acid-ionization constant, Ka. For an acid HA in water, Ka compares the concentrations of the products and the nonionized acid at equilibrium. Water is a reactant in the reaction, but it is not included in the equation because it is the solvent. A larger Ka means more H3O+ and A− relative to HA, so the acid is stronger.

A strong acid is considered completely ionized. In that case, the concentration of HA is zero, and Ka is immeasurably large, or approximately infinite. Weak acids ionize only partially, so their Ka values can be measured in the lab. The relative acid strengths in the examples increase in the order CH3CO2H &lt; HNO2 &lt; HSO4−.

Percent ionization is another way to describe weak acid strength. It is based on the amount of acid that ionizes at equilibrium. For a weak acid, the percent ionization changes with the starting concentration and usually gets smaller as the concentration gets higher. This makes percent ionization different from Ka.

Base strength is measured in a similar way with the base-ionization constant, Kb. For a base B in water, a larger Kb means the base ionizes more and produces a higher hydroxide ion concentration. A stronger base has a larger ionization constant than a weaker base.

The examples of weak bases show the strength order NO2− &lt; CH2CO2− &lt; NH3. Base strength can also be described with percent ionization. As with weak acids, this value depends on both the base-ionization constant and the initial concentration of the solution.

Acid and Base Strength in Water

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...

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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...

strong acid and base solutions

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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/base strengths and dissociation constants

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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key terms and concepts

Learn how acid and base strength in water depends on ionization, Ka, Kb, and percent ionization, with examples of strong and weak acids and bases.

Acid and Base Strength in Water | JoVE K12

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JoVE Core 化学通过简单明了且通俗易懂的动画视频课程，将普通化学概念带入到现实生活当中。此外，在科学家行动视频中，我们演示了在实验室中进行的经典和原始研究实验。

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New York State Science Learning Standards

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SC6.

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Acid and Base Strength in Water

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