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

Os ácidos monopróticos, como o ácido fluorídrico, contêm um único próton ionizável. Em contraste, os ácidos polipróticos contêm dois ou mais prótons ionizáveis. Por exemplo, o ácido sulfuroso tem dois prótons ionizáveis, e o ácido fosfórico tem três.Um ácido poliprótico perde cada um dos seus prótons sequencialmente, e cada reação tem o seu próprio Ka.É mais fácil remover um próton de uma molécula neutra do que uma carregada negativamente porque a carga negativa aumenta a força de a ligação entre o próton e o ânion. Portanto, para o ácido fosfórico, o Ka para a remoção do primeiro próton, Ka1, é mais elevado do que o segundo, Ka2, que é superior a Ka3. O pH de um ácido poliprótico pode ser estimado utilizando apenas a primeira reação se Ka1 for pelo menos mil vezes maior do que os seus posteriores Ka's.Por exemplo, o pH de uma 0, 050 solução molar de ácido ascórbico pode ser determinado utilizando a sua Ka1 e uma tabela ICE. Quando dissolvido em água, o ácido ascórbico dissocia-se no hidrónio e íons ascorbatos. O Ka1 para esta reação é de 8 10-⁵, e é igual à concentração de hidrónio vezes a concentração do monoânion ascorbato, dividido pela concentração de ácido ascórbico.Uma tabela ICE pode ser preparada para esta reação com as concentrações inicial e de equilíbrio. Devido ao pequeno valor de x, 0, 050 menos x é aproximadamente igual a 0, 050. A substituição destes valores na expressão para Ka1, o valor para x é igual a 0, 0020 molar, que é apenas 4%do valor da concentração inicial do ácido ascórbico.Assim, a aproximação é válida. O pH da solução igual a 2, 70. A concentração do monoânion ascorbato formado na segunda etapa da reação também pode ser calculado utilizando o Ka2 e uma tabela ICE.Ka2 é igual a 1, 6 10-¹², e pode ser expresso como a concentração de hidrónio vezes a concentração do monoânion ascorbato dividido pela concentração do monoânion ascorbato. Para esta reação, a tabela ICE é preenchida com a concentração inicial do monoânion ascorbato e hidrónio desde a primeira reação, 0, 002 molar. Devido ao pequeno valor de x, este pode ser omitido das concentrações de equilíbrio do monoânion ascorbato e hidrónio.Após a sua substituição os valores na expressão Ka2 e resolução, o x é 1, 6 10-¹² molar. Uma vez que x é inferior a 5%de 0, 002 molar, a aproximação é válida. Como a concentração de íons hidrônicos que foi gerada durante a segunda etapa do ácido ascórbico a dissociação é insignificante, o primeiro próton ionizável determina o pH da solução.

Ácidos polipróticos

Os ácidos são classificados pelo número de protões por molécula que podem doar em uma reação. Ácidos como HCl, HNO3, e HCN que contêm um átomo de hidrogénio ionizável em cada molécula são chamados ácidos monopróticos. As suas reações com água são: 
<table style="margin-left: auto; margin-right: auto;">
<tbody>
<tr>
<td style="color: black; border: 1px solid black; text-align: center;">Ácidos monopróticos: Reações com água</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>
Embora contenha quatro átomos de hidrogénio, o ácido acético, CH3CO2H, é também monoprótico, porque apenas o átomo de hidrogénio do grupo carboxilo (COOH) reage com bases:
<img alt="Image1" src="/files/ftp_upload/11405/11405_Figure 1.svg" style="height: 125px;" />
De forma semelhante, bases monopróticas são bases que aceitam um único protão.
Os ácidos dipróticos contêm dois átomos de hidrogénio ionizáveis por molécula; a ionização desses ácidos ocorre em dois passos. A primeira ionização ocorre sempre em maior medida do que a segunda ionização. Por exemplo, o ácido sulfúrico, um ácido forte, ioniza da seguinte forma:
<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 diprótico: Ácido sulfúrico (H2SO4)</td>
</tr>
<tr>
<td style="color: black; border: 1px solid black; text-align: center;">1a ionização</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 = mais de 102; dissociação completa</td>
</tr>
<tr>
<td style="color: black; border: 1px solid black; text-align: center;">2a ionização</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>
Este processo de ionização por passos ocorre para todos os ácidos polipróticos. O ácido carbónico, H2CO3, é um exemplo de um ácido diprótico fraco. A primeira ionização do ácido carbónico produz iões hidrónio e iões bicarbonato em pequenas quantidades.
Primeira ionização: H2CO3 (aq) + H2O (l) ⇌ H3O+ (aq) + HCO3− (aq)
<img alt="Eq1" src="/files/ftp_upload/11405/11405_Equation_1.png" style="height: 90px;" />
O ião bicarbonato também pode funcionar como um ácido. Ele ioniza e forma iões hidrónio e iões carbonato em quantidades ainda mais pequenas.
Segunda ionização: HCO3− (aq) + H2O (l) ⇌ H3O+ (aq) + CO32− (aq)
<img alt="Eq2" src="/files/ftp_upload/11405/11405_Equation_2.png" style="height: 90px;" />
KH2CO3 é maior que KHCO3− por um factor de 104, pelo que H2CO3 é o produtor dominante de iões hidrónio na solução. Isto significa que pouco do HCO3− formado pela ionização de H2CO3 ioniza para dar iões hidrónio (e iões carbonato), e as concentrações de H3O+ e HCO3− são praticamente iguais em uma solução aquosa pura de H2CO3.
Se a primeira constante de ionização de um ácido diprótico fraco for superior à segunda por um factor de pelo menos 20, é adequado tratar a primeira ionização separadamente e calcular as concentrações que dela resultam antes de calcular as concentrações de espécies resultantes da ionização subsequente. Esta abordagem é demonstrada no seguinte exemplo de exercício.
Ionização de um Ácido Diprótico
A “água com gás” contém uma quantidade aceitável de dióxido de carbono dissolvido. A solução é ácida porque o CO2 reage com a água para formar ácido carbónico, H2CO3. Quais são [H3O+], [HCO3−], e [CO32−] em uma solução saturada de CO2 com uma inicial [H2CO3] = 0,033?
<img alt="Eq3" src="/files/ftp_upload/11405/11405_Equation_3.png" style="height: 90px;" />
Como indicado pelas constantes de ionização, H2CO3 é um ácido muito mais forte do que HCO3−, portanto as reações de ionização por passos podem ser tratadas separadamente. Usando as informações fornecidas, uma tabela ICE para este primeiro passo de ionização é preparada:
<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;">Concentração 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;">Alteração (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;">Concentração em equilíbrio (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>
Substituindo as concentrações de equilíbrio na equação de equilíbrio obtemos
<img alt="Eq4" src="/files/ftp_upload/11405/11405_Equation_4.png" style="height: 90px;" />
Assumindo que x << 0,033 e resolvendo a equação simplificada obtemos
<img alt="Eq5" src="/files/ftp_upload/11405/11405_Equation_5.png" style="height: 50px;" />
A tabela ICE definiu x como sendo igual à molaridade de iões bicarbonato e à molaridade de iões hidrónio:
<img alt="Eq6" src="/files/ftp_upload/11405/11405_Equation_6.png" style="height: 90px;" />
Utilizando a concentração de iões bicarbonato acima calculada, a segunda ionização é submetida a um cálculo de equilíbrio semelhante: HCO3− (aq) + H2O (l) ⇌ H3O+ (aq) + CO32− (aq)
<img alt="Eq7" src="/files/ftp_upload/11405/11405_Equation_7.png" style="height: 180px;" />
Resumindo: em equilíbrio [H2CO3] = 0,033 M; [H3O+] = 1,2 × 10−4; [HCO3−] = 1,2 × 10−4 M; e [CO32−] = 4,7 × 10−11 M.
Um ácido triprótico é um ácido que tem três átomos de H ionizáveis. O ácido fosfórico é um exemplo:
<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;">1a ionização</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;">2a ionização</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;">3a ionização</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>
Como para os exemplos de ácidos dipróticos, cada reação de ionização sucessiva é menos extensa do que a primeira, refletida em valores decrescentes para as constantes de ionização acídica por passos. Esta é uma característica geral dos ácidos polipróticos e as constantes de ionização sucessivas diferem muitas vezes por um factor de cerca de 105 a 106.
Este conjunto de três reações de dissociação pode parecer complicar os cálculos das concentrações de equilíbrio em uma solução de H3PO4. No entanto, como as constantes de ionização sucessivas diferem por um factor de 105 a 106, existem grandes diferenças nas pequenas alterações na concentração que acompanham as reações de ionização. Isso permite o uso de suposições e processos que simplificam a matemática, como demonstrado nos exemplos acima. As bases polipróticas são capazes de aceitar mais de um ião hidrogénio. O ião carbonato é um exemplo de uma base diprótica, porque pode aceitar dois protões, como mostrado abaixo. Semelhante ao caso dos ácidos polipróticos, observe que as constantes de ionização diminuem com o passo de ionização. Da mesma forma, os cálculos de equilíbrio envolvendo bases polipróticas seguem as mesmas abordagens que os dos ácidos polipróticos.
<img alt="Eq8" src="/files/ftp_upload/11405/11405_Equation_8.png" style="height: 90px;" />
Este texto é adaptado de<a href="https://openstax.org/books/chemistry-2e/pages/14-5-polyprotic-acids"> Openstax, Chemistry 2e, Section 14.5: Polyprotic Acids</a>.

Ácidos Polipróticos

Os ácidos são classificados pelo número de prótons por molécula que podem desistir em uma reação. Ácidos como HCl, HNO3 e HCN que contêm um ...

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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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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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