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

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Polyprotic Acids

Los ácidos monopróticos, como el ácido fluorhídrico, contienen un solo protón ionizable. Por el contrario, los ácidos polipróticos contienen dos o más protones ionizables. Por ejemplo, el ácido sulfuroso tiene dos protones ionizables y el ácido fosfórico tiene tres.Un ácido poliprótico pierde cada uno de sus protones secuencialmente y cada reacción tiene su propio Ka.Es más fácil eliminar un protón de una molécula neutra que de una cargada negativamente porque la carga negativa aumenta la fuerza del enlace entre el protón y el anión. Por lo tanto, para el ácido fosfórico, el Ka para la eliminación del primer protón, Ka1, es más alto que el del segundo, Ka2, que es más alto que Ka3. El pH de un ácido poliprótico se puede calcular usando solo la primera reacción si Ka1 es al menos mil veces mayor que sus Ka posteriores.Por ejemplo, el pH de una solución de ácido ascórbico 0, 050 molar se puede determinar usando su Ka1 y una tabla ICE. Cuando se disuelve en agua, el ácido ascórbico se disocia en iones de hidronio y ascorbato. El Ka1 para esta reacción es 8 10⁻⁵, y es igual a la concentración de hidronio multiplicada por la concentración de monoanión de ascorbato, dividida por la concentración de ácido ascórbico Se puede preparar una tabla ICE para esta reacción con las concentraciones inicial y de equilibrio.Debido al pequeño valor de x, 0, 050 menos x es aproximadamente igual a 0, 050. Si se sustituyen estos valores en la expresión de Ka1, el valor de x es igual a 0, 0020 molar, que es sólo el 4 porciento de la concentración inicial de ácido ascórbico. Por lo tanto, la aproximación es válida.El pH de la solución es igual a 2, 70. La concentración de dianión de ascorbato formada en el segundo paso de la reacción también se puede calcular usando Ka2 y una tabla ICE. Ka2 es igual a 1, 6 10⁻¹², y se puede expresar como la concentración de hidronio multiplicada por la concentración de dianión ascorbato dividida por la concentración de monoanión ascorbato.Para esta reacción, la tabla ICE se completa con la concentración inicial de ascorbato monoanión e hidronio de la primera reacción, 0, 002 molar. Debido al pequeño valor de x, se puede omitir de las concentraciones de equilibrio de ascorbato monoanión e hidronio. Después de sustituir estos valores en la expresión Ka2 y resolver, x es 1, 6 10⁻¹² molar.Dado que x es menos del 5 de 0, 002 molar, la aproximación es válida. Como la concentración de iones de hidronio que se generó durante el segundo paso de la disociación del ácido ascórbico es insignificante, el primer protón ionizable determina el pH de la solución.

Ácidos polipróticos

Los ácidos se clasifican por el número de protones por molécula que pueden ceder en una reacción. Los ácidos como HCl, HNO3 y HCN que contienen un átomo de hidrógeno ionizable en cada molécula se denominan ácidos monopróticos. Sus reacciones con el agua son:
 <table style="margin-left: auto; margin-right: auto;">
 <tbody>
 <tr>
 <td style="color: black; border: 1px solid black; text-align: center;">Ácidos monoproticos: Reacciones con el agua</td>
 </tr>
 <tr>
 <td style="color: black; border: 1px solid black; text-align: center;">HCl (ac) + H2O (l) ⟶ H3O+ (ac) + Cl− (ac)</td>
 </tr>
 <tr>
 <td style="color: black; border: 1px solid black; text-align: center;">HNO3 (ac) + H2O (l) ⟶ H3O+ (ac) + NO3− (ac)</td>
 </tr>
 <tr>
 <td style="color: black; border: 1px solid black; text-align: center;">HCN (ac) + H2O (l) ⇌ H3O+ (ac) + CN− (ac)</td>
 </tr>
 </tbody>
 </table>
 Aunque contiene cuatro átomos de hidrógeno, el ácido acético, CH3CO2H, también es monoprótico porque sólo el átomo de hidrógeno del grupo carboxilo (COOH) reacciona con las bases:
 <img alt="Image1" src="/files/ftp_upload/11405/11405_Figure 1.svg" style="height: 125px;" />
 De manera similar, las bases monopróticas son bases que aceptarán un solo protón.
 Los ácidos dipróticos contienen dos átomos de hidrógeno ionizables por molécula; la ionización de dichos ácidos ocurre en dos pasos. La primera ionización siempre tiene lugar en mayor medida que la segunda ionización. Por ejemplo, el ácido sulfúrico, un ácido fuerte, se ioniza de la siguiente manera:
 <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;">Ácido diprotico: Ácido sulfúrico (H2SO>4)</td>
 </tr>
 <tr>
 <td style="color: black; border: 1px solid black; text-align: center;">1eraionización</td>
 <td style="color: black; border: 1px solid black; text-align: center;">H2SO4 (ac) + H2O (l) ⇌ H3O+ (ac) + HSO4− (ac) </td>
 <td style="color: black; border: 1px solid black; text-align: center;">Ka1 = Más de 102; disociación completa</td>
 </tr>
 <tr>
 <td style="color: black; border: 1px solid black; text-align: center;">2daionización</td>
 <td style="color: black; border: 1px solid black; text-align: center;">HSO4− (ac) + H2O (l) ⇌ H3O+ (ac) + SO42− (ac) </td>
 <td style="color: black; border: 1px solid black; text-align: center;">Ka2 = 1,2 × 10−2 </td>
 </tr>
 </tbody>
 </table>
 Este proceso de ionización escalonado ocurre para todos los ácidos polipróticos. El ácido carbónico, H2CO3, es un ejemplo de un ácido diprotico débil. La primera ionización del ácido carbónico produce iones hidronio e iones bicarbonato en pequeñas cantidades.
 Primera ionización: H2CO3 (ac) + H2O (l) ⇌ H3O+ (ac) + HCO3− (ac)
 <img alt="Eq1" src="/files/ftp_upload/11405/11405_Equation_1.png" style="height: 90px;" />
 El ion bicarbonato también puede actuar como un ácido. Ioniza y forma iones hidronio e iones carbonato en cantidades aún más pequeñas.
 Segunda ionización: HCO3− (ac) + H2O (l) ⇌ H3O+ (ac) + CO32− (ac)
 <img alt="Eq2" src="/files/ftp_upload/11405/11405_Equation_2.png" style="height: 90px;" />
 KH2CO3 es mayor que KHCO3− por un factor de 104, por lo que H2CO3 es el productor dominante de iones de hidronio en la solución. Esto significa que poco de los HCO3− formados por la ionización de H2CO3 ioniza para dar iones hidronio (e iones carbonato), y las concentraciones de H3O+ y HCO3− son prácticamente iguales en una solución acuosa pura de H2CO3.
 Si la primera constante de ionización de un ácido diprótico débil es mayor que la segunda por un factor de al menos 20, es apropiado tratar la primera ionización por separado y calcular las concentraciones resultantes de ella antes de calcular la concentración de especies resultantes de la ionización subsecuente. Este enfoque se demuestra en el siguiente ejercicio de ejemplo.
 Ionización de un ácido diprótico
 El “agua carbonatada” contiene una cantidad aceptable de dióxido de carbono disuelto. La solución es ácida porque el CO2 reacciona con el agua para formar ácido carbónico, H2CO3. ¿Cuáles son las [H3O+], [HCO3−] y [CO32− en una solución saturada de CO2 con un [H2CO3] inicial = 0,033?
 <img alt="Eq3" src="/files/ftp_upload/11405/11405_Equation_3.png" style="height: 90px;" />
 Como indican las constantes de ionización, H2CO3 es un ácido mucho más fuerte que HCO3−, por lo que las reacciones de ionización escalonada pueden tratarse por separado. Utilizando la información proporcionada, se prepara una tabla ICE para este primer paso de ionización:
 <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(ac)</td>
 <td style="color: black; border: 1px solid black; text-align: center;">H3O+(ac)</td>
 <td style="color: black; border: 1px solid black; text-align: center;">HCO3−(ac)</td>
 </tr>
 <tr>
 <td style="color: black; border: 1px solid black; text-align: center;">Concentración inicial (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;">Cambio (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;">Concentración de equilibrio (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>
 Al sustituir las concentraciones de equilibrio en la ecuación de equilibrio queda
 <img alt="Eq4" src="/files/ftp_upload/11405/11405_Equation_4.png" style="height: 90px;" />
 Si se asume que x << 0,033 y se resuelve la ecuación simplificada, se obtiene
 <img alt="Eq5" src="/files/ftp_upload/11405/11405_Equation_5.png" style="height: 50px;" />
 La tabla ICE define x como igual a la molaridad de los iones bicarbonato y a la molaridad de los iones hidronio:
 <img alt="Eq6" src="/files/ftp_upload/11405/11405_Equation_6.png" style="height: 90px;" />
 Utilizando la concentración de iones bicarbonato calculada anteriormente, la segunda ionización es sometida a un cálculo de equilibrio similar: HCO3− (ac) + H2O (l) ⇌ H3O+ (ac) + CO32−(ac)
 <img alt="Eq7" src="/files/ftp_upload/11405/11405_Equation_7.png" style="height: 180px;" />
 En resumen: En equilibrio [H2CO3] = 0,033 M; [H3O+] = 1,2 × 10−4; [HCO3−] = 1,2 × 10−4 M; y [CO32−] = 4,7 × 10−11 M.
 Un ácido triprótico es un ácido que tiene tres átomos de H ionizables. El ácido fosfórico es un ejemplo:
 <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;">Ácido triprótico: Ácido fosfórico (H3PO4)</td>
 </tr>
 <tr>
 <td style="color: black; border: 1px solid black; text-align: center;">1era ionización</td>
 <td style="color: black; border: 1px solid black; text-align: center;">H3PO4(ac) + H2O (l) ⇌ H3O+ (ac) + H2PO4− (ac)  </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;">2da ionización</td>
 <td style="color: black; border: 1px solid black; text-align: center;">H2PO4− (ac) + H2O (l) ⇌ H3O+ (ac) + HPO42− (ac)</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;">3era ionización</td>
 <td style="color: black; border: 1px solid black; text-align: center;">HPO42− (ac) + H2O (l) ⇌ H3O+ (ac) + PO43− (ac)</td>
 <td style="color: black; border: 1px solid black; text-align: center;">Ka3 = 4,2 × 10−13</td>
 </tr>
 </tbody>
 </table>
 En cuanto a los ejemplos de ácido diprótico, cada reacción de ionización sucesiva es menos extensa que la primera, reflejada en valores decrecientes para las constantes de ionización ácida escalonadas. Esta es una característica general de los ácidos polipróticos y las constantes de ionización sucesivas a menudo difieren en un factor de aproximadamente 105 a 106. 
 Este conjunto de tres reacciones de disociación puede dar la impresión de complicar el cálculo de las concentraciones de equilibrio en una solución de H3PO4. Sin embargo, debido a que las constantes de ionización sucesivas difieren en un factor de 105 a 106, existen grandes diferencias en los pequeños cambios de concentración que acompañan a las reacciones de ionización. Esto permite el uso de supuestos y procesos de simplificación matemática, como se demostró en los ejemplos anteriores. Las bases polipróticas son capaces de aceptar más de un ion hidrógeno. El ión carbonato es un ejemplo de base diprótica, ya que puede aceptar dos protones, como se muestra a continuación. Al igual que en el caso de los ácidos polipróticos, observe que las constantes de ionización disminuyen con el paso de ionización. De la misma manera, los cálculos de equilibrio que implican bases polipróticas siguen los mismos enfoques que los de los ácidos polipróticos.
 <img alt="Eq8" src="/files/ftp_upload/11405/11405_Equation_8.png" style="height: 90px;" />
 Este texto es adaptado de<a href="https://openstax.org/books/chemistry-2e/pages/14-5-polyprotic-acids"> Openstax, Química 2e, Sección 14.5: Ácidos Polipróticos</a>.

Ácidos Polipróticos

Los ácidos se clasifican por el número de protones por molécula que pueden ceder en una reacción. Los ácidos como el HCl, el HNO3 y el HCN que contienen ...

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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

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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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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A physical property is ...

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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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