Name: weak acid equilibrium in water
Uploaded: 2020-09-24T15:06:51.000Z
Duration: 4 min 2 s
Channel: JoVE

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Weak Acid Solutions

Un acido debole, come l'acido cianidrico, è un acido di Brønsted poiché dona un protone alla molecola d'acqua e produce lo ione idronio. Un acido debole si dissocia parzialmente in acqua secondo la sua costante di dissociazione acida, Ka, che è 4, 9 10⁻¹⁰ per l'acido cianidrico. Per l'acido cianidrico, il valore della Ka è uguale alla concentrazione di idronio moltiplicata per la concentrazione di ioni cianuro divisa per la concentrazione dell'acido cianidrico.

La costante di dissociazione acida, Ka, può essere usata per determinare la concentrazione di ioni idronio in una soluzione acida debole e, di conseguenza, il pH della soluzione. La concentrazione di ioni idronio e il pH di una soluzione 0, 15 molare di acido cianidrico possono essere calcolati usando la sua espressione di equilibrio e una tabella ICE. Le concentrazioni di acido cianidrico, di idronio e di cianuro in fase di inizio e all'equilibrio possono essere espresse in una tabella, la quale mostra le concentrazioni Iniziale, di Variazione e di Equilibrio di ciascuna delle molecole.

Per raggiungere l'equilibrio, la concentrazione iniziale dei reagenti diminuisce all'aumentare della concentrazione iniziale dei prodotti secondo i loro rapporti molari. Questa variazione nella concentrazione dei reagenti e dei prodotti è indicata con x. Sostituendo le concentrazioni di equilibrio nell'espressione per la Ka si ottiene x volte x diviso 0, 15 meno x.

In molti acidi deboli, x, la quantità di dissociazione, è probabile che sia molto piccola a confronto con la concentrazione iniziale di 0, 15 molare. Si può presumere che 0, 15 meno x sia circa 0, 15. Quando l'equazione è risolta, x è uguale a 8, 6 10⁻⁶ molare.

L'approssimazione, 0, 15 meno x uguale a 0, 15, è valida solo se x è inferiore al 5 percento di 0, 15 molare. Qui, x è 0, 0057 percento di 0, 15 molare, dunque questa approssimazione è valida. Pertanto, la concentrazione di idronio è 8, 6 10⁻⁶ molare.

Per determinare il pH, prendete il log negativo della concentrazione di ioni idronio. Il PH della soluzione di acido cianidrico 0, 15 M è 5, 07. Il pH di una soluzione può essere usato per determinare il Ka di un acido debole.

Per esempio, l'acido acetico si dissocia parzialmente in ioni idronio e ioni acetato quando disciolto in acqua. Il valore della Ka per l'acido acetico può essere espresso come la concentrazione di ioni idronio moltiplicata per la concentrazione di ioni acetato divisa per la concentrazione di acido acetico. Se il pH di una soluzione molare di acido acetico 0, 20 è 2, 72, è possibile calcolare la sua concentrazione di idronio, che è 1, 9 10⁻³ molare.

La tabella ICE può essere costruita dalle concentrazioni iniziali e di equilibrio dell'acido acetico, degli ioni idronio e degli ioni acetato. Utilizzando cifre significative, 0, 20 meno 1, 9 10⁻³ è essenzialmente uguale a 0, 20. Sostituendo i valori di equilibrio nell'espressione Ka, Ka è uguale a 1, 8 10⁻⁵.

soluzioni di acidi deboli

Pochi composti agiscono come acidi forti. Un numero molto maggiore di composti si comportano come acidi deboli e reagiscono solo parzialmente con l'acqua, lasciando la grande maggioranza delle molecole disciolte nella loro forma originale e generando una quantità relativamente piccola di ioni idronio. Gli acidi deboli si trovano comunemente in natura, essendo le sostanze in parte responsabili del sapore piccante degli agrumi, della sensazione pungente delle punture di insetti e degli odori sgradevoli associati all'odore del corpo. Un esempio familiare di acido debole è l’acido acetico, l’ingrediente principale dell’aceto:

<img alt="Eq1" src="/files/ftp_upload/11400/11400_Equation_1.png" style="height: 50px;" />

L'uso di una doppia freccia nell'equazione sopra denota l'aspetto di reazione parziale di questo processo. Quando disciolto in acqua in condizioni tipiche, solo circa l'1% delle molecole di acido acetico sono presenti nella forma ionizzata, CH3COO−.

Calcolo delle concentrazioni di ioni idronio e del pH di una soluzione acida debole

L’acido formico, HCO2H, è un irritante che provoca la reazione del corpo ad alcuni morsi e punture di formiche. Qual è la concentrazione dello ione idronio e il pH di una soluzione 0,534 M di acido formico?

<img alt="Eq2" src="/files/ftp_upload/11400/11400_Equation_2.png" style="height: 50px;" />

La tabella ICE per questo sistema è

<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;"> HCO2H (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;">HCO2− (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;">0.534</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;">0.534 − 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 i termini di concentrazione all'equilibrio nell'espressione Ka si ottiene

<img alt="Eq3" src="/files/ftp_upload/11400/11400_Equation_3.png" style="height: 90px;" />

La concentrazione iniziale relativamente grande e la costante di equilibrio piccola consentono l'assunzione semplificatrice che x sarà molto inferiore a 0,534, e quindi l'equazione diventa

<img alt="Eq4" src="/files/ftp_upload/11400/11400_Equation_4.png" style="height: 90px;" />

Risolvendo l'equazione per x si ottiene

<img alt="Eq5" src="/files/ftp_upload/11400/11400_Equation_5.png" style="height: 90px;" />

Per verificare l'ipotesi che x sia piccolo rispetto a 0,534, si può stimare la sua grandezza relativa:

<img alt="Eq6" src="/files/ftp_upload/11400/11400_Equation_6.png" style="height: 90px;" />

Poiché x è inferiore al 5% della concentrazione iniziale, l'ipotesi è valida. Come definito nella tabella ICE, x è uguale alla concentrazione di equilibrio dello ione idronio:

<img alt="Eq7" src="/files/ftp_upload/11400/11400_Equation_7.png" style="height: 50px;" />

Infine, si calcola che il pH sia

<img alt="Eq8" src="/files/ftp_upload/11400/11400_Equation_8.png" style="height: 50px;" />

Determinazione della Ka dal pH

Il pH di una soluzione 0,0516 M di acido nitroso, HNO2, è 2,34. Qual è il suo Ka?

<img alt="Eq9" src="/files/ftp_upload/11400/11400_Equation_9.png" style="height: 50px;" />

La concentrazione di acido nitroso fornita è una concentrazione formale, che non tiene conto degli eventuali equilibri chimici che possono essere stabiliti in soluzione. Tali concentrazioni vengono trattate come valori “iniziali” per i calcoli di equilibrio utilizzando l’approccio della tabella ICE. Si noti che il valore iniziale dello ione idronio è elencato come circa zero perché è presente una piccola concentrazione di H3O+ (1 × 10−7 M) a causa dell'autoionizzazione dell'acqua. In molti casi, questa concentrazione è molto inferiore a quella generata dalla ionizzazione dell'acido (o della base) in questione e può essere trascurata.

Il pH fornito è una misura logaritmica della concentrazione di ioni idronio risultante dalla ionizzazione acida dell'acido nitroso, e quindi rappresenta un valore di “equilibrio” per la tabella ICE:

<img alt="Eq1" src="/files/ftp_upload/11400/11400_Equation_10.png" style="height: 50px;" />

La tabella ICE per questo sistema è quindi

<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;"> HNO2 (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;">NO2− (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;">0.0516</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;">−0.0046</td>
			<td style="color: black; border: 1px solid black; text-align: center;">+0.0046</td>
			<td style="color: black; border: 1px solid black; text-align: center;">+0.0046</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;">0.0470</td>
			<td style="color: black; border: 1px solid black; text-align: center;">0.0046</td>
			<td style="color: black; border: 1px solid black; text-align: center;">0.0046</td>
		</tr>
	</tbody>
</table>

Infine, calcola il valore della costante di equilibrio utilizzando i dati nella tabella:

<img alt="Eq11" src="/files/ftp_upload/11400/11400_Equation_11.png" style="height: 90px;" />

Questo testo è adattato da <a href="https://openstax.org/books/chemistry-2e/pages/4-2-classifying-chemical-reactions">Openstax, Chemistry 2e, Section 4.2: Classifying Chemical Reactions</a> and <a href="https://openstax.org/books/chemistry-2e/pages/14-3-relative-strengths-of-acids-and-bases">Openstax, Chemistry 2e,14.3 Relative Strengths of Acids and Bases.</a>

Soluzioni di acidi deboli

Pochi composti agiscono come acidi forti. Un numero molto maggiore di composti si comporta come acidi deboli e reagisce solo parzialmente con l'acqua, lasciando una grande maggioranza di ...

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weak acid equilibrium in water

Weak acid equilibrium in water explains why many acids only partially react when dissolved. Most weak acid molecules stay in their original form, and only a small amount of hydronium ions forms. Weak acids are common in nature. They help create the tangy taste of citrus fruits, the stinging sensation from insect bites, and the smells linked to body odor.

Acetic acid is a familiar weak acid. It is the main ingredient in vinegar. Its equation uses a double arrow to show that the reaction with water is only partial. Under typical conditions, only about 1% of acetic acid molecules are present as the ionized form, CH3COO−.

Weak acid problems often use an ICE table to find hydronium ion concentration and pH. Formic acid, HCO2H, is one example because it can cause the irritation from some ant bites and stings. For a 0.534-M formic acid solution, the ICE table starts with formic acid, H3O+, and HCO2−, then tracks the change as x forms and the equilibrium amounts are written in terms of x. The Ka expression is set up from those equilibrium concentrations.

Because the initial formic acid concentration is much larger than the amount that reacts, x is assumed to be much less than 0.534. That lets the expression simplify and makes the calculation easier. After solving for x, the result is checked against the 5% rule. Since x is less than 5% of the initial concentration, the assumption is valid. The value of x equals the equilibrium hydronium ion concentration, and the pH is then calculated from that concentration.

The same ICE table method can also be used in reverse to find Ka from pH. For nitrous acid, HNO2, the given pH is 2.34 for a 0.0516-M solution. That concentration is a formal concentration, which means it does not include any equilibrium changes. In these setups, the initial hydronium ion is shown as approximately zero because water already contains a very small amount, about 1 × 10−7 M, from autoionization. In many cases, that amount is much smaller than the acid-produced hydronium ion and can be ignored.

The pH gives the equilibrium hydronium ion concentration for the nitrous acid solution. The ICE table then shows HNO2, H3O+, and NO2− changing to equilibrium values. Using those values in the equilibrium expression gives the Ka of the acid. This approach shows how pH and Ka connect to the behavior of weak acids in water.

Weak Acid Equilibrium in Water

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Weak acid equilibrium in water explains why many acids only partially react when dissolved. Most weak acid molecules stay in their original form, and only a small amount of hydronium ions forms. Weak acids are common in nature. They help create the tangy taste of citrus fruits, the stinging sensation from insect bites, and the smells linked to body odor. Acetic acid is a familiar weak acid. It is the main ingredient in vinegar. Its equation uses a double arrow to show that the reaction with...

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Electron-pair reactions explain Lewis acid-base chemistry. In 1923, G. N. Lewis described acids and bases by how they accept or donate a pair of electrons. When that electron pair forms a coordinate covalent bond, one atom supplies both bonding electrons. This bond is also called a dative bond. A simple example is the reaction of water with a hydrogen ion to make a hydronium ion. Ammonia can also combine with a hydrogen ion to form an ammonium ion. In both cases, the reaction creates a Lewis...

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Explore how weak acids partially ionize in water, use ICE tables to find hydronium ion concentration and pH, and calculate Ka from pH.

Weak Acid Equilibrium in Water | JoVE K12

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JoVE Core: Chimica illustra concetti chiave di chimica attraverso video animazioni concise e di immediata comprensione. Inoltre, i video della sezione "scienziati in azione" mostrano procedure metodologiche realizzate da ricercatori provenienti dai migliori laboratori internazionali.

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Trends in Lattice Energy: Ion Size and Charge

An ionic compound is stable because of the electrostatic attraction between its positive and negative ions. The lattice energy of a compound is a measure ...

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 ...

Electronegativity

Whether a bond is nonpolar or polar covalent is determined by a property of the bonding atoms called electronegativity.&#xA0;

Electronegativity values of ...

bond-polarity-dipole-moment-and-percent-ionic-character

Bond Polarity, Dipole Moment, and Percent Ionic Character

Bond Polarity

The absolute value of the difference in electronegativity (&Delta;EN) of two bonded atoms provides a rough measure of the polarity to be ...

lewis-structures-of-molecular-compounds-and-polyatomic-ions

Lewis Structures of Molecular Compounds and Polyatomic Ions

To draw Lewis structures for complicated molecules and molecular ions, it is helpful to follow a step-by-step procedure as outlined:


	Determine the ...

Resonance

The Lewis structure of a nitrite anion (NO2&minus;) may actually be drawn in two different ways, distinguished by the locations of the N-O and N=O ...

Formal Charges

In some cases, there are seemingly more than one valid Lewis structures for molecules and polyatomic ions. The concept of formal charges can be used to ...

Exceptions to the Octet Rule

Many covalent molecules have central atoms that do not have eight electrons in their Lewis structures. These molecules fall into three categories:


	...

Bond Energies and Bond Lengths

Stable molecules exist because covalent bonds hold the atoms together. The strength of a covalent bond is measured by the energy required to break it, ...

Analyze and interpret data to explain the mechanisms and properties of the two main types of intramolecular (ionic and covalent) bonds.

PS.Chm.3.1

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 ...

Construct an explanation to summarize the influences intermolecular forces have on the properties of chemical compounds.

PS.Chm.3.2

Analyze the structure of atoms and isotopes.

Weak Acid Equilibrium in Water

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