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

Monoprotische Säuren enthalten wie Flusssäure ein einzelnes ionisierbares Proton. Im Gegensatz dazu enthalten polyprotische Säuren zwei oder mehr ionisierbare Protonen. Zum Beispiel hat schwefelhaltige Säure zwei ionisierbare Protonen und Phosphorsäure drei.
Eine polyprotische Säure verliert jedes ihrer Protonen nacheinander, und jede Reaktion hat ihr eigenes Ka.
Es ist einfacher, ein Proton aus einem neutralen Molekül zu entfernen als aus einem negativ geladenen, da die negative Ladung die Stärke der Bindung zwischen dem Proton und dem Anion erhöht.
Daher ist für Phosphorsäure das Ka für die Entfernung des ersten Protons, Ka1, höher als das zweite, Ka2, das höher als Ka3 ist.
Der pH-Wert einer polyprotischen Säure kann nur mit der ersten Reaktion abgeschätzt werden, wenn Ka1 mindestens tausendmal größer ist als die nachfolgenden Ka.
So kann beispielsweise der pH-Wert einer 0,050 molaren Ascorbinsäurelösung mit deren Ka1 und einer ICE-Tabelle bestimmt werden.
Wenn Ascorbinsäure in Wasser gelöst wird, dissoziiert sie in Hydronium- und Ascorbationen. Der Ka1 für diese Reaktion beträgt 8 × 10−5 und ist gleich der Konzentration von Hydronium multipliziert mit der Konzentration von Ascorbatmonoanion, dividiert durch die Konzentration von Ascorbinsäure.
Für diese Reaktion kann eine ICE-Tabelle mit der Anfangs- und Gleichgewichtskonzentration erstellt werden. Aufgrund des kleinen Wertes von x entspricht 0,050 minus x ungefähr 0,050.
Setzt man diese Werte in den Ausdruck für Ka1 ein, so ist der Wert für x gleich 0,0020 molaren, was nur 4 % der Anfangskonzentration von Ascorbinsäure entspricht. Somit ist die Annäherung gültig.
Der pH-Wert der Lösung beträgt 2,70.
Die Konzentration des im zweiten Reaktionsschritt gebildeten Ascorbatdianions kann auch mit Hilfe von Ka2 und einer ICE-Tabelle berechnet werden.
Ka2 ist gleich 1,6 × 10−12 und kann ausgedrückt werden als die Konzentration von Hydronium multipliziert mit der Konzentration von Ascorbatdianion dividiert durch die Konzentration von Ascorbatmonoanion.
Für diese Reaktion wird die ICE-Tabelle mit der Anfangskonzentration von Ascorbat-Monoanion und Hydronium aus der ersten Reaktion, 0,002 molaren, ausgefüllt. Aufgrund des kleinen Wertes von x kann es bei den Gleichgewichtskonzentrationen von Ascorbatmonoanion und Hydronium weggelassen werden.
Nach dem Einsetzen dieser Werte in den K a2-Ausdruck und dem Lösen beträgt x 1,6 ×10−12 molaren Durchmesser.
Da x weniger als 5 % von 0,002 molar ist, ist die Näherung gültig.
Da die Konzentration der Hydroniumionen, die während des zweiten Schritts der Ascorbinsäuredissoziation erzeugt wurden, vernachlässigbar ist, bestimmt das erste ionisierbare Proton den pH-Wert der Lösung.

polyprotische säuren

Säuren werden nach der Anzahl der Protonen pro Molekül klassifiziert, die sie bei einer Reaktion abgeben können. Säuren wie HCl, HNO3 und HCN, die in jedem Molekül ein ionisierbares Wasserstoffatom enthalten, werden monoprotische Säuren genannt. Ihre Reaktionen mit Wasser sind:

 <table style="margin-left: auto; margin-right: auto;"> <tbody> <tr> <td style="color: black; border: 1px solid black; text-align: center;">Monoprotic acids: Reactions with water</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> 

 Obwohl Essigsäure, CH3CO2H, vier Wasserstoffatome enthält, ist sie ebenfalls monoprotisch, da nur das Wasserstoffatom der Carboxylgruppe (COOH) mit Basen reagiert:

 <img alt="Image1" src="/files/ftp_upload/11405/11405_Figure 1.svg" style="height: 125px;">

 Ebenso sind monoprotische Basen Basen, die ein einzelnes Proton aufnehmen.

 Diprotonische Säuren enthalten zwei ionisierbare Wasserstoffatome pro Molekül; Die Ionisierung solcher Säuren erfolgt in zwei Schritten. Die erste Ionisation erfolgt immer in einem größeren Ausmaß als die zweite Ionisation. Beispielsweise ionisiert Schwefelsäure, eine starke Säure, wie folgt:

 <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;">Diprotic acid: Sulfuric acid (H2SO4)</td> </tr> <tr> <td style="color: black; border: 1px solid black; text-align: center;">1st ionization</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 = more than 102; complete dissociation</td> </tr> <tr> <td style="color: black; border: 1px solid black; text-align: center;">2nd ionization</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> 

 Dieser schrittweise Ionisierungsprozess findet bei allen mehrprotonigen Säuren statt. Kohlensäure, H2CO3, ist ein Beispiel für eine schwache diprotische Säure. Bei der ersten Ionisierung von Kohlensäure entstehen in geringen Mengen Hydroniumionen und Bicarbonationen.

 Erste Ionisierung: H2CO3 (aq) + H2O (l) ⇌ H3O+ (aq) + HCO3− (aq)

 <img alt="Eq1" src="/files/ftp_upload/11405/11405_Equation_1.png" style="height: 90px;">

 Das Bicarbonat-Ion kann auch als Säure wirken. Es ionisiert und bildet Hydroniumionen und Carbonationen in noch geringeren Mengen.

 Zweite Ionisierung: HCO3− (aq) + H2O (l) ⇌ H3O+ (aq) + CO32− (aq)

 <img alt="Eq2" src="/files/ftp_upload/11405/11405_Equation_2.png" style="height: 90px;">

 KH2CO3 ist um den Faktor 104 größer als KHCO3−, daher ist H2CO3 der Hauptproduzent von Hydroniumionen in der Lösung. Dies bedeutet, dass nur wenig von dem durch die Ionisierung von H2CO3 gebildeten HCO3− zu Hydroniumionen (und Carbonationen) ionisiert und die Konzentrationen von H3O+ und HCO3− in einer reinen wässrigen Lösung von H2CO3 praktisch gleich sind.

 Wenn die erste Ionisationskonstante einer schwachen zweiprotonigen Säure mindestens um den Faktor 20 größer als die zweite ist, ist es angebracht, die erste Ionisation separat zu behandeln und die daraus resultierenden Konzentrationen zu berechnen, bevor die Konzentrationen der Spezies berechnet werden, die aus der nachfolgenden Ionisation resultieren. Dieser Ansatz wird in der folgenden Beispielübung demonstriert.

 Ionisierung einer zweiprotonigen Säure

 „Kohlensäurehaltiges Wasser“ enthält eine wohlschmeckende Menge gelöstes Kohlendioxid. Die Lösung ist sauer, da CO2 mit Wasser unter Bildung von Kohlensäure H2CO3 reagiert. Was sind [H3O+], [HCO3−], und [CO32−] in einer gesättigten CO2-Lösung mit einem anfänglichen [H2CO3] = 0,033?

 <img alt="Eq3" src="/files/ftp_upload/11405/11405_Equation_3.png" style="height: 90px;">

 Wie aus den Ionisierungskonstanten hervorgeht, ist H2CO3 eine viel stärkere Säure als HCO3−, sodass die schrittweisen Ionisierungsreaktionen separat behandelt werden können. Anhand der bereitgestellten Informationen wird eine ICE-Tabelle für diesen ersten Ionisierungsschritt erstellt:

 <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;">Initial concentration (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;">Change (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;">Equilibrium concentration (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> 

 Das Einsetzen der Gleichgewichtskonzentrationen in die Gleichgewichtsgleichung ergibt

 <img alt="Eq4" src="/files/ftp_upload/11405/11405_Equation_4.png" style="height: 90px;">

 Unter der Annahme, dass x << 0,033 ist, und die Lösung der vereinfachten Gleichung ergibt sich das Ergebnis

 <img alt="Eq5" src="/files/ftp_upload/11405/11405_Equation_5.png" style="height: 50px;">

 In der ICE-Tabelle wurde x als gleich der Bicarbonat-Ionen-Molarität und der Hydronium-Ionen-Molarität definiert:

 <img alt="Eq6" src="/files/ftp_upload/11405/11405_Equation_6.png" style="height: 90px;">

 Unter Verwendung der oben berechneten Bicarbonat-Ionenkonzentration wird die zweite Ionisierung einer ähnlichen Gleichgewichtsberechnung unterzogen: HCO3− (aq) + H2O (l) ⇌ H3O+ (aq) + CO32− (aq)

 <img alt="Eq7" src="/files/ftp_upload/11405/11405_Equation_7.png" style="height: 180px;">

 Zusammenfassend: im Gleichgewicht [H2CO3] = 0,033 M; [H3O+] = 1,2 × 10−4; [HCO3−] = 1,2 × 10−4 M; und [CO32−] = 4,7 × 10−11 M.

 Eine triprotische Säure ist eine Säure mit drei ionisierbaren H-Atomen. Phosphorsäure ist ein Beispiel:

 <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;">Triprotic acid: Phosphoric acid (H3PO4)</td> </tr> <tr> <td style="color: black; border: 1px solid black; text-align: center;">1st ionization</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 ionization</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 ionization</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> 

 Was die Beispiele für zweiprotonige Säuren betrifft, so ist jede aufeinanderfolgende Ionisierungsreaktion weniger umfangreich als die erstere, was sich in abnehmenden Werten für die stufenweisen Säureionisierungskonstanten widerspiegelt. Dies ist ein allgemeines Merkmal polyprotonischer Säuren und aufeinanderfolgende Ionisationskonstanten unterscheiden sich häufig um den Faktor 105 bis 106.

Dieser Satz von drei Dissoziationsreaktionen scheint die Berechnung der Gleichgewichtskonzentrationen in einer H3PO4-Lösung kompliziert zu machen. Da sich die aufeinanderfolgenden Ionisationskonstanten jedoch um den Faktor 105 bis 106 unterscheiden, bestehen große Unterschiede bei den kleinen Konzentrationsänderungen, die mit den Ionisationsreaktionen einhergehen. Dies ermöglicht die Verwendung mathematisch vereinfachender Annahmen und Prozesse, wie in den obigen Beispielen gezeigt. Polyprotische Basen sind in der Lage, mehr als ein Wasserstoffion aufzunehmen. Das Carbonation ist ein Beispiel für eine diprotische Base, da es zwei Protonen aufnehmen kann, wie unten gezeigt. Beachten Sie, dass die Ionisationskonstanten ähnlich wie bei polyprotonen Säuren mit dem Ionisationsschritt abnehmen. Ebenso folgen Gleichgewichtsberechnungen für polyprotische Basen den gleichen Ansätzen wie für polyprotische Säuren.

<img alt="Eq8" src="/files/ftp_upload/11405/11405_Equation_8.png" style="height: 90px;">

Dieser Text ist adaptiert von <a href="https://openstax.org/books/chemistry-2e/pages/14-5-polyprotic-acids"> Openstax, Chemistry 2e, Section 14.5: Polyprotic Acids</a>

Polyprotische Säuren

Säuren werden nach der Anzahl der Protonen pro Molekül klassifiziert, die sie in einer Reaktion abgeben können. Säuren wie HCl, HNO3 und HCN, die eine ...

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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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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 ...

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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