Name: stepwise ionization of polyprotic acids
Uploaded: 2020-09-24T15:06:51.000Z
Duration: 3 min 38 s
Channel: JoVE

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North Carolina K-12 Science Standard Course of Study

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Physical Science

Matter and its Interactions

PS.PSc.2.5

Polyprotic Acids

一元酸，像氢氟酸，含有一个可电离的质子。相反，多质子酸含有 两个或两个以上的可电离质子。例如，亚硫酸有两个可电离质子，磷酸有三个。一种多质子酸会依次失去它的每一个质子，每一个反应都有自己的 Ka。从中性分子中除去一个质子比从带负电荷的分子中 除去一个质子更容易，因为负电荷 增加了质子和阴离子之间的 键的强度。因此，对于磷酸而言，去除 第一个质子的 Ka，Ka1，高于去除第二个质子的 Ka，Ka2，而 Ka2 又高于 Ka3。如果 Ka1 比其随后的 Ka 至少大 1000 倍，则仅使用第一个反应就可以估算 多元酸的 pH 值。例如，0.050M 抗坏血酸溶液的 pH 值 可以使用其 Ka1 和 ICE 表来确定。当抗坏血酸溶解在水中时，会分解成水合氢离子和抗坏血酸离子。该反应的 Ka1 为 8×10⁻⁵，它等于水合氢离子浓度 乘以抗坏血酸单阴离子浓度 除以抗坏血酸浓度。用初始浓度和平衡浓度可以 为这个反应准备一个 ICE 表。由于 x 的值很小，0.050 减去 x 大约 等于 0.050。将这些值代入 Ka1 的表达式中，x 的值等于 0.0020M，仅为抗坏血酸初始浓度的 4%因此，该近似是有效的。溶液的 pH 值等于 2.70。在反应的第二步中形成的 抗坏血酸二价阴离子的浓度 也可以用 Ka2 和 ICE 表来计算。Ka2 等于 1.6×10⁻¹²，它可以表示为水合氢离子浓度 乘以抗坏血酸二价阴离子浓度 除以抗坏血酸单阴离子浓度。对于这个反应，ICE 表是用 第一个反应产生的抗坏血酸单阴离子 和水合氢离子的初始浓度 进行填充，0.002M。由于 x 值很小，它可以 从抗坏血酸单阴离子和 水合氢离子的平衡浓度中忽略。将这些值代入 Ka2 表达式 并进行求解，x 为 1.6×10⁻¹²M。因为 x 小于 0.002M 的 5%该近似是有效的。由于在抗坏血酸解离的第二步中 产生的水合氢离子浓度 可以忽略不计，因此 第一个可电离质子决定了 溶液的 pH 值。

多质子酸

酸按照每一分子在反应中可以放弃的质子数量进行分类。 HCL ， HNO3 和 HPN 等酸在每个分子中含有一个可电离氢原子，称为单原子酸。 他们对水的反应如下： 
 <table style="margin-left: auto; margin-right: auto;">
 <tbody>
 <tr>
 <td style="color: black; border: 1px solid black; text-align: center;">单质子酸：与水的反应 </td>
 </tr>
 <tr>
 <td style="color: black; border: 1px solid black; text-align: center;">HCl (aq) + H2O (l) ⟶ H3O+ (aq) + Cl− (aq)</td>
 </tr>
 <tr>
 <td style="color: black; border: 1px solid black; text-align: center;">HNO3 (aq) + H2O (l) ⟶ H3O+ (aq) + NO3− (aq)</td>
 </tr>
 <tr>
 <td style="color: black; border: 1px solid black; text-align: center;">HCN (aq) + H2O (l) ⇌ H3O+ (aq) + CN− (aq)</td>
 </tr>
 </tbody>
 </table>
 尽管它含有四个氢原子，但乙酸 CH3CO2H 也是单质子，因为只有羧基组的氢原子会与碱发生反应： 
 <img alt="Image1" src="/files/ftp_upload/11405/11405_Figure 1.svg" style="height: 125px;" />
 同样，单质子碱是接受单个质子的碱。 
 双质子酸每分子含有两个可电离氢原子；此类酸的电离发生在两个步骤中。 第一次电离的发生范围总是大于第二次电离。 例如，硫酸 (强酸) 的电离如下： 
 <table style="margin-left: auto; margin-right: auto;">
 <tbody>
 <tr>
 <td style="color: black; border: 1px solid black; text-align: center;"></td>
 <td colspan="2" style="color: black; border: 1px solid black; text-align: center;">双质子酸：硫酸 (H2SO4)</td>
 </tr>
 <tr>
 <td style="color: black; border: 1px solid black; text-align: center;">1st 首次电离</td>
 <td style="color: black; border: 1px solid black; text-align: center;">H2SO4 (aq) + H2O (l) ⇌ H3O+ (aq) + HSO4− (aq)</td>
 <td style="color: black; border: 1px solid black; text-align: center;">Ka1 = 102 以上; 完全解离</td>
 </tr>
 <tr>
 <td style="color: black; border: 1px solid black; text-align: center;">2nd 二次电离</td>
 <td style="color: black; border: 1px solid black; text-align: center;">HSO4− (aq) + H2O (l) ⇌ H3O+ (aq) + SO42− (aq)</td>
 <td style="color: black; border: 1px solid black; text-align: center;">Ka2 = 1.2 × 10−2</td>
 </tr>
 </tbody>
 </table>
 所有多质子酸都将执行此分步电离过程。 粘性酸H2CO3是弱双质子酸的一个示例。 碳酸的首次电离会产生少量的水合氢离子和碳酸二酯离子。 
 首次电离: H2CO3 (aq) + H2O (l) ⇌ H3O+ (aq) + HCO3− (aq)
 <img alt="Eq1" src="/files/ftp_upload/11405/11405_Equation_1.png" style="height: 90px;" />
 二碳酸盐离子也可用作酸。 它可以使水合氢离子和碳酸盐离子的电离和形成更小的数量。 
 二次电离: HCO3− (aq) + H2O (l) ⇌ H3O+ (aq) + CO32− (aq)
 <img alt="Eq2" src="/files/ftp_upload/11405/11405_Equation_2.png" style="height: 90px;" />
 KH2CO3 比 KHCO3− 大 104 倍，因此 H2CO3 是溶液中水合氢离子的主要生产者. 这意味着会形成很少的 HCO3− 由电离 H2CO3 离子产生水合氢离子 (和碳酸盐离子)，H3O+ 和 HCO3− 的浓度基本上等同于纯水溶液中的 H2CO3。
 如果弱双质子酸的第一个电离常数大于第二个，系数至少为 20 ，则应分别处理第一个电离并计算其产生的浓度，然后再计算随后电离产生的物种的计算浓度。 这种方法在以下示例练习中得到了证明。 
 双质子酸电离 
 “碳化水”含有一个可调的数量二氧化碳溶解。 由于CO2与水发生反应形成碳酸 H2CO3，因此溶液酸酸性强。
 在初始值为[H2CO3] = 0.033的CO2 饱和溶液中，[H3O+], [HCO3−], 和 [CO32−] 的值是什么？
 <img alt="Eq3" src="/files/ftp_upload/11405/11405_Equation_3.png" style="height: 90px;" />
 如电离常数所示，H2CO3的酸比HCO3−强得多，因此可以单独处理逐步电离反应。 使用提供的信息，为这一第一个电离步骤准备了一个 ICE表格 ： 
 <table style="margin-left: auto; margin-right: auto;">
 <tbody>
 <tr>
 <td style="color: black; border: 1px solid black; text-align: center;"></td>
 <td style="color: black; border: 1px solid black; text-align: center;">H2CO3 (aq) </td>
 <td style="color: black; border: 1px solid black; text-align: center;">H3O+ (aq)</td>
 <td style="color: black; border: 1px solid black; text-align: center;">HCO3− (aq)</td>
 </tr>
 <tr>
 <td style="color: black; border: 1px solid black; text-align: center;">初始浓度 (M)</td>
 <td style="color: black; border: 1px solid black; text-align: center;">0.033</td>
 <td style="color: black; border: 1px solid black; text-align: center;">~0</td>
 <td style="color: black; border: 1px solid black; text-align: center;">0</td>
 </tr>
 <tr>
 <td style="color: black; border: 1px solid black; text-align: center;">变化 (M)</td>
 <td style="color: black; border: 1px solid black; text-align: center;">−x</td>
 <td style="color: black; border: 1px solid black; text-align: center;">+x</td>
 <td style="color: black; border: 1px solid black; text-align: center;">+x</td>
 </tr>
 <tr>
 <td style="color: black; border: 1px solid black; text-align: center;">平衡浓度 (M)</td>
 <td style="color: black; border: 1px solid black; text-align: center;">0.033 − x</td>
 <td style="color: black; border: 1px solid black; text-align: center;">x</td>
 <td style="color: black; border: 1px solid black; text-align: center;">x</td>
 </tr>
 </tbody>
 </table>
 将平衡浓度替换为方程提供的平衡浓度 
 <img alt="Eq4" src="/files/ftp_upload/11405/11405_Equation_4.png" style="height: 90px;" />
 假设x << 0.033并解简化的方程得出 
 <img alt="Eq5" src="/files/ftp_upload/11405/11405_Equation_5.png" style="height: 50px;" />
 ICE表格 将 x 定义为等于碳酸氢离子体积摩尔浓度和水合氢离子体积摩尔浓度： 
 <img alt="Eq6" src="/files/ftp_upload/11405/11405_Equation_6.png" style="height: 90px;" />
 使用上述计算的碳酸氢离子浓度，第二次电离会受到类似的平衡计算： HCO3− (aq) + H2O (l) ⇌ H3O+ (aq) + CO32− (aq)
 <img alt="Eq7" src="/files/ftp_upload/11405/11405_Equation_7.png" style="height: 180px;" />
 总结：处于平衡时 [H2CO3] = 0.033 M; [H3O+] = 1.2 × 10−4; [HCO3−] = 1.2 × 10−4 M; 且 [CO32−] = 4.7 × 10−11 M.
 三质子酸是一种具有三个可电离 H 原子的酸。 磷酸就是一个例子： 
 <table style="margin-left: auto; margin-right: auto;">
 <tbody>
 <tr>
 <td style="color: black; border: 1px solid black; text-align: center;"></td>
 <td colspan="2" style="color: black; border: 1px solid black; text-align: center;">三质子酸：磷酸 (H3PO4)</td>
 </tr>
 <tr>
 <td style="color: black; border: 1px solid black; text-align: center;">1st 首次电离</td>
 <td style="color: black; border: 1px solid black; text-align: center;">H3PO4 (aq) + H2O (l) ⇌ H3O+ (aq) + H2PO4− (aq) </td>
 <td style="color: black; border: 1px solid black; text-align: center;">Ka1 = 7.5 × 10−3</td>
 </tr>
 <tr>
 <td style="color: black; border: 1px solid black; text-align: center;">2nd 二次电离</td>
 <td style="color: black; border: 1px solid black; text-align: center;">H2PO4− (aq) + H2O (l) ⇌ H3O+ (aq) + HPO42− (aq)</td>
 <td style="color: black; border: 1px solid black; text-align: center;">Ka2 = 6.2 × 10−8</td>
 </tr>
 <tr>
 <td style="color: black; border: 1px solid black; text-align: center;">3rd 三次电离</td>
 <td style="color: black; border: 1px solid black; text-align: center;">HPO42− (aq) + H2O (l) ⇌ H3O+ (aq) + PO43− (aq)</td>
 <td style="color: black; border: 1px solid black; text-align: center;">Ka3 = 4.2 × 10−13</td>
 </tr>
 </tbody>
 </table>
 至于双质子酸示例，每一个连续电离反应都比前者的外延的小，反映在逐步酸电离常数的数值下降中。 这是多质子酸的一个一般特性，连续电离常数通常相差约 105 到 106。 
 这组三种离解反应似乎使 H3PO4 溶液中的平衡浓度计算变得复杂。 但是，由于连续电离常数相差 105 到 106，因此伴随电离反应的浓度小变化中存在很大差异。 这允许使用数学简化的假设和流程，如以上示例所示。 多质子碱能够接受多个氢离子。 碳酸盐离子是双质子碱的一个示例，因为它可以接受两个质子，如下所示。 与多质子酸的情况类似，请注意电离常数随着电离步骤的增加而减少。 同样，涉及多质子碱的平衡计算遵循与多质子酸相同的方法。 
 <img alt="Eq8" src="/files/ftp_upload/11405/11405_Equation_8.png" style="height: 90px;" />
 本文改编自<a href="https://openstax.org/books/chemistry-2e/pages/14-5-polyprotic-acids"> Openstax, 化学 2e, 第14.5节：多质子酸</a>。

酸根据每分子在反应中能够释放的质子数量进行分类。例如 HCl、HNO₃ 和 HCN 等含有一个质子的酸……

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KEY TERMS AND CONCEPTS

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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 reacts with bases. Monoprotic bases accept a single proton.

Diprotic acids have two ionizable hydrogen atoms, so they ionize in two steps. The first ionization is greater than the second. Sulfuric acid is a strong example, with complete first dissociation and a much smaller second dissociation. Carbonic acid, H2CO3, is a weak diprotic acid with the same stepwise pattern. Its first ionization forms hydronium and bicarbonate ions, and its second ionization converts bicarbonate into carbonate in even smaller amounts.

For carbonic acid, the first ionization constant is much larger than the second. In a pure aqueous solution of H2CO3, H3O+ and HCO3− are practically equal, while only a tiny amount of CO3 2− forms. When the first ionization constant is at least 20 times larger than the second, the two steps can be treated separately in equilibrium calculations. The example of saturated CO2 in water uses an ICE table for the first step, then a second equilibrium calculation for bicarbonate.

At equilibrium in that example, [H2CO3] is 0.033 M, [H3O+] is 1.2 × 10−4 M, [HCO3−] is 1.2 × 10−4 M, and [CO3 2−] is 4.7 × 10−11 M. This shows why the first ionization dominates the solution. It also shows how stepwise calculations can simplify polyprotic acid problems.

Triprotic acids have three ionizable hydrogen atoms. Phosphoric acid, H3PO4, is one example. Its three ionization steps produce H2PO4−, then HPO4 2−, and finally PO4 3−. Each successive ionization is less extensive than the one before it. The stepwise acid ionization constants decrease, often by a factor of about 10^5 to 10^6.

That large difference in ionization constants makes equilibrium work easier. Even though a triprotic acid has three dissociation reactions, the small concentration changes can still be handled with simplifying assumptions. Polyprotic bases follow the same idea. The carbonate ion is a diprotic base because it can accept two protons, and its ionization constants also decrease with each step.

Stepwise Ionization of Polyprotic Acids

bronsted-lowry acids and bases

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strong acid and base solutions

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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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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...

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key terms and concepts

Explore how monoprotic, diprotic, and triprotic acids ionize in steps, plus how polyprotic bases accept protons and how to use ICE tables.

Stepwise Ionization of Polyprotic Acids | 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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When a liquid vaporizes in a closed container, gas molecules cannot escape. As these gas phase molecules move randomly about, they will occasionally ...

Clausius-Clapeyron Equation

The equilibrium between a liquid and its vapor depends on the temperature of the system; a rise in temperature causes a corresponding rise in the vapor ...

Phase Transitions: Melting and Freezing

Heating a crystalline solid increases the average energy of its atoms, molecules, or ions, and the solid gets hotter. At some point, the added energy ...

phase-transitions-sublimation-and-deposition

Phase Transitions: Sublimation and Deposition

Some solids can transition directly into the gaseous state, bypassing the liquid state, via a process known as sublimation. At room temperature and ...

Heating and Cooling Curves

When a substance&#8212;isolated from its environment&#8212;is subjected to heat changes, corresponding changes in temperature and phase of the substance ...

Phase Diagrams

A phase diagram combines plots of pressure versus temperature for the liquid-gas, solid-liquid, and solid-gas phase-transition equilibria of a substance. ...

Use models to compare the phases of matter and the physical changes they undergo.

PS.PSc.1.2

physical-and-chemical-properties-of-matter

Physical and Chemical Properties of Matter

The characteristics that enable us to distinguish one substance from another are called properties.

Physical Properties of Matter

A physical property is ...

molecular-comparison-of-gases-liquids-and-solids

Molecular Comparison of Gases, Liquids, and Solids

Particles in a solid are tightly packed together (fixed shape) and often arranged in a regular pattern; in a liquid, they are close together with no ...

Intermolecular vs Intramolecular Forces

Intermolecular forces (IMF) are electrostatic attractions arising from charge-charge interactions between molecules. The strength of the intermolecular ...

Intermolecular Forces

Atoms and molecules interact through bonds (or forces): intramolecular and intermolecular. The forces are electrostatic as they arise from interactions ...

comparing-intermolecular-forces-melting-point-boiling-point

Comparing Intermolecular Forces: Melting Point, Boiling Point, and Miscibility

Intermolecular forces are attractive forces that exist between molecules. They dictate several bulk properties, such as melting points, boiling points, ...

Carry out investigations to compare physical and chemical properties of matter.

PS.PSc.1.3

The Atomic Theory of Matter

The earliest recorded discussion of the basic structure of matter comes from ancient Greek philosophers. Leucippus and Democritus argued that all matter ...

Subatomic Particles

Dalton was only partially correct about the particles that make up matter. All matter is composed of atoms, and atoms are composed of three smaller ...

The Bohr Model

Following the work of Ernest Rutherford and his colleagues in the early twentieth century, the picture of atoms consisting of tiny dense nuclei surrounded ...

Emission Spectra

When solids, liquids, or condensed gases are heated sufficiently, they radiate some of the excess energy as light. Photons produced in this manner have a ...

The de Broglie Wavelength

In the macroscopic world, objects that are large enough to be seen by the naked eye follow the rules of classical physics. A billiard ball moving on a ...

The Uncertainty Principle

Werner Heisenberg considered the limits of how accurately one can measure properties of an electron or other microscopic particles. He determined that ...

Use models to interpret the data presented in Bohr diagrams and electron dot diagrams for neutral atoms of elements 1 through 18.

PS.PSc.1.4

Ions and Ionic Charges

In ordinary chemical reactions, the nucleus &mdash; which contains the protons and neutrons of each atom and thus identifies the element &mdash; remains ...

Use models to compare representations of atoms, ions, and isotopes.

PS.PSc.1.5

The Periodic Table

As early chemists discovered more elements, they realized that various elements could be grouped by their similar chemical behaviors. One such grouping ...

Periodic Classification of the Elements

The periodic table arranges atoms based on increasing atomic number so that elements with the same chemical properties recur periodically. When their ...

atomic-radii-and-effective-nuclear-charge

Atomic Radii and Effective Nuclear Charge

The elements in groups of the periodic table exhibit similar chemical behavior. This similarity occurs because the members of a group have the same number ...

Ionic Radii

Ionic radius is the measure used to describe the size of an ion. A cation always has fewer electrons and the same number of protons as the parent atom; it ...

Ionization Energy

The amount of energy required to remove the most loosely bound electron from a gaseous atom in its ground state is called its first ionization energy ...

Electron Affinity

The electron affinity (EA) is the energy change for adding an electron to a gaseous atom to form an anion (negative ion).



This process can be either ...

Alkali Metals

Group 1 elements are soft and shiny metallic solids. They are malleable, ductile, and good conductors of heat and electricity. The melting points of the ...

Halogens

Group 17 elements, known as halogens, are nonmetals. At room temperature, fluorine and chlorine are gases, bromine is a liquid, and iodine a solid. ...

Noble Gases


The elements in group 18 are noble gases (helium, neon, argon, krypton, xenon, and radon). They earned the name &ldquo;noble&rdquo; because they were ...

Use the Periodic Table as a model to predict the relative properties (metallic/nonmetallic character, ionic charge, and reactivity) and arrangement of elements based on the pattern of valence electrons in the outermost energy levels of atoms.

PS.PSc.1.6

Types of Chemical Bonds

Chemical bonding theories were pioneered by American chemist Gilbert&#xA0;N. Lewis. He developed a model called the Lewis model to explain the type and ...

Ionic Bonding and Electron Transfer

Ions are atoms or molecules bearing an electrical charge. A cation (a positive ion) forms when a neutral atom loses one or more electrons from its valence ...

Covalent Bonding and Lewis Structures

Compared to ionic bonds, which results from the transfer of electrons between metallic and nonmetallic atoms, covalent bonds result from the mutual ...

Bonding in Metals

Metallic bonds are formed between two metal atoms. A simplified model to describe metallic bonding has been developed by Paul Dr&#xFC;de called the ...

Construct an explanation to classify the type of chemical bond that occurs (covalent, ionic, or metallic) in a given substance.

PS.PSc.2.1

Nomenclature of Alkanes

In the late 19th-century, the number of new chemical compounds discovered increased tremendously. Hence, the necessity arose to develop a naming system ...

Nomenclature of Alkenes

The IUPAC naming system for alkenes replaces -an- with -en- in the corresponding parent alkanes. Accordingly, a simple alkene replaces the -ane suffix of ...

Nomenclature of Alkynes

Alkynes are unsaturated hydrocarbons characterized by the presence of carbon-carbon triple bonds and have a general formula CnH2n-2. The nomenclature of ...

structure-and-nomenclature-of-alcohols-and-phenols

Structure and Nomenclature of Alcohols and Phenols

Overview 
Alcohols are one of the most important functional groups in organic chemistry. The name of alcohol comes from the hydrocarbon from which it is ...

Structure and Nomenclature of Ethers

Structure and Bonding
Ethers are organic compounds with an ether functional group which is characterized by an oxygen atom connected to two &#8212; ...

IUPAC Nomenclature of Aldehydes

Aldehydes are named based on the systematic nomenclature rules set by the IUPAC. For acyclic aldehydes, the longest carbon chain containing the aldehydic ...

IUPAC Nomenclature of Ketones

Like aldehydes, ketones are named using IUPAC rules; in this case, by replacing &#8220;e&#8221; in the name of the longest hydrocarbon chain with ...

IUPAC Nomenclature of Carboxylic Acids

IUPAC names of carboxylic acids are systematically derived following a few rules discussed below.
For acyclic saturated monocarboxylic acids, the longest ...

Use models to apply International Union of Pure and Applied Chemistry (IUPAC) conventions to name and write formulas for simple compounds.

PS.PSc.2.2

Understand types, properties, and structure of matter.

Stepwise Ionization of Polyprotic Acids

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