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

Gli acidi monoprotici, come l'acido fluoridrico, contengono un singolo protone ionizzabile. Al contrario, gli acidi poliprotici contengono due o più protoni ionizzabili. Per esempio, l'acido solforoso ha due protoni ionizzabili, e l'acido fosforico ne ha tre.

Un acido poliprotico perde ciascuno dei suoi protoni in sequenza, e ogni reazione ha il suo valore della Ka.È più facile rimuovere un protone da una molecola neutra rispetto a una carica negativa, perché la carica negativa aumenta la forza del legame fra il protone e l'anione. Pertanto, per l'acido fosforico, la Ka per la rimozione del primo protone, Ka1, è maggiore dellal seconda, Ka2, che è maggiore di Ka3. Il pH di un acido poliprotico può essere stimato usando solo la prima reazione se Ka1 è almeno mille volte più grande dei suoi successivi Ka.Per esempio, il pH di una soluzione di acido ascorbico molare 0, 050 può essere determinato utilizzando il suo Ka1 e una tabella ICE.

Quando disciolto in acqua, l'acido ascorbico si dissocia in idronio e ioni ascorbato. La Ka1 per questa reazione è 8 10⁻⁵, ed è uguale alla concentrazione di idronio moltiplicata per la concentrazione del monoanione ascorbato, divisa per la concentrazione di acido ascorbico. Una tabella ICE può essere preparata per questa reazione con le concentrazioni iniziali e di equilibrio.

A causa del piccolo valore di x, 0, 050 meno x è approssimativamente uguale a 0, 050. Sostituendo questi valori nell'espressione per Ka1, il valore di x è uguale a 0, 0020 molare, che è solo il 4 percento della concentrazione iniziale di acido ascorbico. Pertanto, l'approssimazione è valida.

Il pH della soluzione è pari a 2, 70. La concentrazione di ascorbato dianione, formata nella seconda fase della reazione, può anche essere calcolata usando Ka2 e una tabella ICE. Ka2 è uguale a 1, 6 10⁻¹², e può essere espresso come la concentrazione di idronio moltiplicata per la concentrazione di ascorbato dianione divisa per la concentrazione di ascorbato monoanione.

Per questa reazione, la tabella ICE è completata con la concentrazione iniziale di ascorbato monoanione e idronio dalla prima reazione, 0, 002 molare. A causa del piccolo valore di x, può essere omesso dalle concentrazioni in equilibrio del monoanione ascorbato e dell'idronio. Dopo aver sostituito questi valori nell'espressione Ka2 e risolta l'equazione, x è 1, 6 10⁻¹² molare.

Poiché x è inferiore al 5 percento di 0, 002 molare, l'approssimazione è valida. Poiché la concentrazione di ioni idronio che sono stati generati durante la seconda fase della dissociazione dell'acido ascorbico è trascurabile, il primo protone ionizzabile determina il pH della soluzione.

acidi poliprotici

Gli acidi sono classificati in base al numero di protoni per molecola che possono cedere in una reazione. Acidi come HCl, HNO_3 e HCN che contengono un atomo di idrogeno ionizzabile in ciascuna molecola sono chiamati acidi monoprotici. Le loro reazioni con l'acqua sono:

<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>

Anche se contiene quattro atomi di idrogeno, anche l'acido acetico, CH_3CO_2H, è monoprotico perché solo l'atomo di idrogeno del gruppo carbossilico (COOH) reagisce con le basi:

<img alt="Image1" src="/files/ftp_upload/11405/11405_Figure 1.svg" style="height: 125px;" />

Allo stesso modo, le basi monoprotiche sono basi che accetteranno un singolo protone.

Gli acidi diprotici contengono due atomi di idrogeno ionizzabili per molecola; la ionizzazione di tali acidi avviene in due fasi. La prima ionizzazione avviene sempre in misura maggiore rispetto alla seconda ionizzazione. Ad esempio, l'acido solforico, un acido forte, ionizza come segue:

<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>

Questo processo di ionizzazione graduale avviene per tutti gli acidi poliprotici. L'acido carbonico, H_2CO_3, è un esempio di acido diprotico debole. La prima ionizzazione dell'acido carbonico produce ioni idronio e ioni bicarbonato in piccole quantità.

Prima ionizzazione: H_2CO_3 (aq) + H_2O (l) ⇌ H_3O+ (aq) + HCO_3− (aq)

<img alt="Eq1" src="/files/ftp_upload/11405/11405_Equation_1.png" style="height: 90px;" />

Lo ione bicarbonato può anche agire come un acido. Ionizza e forma ioni idronio e ioni carbonato in quantità ancora minori.

Seconda ionizzazione: HCO_3− (aq) + H_2O (l) ⇌ H_3O+ (aq) + CO_3_2− (aq)

<img alt="Eq2" src="/files/ftp_upload/11405/11405_Equation_2.png" style="height: 90px;" />

K_H2CO3 è più grande di K_HCO3− di un fattore 10_4, quindi H_2CO_3 è il produttore dominante di ione idronio nella soluzione. Ciò significa che una piccola parte dell'HCO_3− formato dalla ionizzazione di H_2CO_3 si ionizza per dare ioni idronio (e ioni carbonato) e le concentrazioni di H_3O+ e HCO_3− sono praticamente uguali in una soluzione acquosa pura di H_2CO_3.

Se la prima costante di ionizzazione di un acido diprotico debole è maggiore del secondo di un fattore almeno 20, è opportuno trattare separatamente la prima ionizzazione e calcolare le concentrazioni risultanti da essa prima di calcolare le concentrazioni delle specie risultanti dalla successiva ionizzazione. Questo approccio è dimostrato nell'esercizio di esempio seguente.

Ionizzazione di un acido diprotico

L’“acqua gassata” contiene una quantità gradevole di anidride carbonica disciolta. La soluzione è acida perché la CO_2 reagisce con l'acqua per formare acido carbonico, H_2CO_3. Cosa sono [H_3O+], [HCO_3−] e [CO_3_2−] in una soluzione satura di CO_2 con un [H_2CO_3] iniziale = 0,033?

<img alt="Eq3" src="/files/ftp_upload/11405/11405_Equation_3.png" style="height: 90px;" />

Come indicato dalle costanti di ionizzazione, H_2CO_3 è un acido molto più forte di HCO_3−, quindi le reazioni di ionizzazione graduali possono essere trattate separatamente. Utilizzando le informazioni fornite, viene preparata una tabella ICE per questa prima fase di ionizzazione:

<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>

Sostituendo le concentrazioni di equilibrio nell'equazione di equilibrio si ottiene

<img alt="Eq4" src="/files/ftp_upload/11405/11405_Equation_4.png" style="height: 90px;" />

Supponendo x << 0,033 e risolvendo l'equazione semplificata si ottiene

<img alt="Eq5" src="/files/ftp_upload/11405/11405_Equation_5.png" style="height: 50px;" />

La tabella ICE definisce x come uguale alla molarità degli ioni bicarbonato e alla molarità degli ioni idronio:

<img alt="Eq6" src="/files/ftp_upload/11405/11405_Equation_6.png" style="height: 90px;" />

Utilizzando la concentrazione di ioni bicarbonato calcolata sopra, la seconda ionizzazione è soggetta a un calcolo di equilibrio simile: HCO_3− (aq) + H_2O (l) ⇌ H_3O+ (aq) + CO_3_2− (aq)

<img alt="Eq7" src="/files/ftp_upload/11405/11405_Equation_7.png" style="height: 180px;" />

Riassumendo: all'equilibrio [H_2CO_3] = 0,033 M; [H_3O+] = 1,2 × 10^−4; [HCO_3−] = 1,2 × 10^−4 M; e [CO_3_2−] = 4,7 × 10^−11 M.

Un acido triprotico è un acido che ha tre atomi di H ionizzabili. L'acido fosforico è un esempio:

<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>

Per quanto riguarda gli esempi di acido diprotico, ciascuna reazione di ionizzazione successiva è meno estesa della precedente, il che si riflette in valori decrescenti per le costanti di ionizzazione acida graduale. Questa è una caratteristica generale degli acidi poliprotici e le costanti di ionizzazione successive spesso differiscono di un fattore compreso tra 10_5 e 10_6.

Può sembrare che questo insieme di tre reazioni di dissociazione renda complicato il calcolo delle concentrazioni di equilibrio in una soluzione di H_3PO_4. Tuttavia, poiché le costanti di ionizzazione successive differiscono di un fattore compreso tra 10_5 e 10_6, esistono grandi differenze nei piccoli cambiamenti di concentrazione che accompagnano le reazioni di ionizzazione. Ciò consente l'uso di ipotesi e processi che semplificano la matematica, come dimostrato negli esempi precedenti. Le basi poliprotiche sono in grado di accettare più di uno ione idrogeno. Lo ione carbonato è un esempio di base diprotica, perché può accettare due protoni, come mostrato di seguito. Similmente al caso degli acidi poliprotici, si noti che le costanti di ionizzazione diminuiscono con la fase di ionizzazione. Allo stesso modo, i calcoli dell'equilibrio che coinvolgono le basi poliprotiche seguono gli stessi approcci di quelli per gli acidi poliprotici.

<img alt="Eq8" src="/files/ftp_upload/11405/11405_Equation_8.png" style="height: 90px;" />

Questo testo è adattato da <a href="https://openstax.org/books/chemistry-2e/pages/14-5-polyprotic-acids"> Openstax, Chemistry 2e, Section 14.5: Polyprotic Acids</a>

Acidi poliprotici

Gli acidi sono classificati in base al numero di protoni per molecola che possono cedere in una reazione. Acidi come HCl, HNO3 e HCN che contengono uno ...

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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.

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When a liquid vaporizes in a closed container, gas molecules cannot escape. As these gas phase molecules move randomly about, they will occasionally ...

Clausius-Clapeyron Equation

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

Phase Transitions: Melting and Freezing

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

phase-transitions-sublimation-and-deposition

Phase Transitions: Sublimation and Deposition

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

Heating and Cooling Curves

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

Phase Diagrams

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

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

PS.PSc.1.2

physical-and-chemical-properties-of-matter

Physical and Chemical Properties of Matter

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

Physical Properties of Matter

A physical property is ...

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

Molecular Comparison of Gases, Liquids, and Solids

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

Intermolecular vs Intramolecular Forces

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

Intermolecular Forces

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

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

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

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

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

PS.PSc.1.3

The Atomic Theory of Matter

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

Subatomic Particles

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

The Bohr Model

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

Emission Spectra

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

The de Broglie Wavelength

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

The Uncertainty Principle

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

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

PS.PSc.1.4

Ions and Ionic Charges

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

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

PS.PSc.1.5

The Periodic Table

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

Periodic Classification of the Elements

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

atomic-radii-and-effective-nuclear-charge

Atomic Radii and Effective Nuclear Charge

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

Ionic Radii

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

Ionization Energy

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

Electron Affinity

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



This process can be either ...

Alkali Metals

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

Halogens

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

Noble Gases


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

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

PS.PSc.1.6

Types of Chemical Bonds

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

Ionic Bonding and Electron Transfer

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

Covalent Bonding and Lewis Structures

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

Bonding in Metals

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

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

PS.PSc.2.1

Nomenclature of Alkanes

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

Nomenclature of Alkenes

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

Nomenclature of Alkynes

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

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

Structure and Nomenclature of Alcohols and Phenols

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

Structure and Nomenclature of Ethers

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

IUPAC Nomenclature of Aldehydes

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

IUPAC Nomenclature of Ketones

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

IUPAC Nomenclature of Carboxylic Acids

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

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

PS.PSc.2.2

Understand types, properties, and structure of matter.

Stepwise Ionization of Polyprotic Acids

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