World Library  
Flag as Inappropriate
Email this Article

Brønsted acid

Article Id: WHEBN0001792769
Reproduction Date:

Title: Brønsted acid  
Author: World Heritage Encyclopedia
Language: English
Subject: Acid, Boric acid, Azide, Sodium hydride, Superacid, Decaborane, Fischer indole synthesis, Enol, Dehydration reaction, N-Butyllithium
Collection:
Publisher: World Heritage Encyclopedia
Publication
Date:
 

Brønsted acid

Independently, Johannes Nicolaus Brønsted and Thomas Martin Lowry formulated the idea that acids are proton (H+) donors while bases are proton acceptors.

In chemistry, the Brønsted–Lowry theory is an acid–base reaction theory, proposed independently by Johannes Nicolaus Brønsted and Thomas Martin Lowry in 1923.[1][2] The fundamental concept of this theory is that an acid (or Brønsted acid) is defined as being able to lose, or "donate" a proton (the hydrogen cation, or H+) while a base (or Brønsted base) is defined as a species with the ability to gain, or "accept," a proton.

Properties of acids and bases

The Brønsted–Lowry model of proton donors and proton acceptors in acid–base reactions is an improvement over the Arrhenius theory, which was limited for it stated that bases had to contain the hydroxyl group. The main effect of the Brønsted–Lowry definition is to identify the proton (H+) transfer occurring in the acid–base reaction.

In the Brønsted–Lowry theory, an acid donates a proton and the base accepts it. The ion or molecule remaining after the acid has lost a proton is known as that acid's conjugate base, and the species created when the base accepts the proton is known as the conjugate acid. This is expressed in the following reaction:

acid + base is in equilibrium with conjugate base + conjugate acid.

Notice how this reaction can proceed in either forward or backward direction; in each case, the acid donates a proton to the base.

With letters, the above equation can be written as:

HA + B is in equilibrium with A + HB+

The acid, HA, donates a H+ ion to become A−, its conjugate base. The base, B, accepts the proton to become HB+, its conjugate acid. In the reverse reaction, A− it accepts a H+ from HB+ to recreate HA in order to remain in equilibrium. In the reverse reaction, as HB+ has donated a H+ to A, it therefore recreates B and remains in equilibrium.

Example


Consider the following acid–base reaction, seen in the image to the right:

Template:Chem/link + Template:Chem/link is in equilibrium with Template:Chem/link + Template:Chem/link

Acetic acid, Template:Chem/link, is an acid because it donates a proton to water (Template:Chem/link) and becomes its conjugate base: the acetate ion (Template:Chem/link). In the same sense, Template:Chem/link is the base because it accepts a proton from Template:Chem/link and becomes its conjugate acid: the hydronium (Template:Chem/link).

Hydronium, Template:Chem/link, is the conjugate acid of water because, in the reverse reaction, it donates a proton to the acetate ion, Template:Chem/link, and becomes water. The acetate ion, Template:Chem/link, is the conjugate base of acetic acid because, in the reverse reaction, it accepts an proton from Template:Chem/link to become the acid.

Both of these processes demonstrate the equilibrium nature of the acid–base reaction.

Amphoteric substances

Water is amphoteric and can act as an acid or as a base. In the reaction between acetic acid, CH3CO2H, and water, H2O, discussed in the above section, water acts as a base.

CH3COOH + H2O is in equilibrium with CH3COO + H3O+

The acetate ion, CH3CO2, is the conjugate base of acetic acid and the hydronium ion, H3O+, is the conjugate acid of the base, water.

Water can also act as an acid, for instance when it reacts with ammonia. The equation given for this reaction is:

H2O + NH3 is in equilibrium with OH + NH4+

in which H2O donates a proton to NH3. The hydroxide ion is the conjugate base of water acting as an acid and the ammonium ion is the conjugate acid of the base, ammonia.

Acid strength

A strong acid, such as hydrochloric acid, dissociates completely. A weak acid, such as acetic acid, may be partially dissociated; the acid dissociation constant, pKa, is a quantitative measure of the strength of the acid.

A wide range of compounds can be classified in the Brønsted–Lowry framework: mineral acids and derivatives such as sulfonates, phosphonates, etc., carboxylic acids, amines, carbon acids, 1,3-diketones such as acetylacetone, ethyl acetoacetate, and Meldrum's acid, and many more.

Brønsted concept and Lewis acids/bases

A Lewis base, defined as an electron-pair donor, can act as a Brønsted–Lowry base as the pair of electrons can be donated to a proton. This means that the Brønsted–Lowry concept is not limited to aqueous solutions. Any donor solvent S can act as a proton acceptor.

AH + S: is in equilibrium with A + SH+

Typical donor solvents used in acid–base chemistry, such as dimethyl sulfoxide or liquid ammonia have an oxygen or nitrogen atom with a lone pair of electrons that can be used to form a bond with a proton.

Some Lewis acids, defined as electron-pair acceptors, also act as Brønsted–Lowry acids. For example, the aluminium ion, Al3+ can accept electron pairs from water molecules, as in the reaction

Al3+ + 6H2O → Al(H2O)63+

The aqua ion formed is a weak Brønsted–Lowry acid.

Al(H2O)63+ + H2O is in equilibrium with Al(H2O)5OH2+ + H3O+...........Ka = 1.2 × 10−5 [3]

The overall reaction is described as acid hydrolysis of the aluminium ion.

However not all Lewis acids generate Brønsted–Lowry acidity. The magnesium ion similarly reacts as a Lewis acid with six water molecules

Mg2+ + 6H2O → Mg(H2O)62+

but here very few protons are exchanged since the Brønsted–Lowry acidity of the aqua ion is negligible (Ka = 3.0 × 10−12).[3]

Boric acid also exemplifies the usefulness of the Brønsted–Lowry concept for an acid that does not dissociate but does effectively donate a proton to the base, water. The reaction is

B(OH)3 + 2H2O is in equilibrium with B(OH)4 + H3O+

Here boric acid acts as a Lewis acid and accepts an electron pair from the oxygen of one water molecule. The water molecule in turn donates a proton to a second water molecule and, therefore, acts as a Brønsted acid.

See also

References

de:Säure-Base-Konzepte#Definition nach Brønsted und Lowry
This article was sourced from Creative Commons Attribution-ShareAlike License; additional terms may apply. World Heritage Encyclopedia content is assembled from numerous content providers, Open Access Publishing, and in compliance with The Fair Access to Science and Technology Research Act (FASTR), Wikimedia Foundation, Inc., Public Library of Science, The Encyclopedia of Life, Open Book Publishers (OBP), PubMed, U.S. National Library of Medicine, National Center for Biotechnology Information, U.S. National Library of Medicine, National Institutes of Health (NIH), U.S. Department of Health & Human Services, and USA.gov, which sources content from all federal, state, local, tribal, and territorial government publication portals (.gov, .mil, .edu). Funding for USA.gov and content contributors is made possible from the U.S. Congress, E-Government Act of 2002.
 
Crowd sourced content that is contributed to World Heritage Encyclopedia is peer reviewed and edited by our editorial staff to ensure quality scholarly research articles.
 
By using this site, you agree to the Terms of Use and Privacy Policy. World Heritage Encyclopedia™ is a registered trademark of the World Public Library Association, a non-profit organization.
 



Copyright © World Library Foundation. All rights reserved. eBooks from World eBook Library are sponsored by the World Library Foundation,
a 501c(4) Member's Support Non-Profit Organization, and is NOT affiliated with any governmental agency or department.