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

Acides monoprotiques, comme l'acide fluorhydrique, contiennent un seul proton ionisable. En revanche, les acides polyprotiques contiennent deux ou plus de protons ionisables. Par exemple, l'acide sulfureux a deux protons ionisables, et l'acide phosphorique en a trois.Un acide polyprotique perd chacun de ses protons séquentiellement, et chaque réaction a son propre Ka.Il est plus facile d'éliminer un proton d'une molécule neutre que celle chargée négative parce que la charge négative augmente la force de la liaison entre le proton et l'anion. Par conséquent, pour l'acide phosphorique, le Ka pour l'élimination du premier proton, Ka1, est supérieur au deuxième, Ka2, qui est supérieur à Ka3. Le pH d'un acide polyprotique peut être estimé en utilisant uniquement la première réaction si Ka1 est au moins mille fois plus grand que ses Ka ultérieurs.Par exemple, le pH d'une solution d'acide ascorbique de 0, 050 molaire peut être déterminée à l'aide de son Ka1 et d'une table ICE. Lorsqu'il est dissous dans l'eau, l'acide ascorbique se dissocie en ions hydronium et ascorbate. Le Ka1 pour cette réaction est de 8 10⁻⁵, et il est égal à la concentration d'hydronium fois la concentration de monoanion ascorbate, divisée par la concentration d'acide ascorbique.Une table ICE peut être préparée pour cette réaction avec les concentrations initiale et d'équilibre. En raison de la faible valeur de x, 0, 050 moins x est approximativement égal à 0, 050. En remplaçant ces valeurs dans l'expression de Ka1, la valeur de x est égale à 0, 0020 molaire, qui ne représente que 4%de la concentration initiale d'acide ascorbique.Ainsi, l'approximation est valide. Le pH de la solution est égal à 2, 70. La concentration de dianion ascorbate formé dans la deuxième étape de la réaction peut également être calculé à l'aide de Ka2 et d'une table ICE.Ka2 est égal à 1, 6 10⁻¹², et il peut être exprimé comme la concentration d'hydronium fois la concentration de dianion ascorbate divisée par la concentration de monoanion ascorbate. Pour cette réaction, la table ICE est remplie avec la concentration initiale de monoanion ascorbate et d'hydronium dès la première réaction, 0, 002 molaire. En raison de la faible valeur de x, il peut être omis des concentrations d'équilibre de monoanion ascorbate et d'hydronium.Après avoir remplacé ces valeurs dans l'expression Ka2 et en résolvant, x est de 1, 6 10⁻¹² molaire. Puisque x est inférieur à 5%de 0, 002 molaire, l'approximation est valide. Comme la concentration d'ions hydronium qui ont été générés lors de la deuxième étape de dissociation de l'acide ascorbique est négligeable, le premier proton ionisable détermine le pH de la solution.

polyacides

Les acides sont classés selon le nombre de protons par molécule qu'ils peuvent céder dans une réaction. Les acides tels que HCl, HNO3 et HCN qui contiennent un atome d'hydrogène ionisable dans chaque molécule sont appelés des acides monoprotiques. Leurs réactions avec l'eau sont : 
 <table style="margin-left: auto; margin-right: auto;">
 <tbody>
 <tr>
 <td style="color: black; border: 1px solid black; text-align: center;">Acides monoprotiques : réactions avec l'eau </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>
 Même s'il contient quatre atomes d'hydrogène, l'acide acétique CH3CO2H est également un acide monoprotique car seul l'atome d'hydrogène du groupe carboxyle (COOH) réagit avec les bases : 
 <img alt="Image1" src="/files/ftp_upload/11405/11405_Figure 1.svg" style="height: 125px;" />
 De même, les bases monoprotiques sont des bases qui acceptent un seul proton. 
 Les acides diprotiques contiennent deux atomes d'hydrogène ionisables par molécule ; l'ionisation de ces acides se produit en deux étapes. La première ionisation a toujours lieu dans une plus grande mesure que la seconde ionisation. Par exemple, l'acide sulfurique, un acide fort, s'ionise comme suit : 
 <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;">Acide diprotique : acide sulfurique (H2SO4) </td>
 </tr>
 <tr>
 <td style="color: black; border: 1px solid black; text-align: center;">1re ionisation </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 = plus de 102 ; dissociation totale </td>
 </tr>
 <tr>
 <td style="color: black; border: 1px solid black; text-align: center;">2nde ionisation </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>
 Ce processus d'ionisation par étapes se produit pour tous les acides polyprotiques. L'acide carbonique, H2CO3, est un exemple d’acide diprotique faible. La première ionisation de l'acide carbonique produit des ions hydronium et des ions bicarbonate en petites quantités. 
 Première ionisation : H2CO3 (aq) + H2O (l) ⇌ H3O+ (aq) + HCO3− (aq) 
 <img alt="Eq1" src="/files/ftp_upload/11405/11405_Equation_1.png" style="height: 90px;" />
 L'ion bicarbonate peut également agir comme un acide. Il s'ionise et forme des ions hydronium et des ions carbonate en quantités encore plus petites. 
 Seconde ionisation : HCO3− (aq) + H2O (l) ⇌ H3O+ (aq) + CO32− (aq) 
 <img alt="Eq2" src="/files/ftp_upload/11405/11405_Equation_2.png" style="height: 90px;" />
 KH2CO3 est plus grand que KHCO3− par un facteur de 104, donc H2CO3 est le principal producteur d'ion hydronium dans la solution. Cela signifie que peu de HCO3− formé par l'ionisation de H2CO3 s'ionise pour donner des ions hydronium (et des ions carbonate), et les concentrations de H3O+ et HCO3&#8722 sont pratiquement égales dans une solution aqueuse pure de H2CO3. 
 Si la première constante d'ionisation d'un acide diprotique faible est plus grande que la seconde par un facteur d'au moins 20, il convient de traiter la première ionisation séparément et de calculer les concentrations qui en résultent avant de calculer les concentrations d'espèces qui résultent de l'ionisation suivante. Cette approche est démontrée dans l'exemple suivant. 
 Ionisation d'un acide diprotique 
 “ L'eau gazeuse ” contient une quantité convenable de dioxyde de carbone dissous. La solution est acide parce que le CO2 réagit avec l'eau pour former de l'acide carbonique, H2CO3. Que sont [H3O+], [HCO3−] et [CO32−] dans une solution saturée de CO2 avec une [H2CO3] initiale = 0,033 ? 
 <img alt="Eq3" src="/files/ftp_upload/11405/11405_Equation_3.png" style="height: 90px;" />
 Comme l'indiquent les constantes d'ionisation, H2CO3 est un acide beaucoup plus fort que HCO3−, de sorte que les réactions d'ionisation par étapes peuvent être traitées séparément. À l'aide des informations fournies, on prépare un tableau ICE pour cette première étape d'ionisation : 
 <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;">Concentration initiale (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;">Changement (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;">Concentration à l'équilibre (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>
 Le remplacement des concentrations à l'équilibre dans l'équation de l'équilibre donne 
 <img alt="Eq4" src="/files/ftp_upload/11405/11405_Equation_4.png" style="height: 90px;" />
 En supposant que x << 0,033 et en résolvant l'équation simplifiée donne 
 <img alt="Eq5" src="/files/ftp_upload/11405/11405_Equation_5.png" style="height: 50px;" />
 Le tableau ICE définit x comme étant égal à la molarité de l'ion bicarbonate et à la molarité de l'ion hydronium : 
 <img alt="Eq6" src="/files/ftp_upload/11405/11405_Equation_6.png" style="height: 90px;" />
 En utilisant la concentration en ions bicarbonate calculée ci-dessus, la seconde ionisation est soumise à un calcul d'équilibre semblable : HCO3− (aq) + H2O (l) ⇌ H3O+ (aq) + CO32− (aq) 
 <img alt="Eq7" src="/files/ftp_upload/11405/11405_Equation_7.png" style="height: 180px;" />
 En résumé : à l'équilibre [H2CO3] = 0,033 M ; [H3O+] = 1,2 × 10−4 ; [HCO3−] = 1,2 × 10−4 M ; et [CO32−] = 4,7 × 10−11 M. 
 Un acide triprotique est un acide qui possède trois atomes H ionisables. L'acide phosphorique en est un exemple : 
 <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;">Acide triprotique : acide phosphorique (H3PO4) </td>
 </tr>
 <tr>
 <td style="color: black; border: 1px solid black; text-align: center;">1re ionisation </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;">2e ionisation </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;">3e ionisation </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>
 Comme pour les exemples d’acides diprotiques, chaque réaction d'ionisation successive est moins étendue que la première, ce qui se reflète dans les valeurs décroissantes des constantes d'ionisation acide progressives. Il s'agit d'une caractéristique générale des acides polyprotiques et les constantes d'ionisation successives diffèrent souvent par un facteur d'environ 105 à 106. 
 Cet ensemble de trois réactions de dissociation peut sembler compliquer les calculs des concentrations à l'équilibre dans une solution de H3PO4. Cependant, comme les constantes d'ionisation successives diffèrent d'un facteur allant de 105 à 106, il existe de grandes différences dans les petits changements de concentration qui accompagnent les réactions d'ionisation. Cela permet d'utiliser des suppositions et des processus de simplification mathématique, comme le montrent les exemples ci-dessus. Les bases polyprotiques sont capables d'accepter plus d'un ion hydrogène. L'ion carbonate est un exemple de base diprotique, car il peut accepter deux protons, comme illustré ci-dessous. Comme dans le cas des acides polyprotiques, notez que les constantes d'ionisation diminuent avec l'étape d'ionisation. De même, les calculs d'équilibre impliquant des bases polyprotiques suivent les mêmes approches que ceux des acides polyprotiques. 
 <img alt="Eq8" src="/files/ftp_upload/11405/11405_Equation_8.png" style="height: 90px;" />
 Ce texte est adapté de<a href="https://openstax.org/books/chemistry-2e/pages/14-5-polyprotic-acids"> Openstax, Chimie 2e, Section 14.5: Acides polyprotiques</a>.

Polyacides

Les acides sont classés en fonction du nombre de protons par molécule qu’ils peuvent céder lors d’une réaction. Les acides tels que HCl, HNO3 et HCN qui contiennent un ...

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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

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polyprotic acids

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

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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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