Name: proton transfer in acids and bases
Uploaded: 2020-09-24T15:06:51.000Z
Duration: 2 min 58 s
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Bronsted-Lowry Acids and Bases

Un acido di Arrhenius è una sostanza che produce uno ione idrogeno quando disciolto in acqua, e una base di Arrhenius è una sostanza che produce uno OH, o ione idrossido. L'acido cloridrico è un acido di Arrhenius, poiché si dissocia in uno ione idrogeno e uno ione cloruro quando viene disciolto in acqua. L'idrossido di sodio è una base di Arrhenius, poiché si dissocia in uno ione sodio e uno ione idrossido quando viene disciolto in acqua.

Tuttavia, questa definizione non può essere usata per descrivere acidi e basi che non si trovano in una soluzione acquosa o basi che non contengono ioni idrossido. Una definizione più vasta di Brønsted e Lowry definisce un acido come uno ione idrogeno o donatore di protoni, mentre una base è un accettore di protoni. Quando l'acido cloridrico viene disciolto in acqua, agisce come un acido donando un protone all'acqua, producendo uno ione idronio e uno ione cloruro.

Quando l'ammoniaca viene disciolta in acqua, agisce come base e accetta un protone dall'acqua, producendo uno ione ammonio e uno ione idrossido. Un acido di Brønsted-Lowry reagirà sempre con una base di Brønsted-Lowry, e viceversa. Quando un acido, per esempio l'acido acetico, dona il suo protone, l'acqua agisce da base e accetta il protone.

L'acido acetico viene convertito in una base coniugata, l'acetato, e l'acqua viene convertita in un acido coniugato, uno ione idronio. Gli acidi e le basi che differiscono fra loro a causa del trasferimento di un protone sono detti coppie coniugate acido-base. Nella reazione inversa, l'acido coniugato, l'idronio, agisce come un donatore di protoni e la base coniugata, l'acetato, accetterà un protone.

La forza di un acido è determinata dalla sua capacità di donare un protone, mentre la forza di una base è determinata dalla sua capacità di accettare un protone. È più probabile che un acido più forte doni un protone rispetto ad un acido più debole. Allo stesso modo, è più probabile che una base più forte accetti un protone rispetto ad una base più debole.

La forza di un acido e della la sua base coniugata sono inversamente correlate. Un acido forte si dissocia completamente in una soluzione e la base coniugata risultante è troppo debole per accettare un protone. Lo stesso vale nel caso di una base forte e del suo acido coniugato.

D'altro canto, un acido debole si dissocia parzialmente in una soluzione. Anche la base coniugata di un acido debole è relativamente debole;pertanto, una miscela dell'acido debole non dissociato e della sua base coniugata debole si troverà in equilibrio. Lo stesso fenomeno si verifica nel caso di una base debole e del suo acido coniugato debole.

<ol>
	<li>Storia, David A. "Revisione dal banco al letto del paziente: una breve storia della clinica acido-base". Terapia intensiva 8, n. 4 (2004): 253.</li>
</ol>

acidi e basi di bronsted-lowry

La classe delle reazioni acido-base è studiata da diverso tempo. Nel 1680, Robert Boyle riportò caratteristiche delle soluzioni acide che includevano la loro capacità di dissolvere molte sostanze, di cambiare i colori di alcuni coloranti naturali e di perdere queste caratteristiche dopo essere entrate in contatto con soluzioni alcaline (basi). Nel XVIII secolo si riconobbe che gli acidi hanno un sapore aspro, reagiscono con il calcare per liberare una sostanza gassosa (ora nota come CO2) e interagiscono con gli alcali per formare sostanze neutre. Nel 1815, Humphry Davy contribuì notevolmente allo sviluppo del moderno concetto acido-base dimostrando che l'idrogeno è il costituente essenziale degli acidi. Nello stesso periodo Joseph Louis Gay-Lussac concluse che gli acidi sono sostanze in grado di neutralizzare le basi e che queste due classi di sostanze possono essere definite solo l'una rispetto all'altra. Il significato dell'idrogeno fu enfatizzato nuovamente nel 1884 quando Svante Arrhenius definì un acido come un composto che si dissolve in acqua per produrre cationi idrogeno (ora riconosciuti come ioni idronio) e una base come un composto che si dissolve in acqua per produrre anioni idrossido.

Acidi e basi di Brønsted-Lowry

Johannes Brønsted e Thomas Lowry proposero una descrizione più generale nel 1923 in cui gli acidi e le basi erano definiti in termini di trasferimento di ioni idrogeno, H+. (Si noti che questi ioni idrogeno sono spesso indicati semplicemente come protoni, poiché quella particella subatomica è l'unico componente dei cationi derivati dall'isotopo più abbondante dell'idrogeno, 1H.) Un composto che dona un protone a un altro composto è chiamato Brønsted-Lowry acido e un composto che accetta un protone è chiamato base di Brønsted-Lowry. Una reazione acido-base è, quindi, il trasferimento di un protone da un donatore (acido) ad un accettore (base).

Il concetto di coppie coniugate è utile per descrivere le reazioni acido-base di Brønsted-Lowry (e anche altre reazioni reversibili). Quando un acido dona H+, la specie che rimane è chiamata base coniugata dell'acido perché reagisce come accettore di protoni nella reazione inversa. Allo stesso modo, quando una base accetta H+, viene convertita nel suo acido coniugato. La reazione tra acqua e ammoniaca illustra questa idea come mostrato di seguito.

 <img alt="Eq1" src="/files/ftp_upload/11394/11394_Equation_1.png" style="height: 50px;" />

Nella direzione in avanti, l'acqua agisce come un acido donando un protone all'ammoniaca e successivamente diventando uno ione idrossido, OH^−, la base coniugata dell'acqua. L'ammoniaca agisce come base nell'accettare questo protone, diventando uno ione ammonio, NH_4^+, l'acido coniugato dell'ammoniaca. Nella direzione opposta, uno ione idrossido agisce come base accettando un protone dallo ione ammonio, che agisce come un acido.

Gli acidi e le basi forti si dissociano completamente in una soluzione. I loro acidi e le loro basi coniugate sono estremamente deboli e non possono rispettivamente donare o accettare i protoni per effettuare la reazione inversa; pertanto, le reazioni che coinvolgono acidi e basi forti si completano essenzialmente quando si trovano in una soluzione acquosa. D'altra parte, gli acidi e le basi deboli si dissociano parzialmente nelle soluzioni e producono rispettivamente basi e acidi deboli coniugati. Questi acidi o basi coniugati deboli possono effettuare la reazione inversa, e quindi le reazioni di acido e base deboli raggiungono un equilibrio a seconda delle forze relative degli acidi e delle basi deboli. Riassumendo, un acido più forte produrrà una base coniugata altrettanto più debole mentre una base più forte produrrà un acido coniugato altrettanto più debole e viceversa. La tabella 1 illustra la relazione tra diverse coppie acido-base coniugate.

<table style="margin-left: auto; margin-right: auto;">
	<tbody>
		<tr>
			<td style="color: black; border: 1px solid black; text-align: center;">Strong Acid</td>
			<td style="color: black; border: 1px solid black; text-align: center;">Very Weak Conjugate Base</td>
		</tr>
		<tr>
			<td style="color: black; border: 1px solid black; text-align: center;">HCl</td>
			<td style="color: black; border: 1px solid black; text-align: center;">Cl−</td>
		</tr>
		<tr>
			<td style="color: black; border: 1px solid black; text-align: center;">HNO3</td>
			<td style="color: black; border: 1px solid black; text-align: center;">NO3−</td>
		</tr>
		<tr>
			<td colspan="2"></td>
		</tr>
		<tr>
			<td style="color: black; border: 1px solid black; text-align: center;">Weak Acid</td>
			<td style="color: black; border: 1px solid black; text-align: center;">Weak Conjugate Base</td>
		</tr>
		<tr>
			<td style="color: black; border: 1px solid black; text-align: center;">HF</td>
			<td style="color: black; border: 1px solid black; text-align: center;">F−</td>
		</tr>
		<tr>
			<td style="color: black; border: 1px solid black; text-align: center;">NH4+</td>
			<td style="color: black; border: 1px solid black; text-align: center;">NH3</td>
		</tr>
		<tr>
			<td colspan="2"></td>
		</tr>
		<tr>
			<td style="color: black; border: 1px solid black; text-align: center;">Very Weak Acid</td>
			<td style="color: black; border: 1px solid black; text-align: center;">Strong Conjugate Base</td>
		</tr>
		<tr>
			<td style="color: black; border: 1px solid black; text-align: center;">OH−</td>
			<td style="color: black; border: 1px solid black; text-align: center;">O2−</td>
		</tr>
		<tr>
			<td style="color: black; border: 1px solid black; text-align: center;">CH4</td>
			<td style="color: black; border: 1px solid black; text-align: center;">CH3−</td>
		</tr>
	</tbody>
</table>

Tabella 1: Forza relativa di alcune coppie acido-base coniugate.

Questo testo è adattato da <a href="https://openstax.org/books/chemistry-2e/pages/14-1-bronsted-lowry-acids-and-bases">Openstax, Chemistry 2e, Section 14.4 Brønsted-Lowry Acid and Bases</a>

Acidi e basi di Bronsted-Lowry

La classe di reazione acido-base è stata studiata per un bel po' di tempo. Nel 1680, Robert Boyle riportò i tratti delle soluzioni acide che includevano la loro capacità di ...

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proton transfer in acids and bases

Acids and bases can be understood through proton transfer. Early scientists described acids by their sour taste, their effect on dyes, and their reaction with limestone, which releases carbon dioxide. Robert Boyle also noted that acid solutions can lose these properties after contact with alkali, or base, solutions.

The modern view developed step by step. Humphry Davy showed that hydrogen is the essential part of acids. Joseph Louis Gay-Lussac then described acids as substances that neutralize bases. In 1884, Svante Arrhenius defined acids as compounds that produce hydrogen cations in water and bases as compounds that produce hydroxide anions.

Brønsted and Lowry gave a broader definition in 1923. A Brønsted-Lowry acid donates a proton, and a Brønsted-Lowry base accepts a proton. An acid-base reaction is the transfer of that proton from the donor to the acceptor. The hydrogen ion, H+, is often called a proton because it is the only particle in the most common hydrogen isotope.

Conjugate pairs help describe these reactions. After an acid donates H+, the species left behind is its conjugate base. After a base accepts H+, it becomes its conjugate acid. Water and ammonia show this clearly. In the forward direction, water donates a proton to ammonia, so water acts as an acid and becomes hydroxide, OH−, while ammonia acts as a base and becomes ammonium, NH4+.

The reverse reaction also fits the same pattern. Hydroxide can accept a proton, so it acts as a base. Ammonium can donate a proton, so it acts as an acid. Strong acids and strong bases dissociate completely in solution, and their conjugate acids and bases are extremely weak. That is why reactions with strong acids and bases in water usually go to completion.

Weak acids and weak bases behave differently. They only partially dissociate in solution and form weak conjugate bases or weak conjugate acids. These conjugate species can take part in the reverse reaction, so weak acid-base reactions reach equilibrium. In general, a stronger acid gives a weaker conjugate base, and a stronger base gives a weaker conjugate acid.

Proton Transfer in Acids and Bases

bronsted-lowry acids and bases

Acids and bases can be understood through proton transfer. Early scientists described acids by their sour taste, their effect on dyes, and their reaction with limestone, which releases carbon dioxide. Robert Boyle also noted that acid solutions can lose these properties after contact with alkali, or base, solutions. The modern view developed step by step. Humphry Davy showed that hydrogen is the essential part of acids. Joseph Louis Gay-Lussac then described acids as substances that neutralize...

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Brønsted-Lowry acid-base chemistry is based on proton transfer. That idea helps show how the strength of an acid is linked to the strength of its conjugate base, and vice versa. Acid and base strength is measured with ionization constants, Ka and Kb, which tell how far an ionization reaction goes. For the conjugate pair HA and A−, the acid ionizes in water and the base reaction can be written for the partner species. When the two reactions are added, they give the autoionization of water. The...

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Learn how Brønsted-Lowry acids and bases work through proton transfer, conjugate pairs, and the difference between strong and weak acid-base reactions.

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Proton Transfer in Acids and Bases

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