Brønsted–Lowry acid–base theory
|Independently, Johannes Nicolaus Brønsted and Thomas Martin Lowry formulated the idea that acids are proton (H+) donors while bases are proton acceptors.|
|Acids and bases|
The Brønsted–Lowry theory is an acid–base reaction theory. It was proposed independently by Johannes Nicolaus Brønsted and Thomas Martin Lowry in 1923. The fundamental concept of this theory is that when an acid and a base react with each other, the acid forms its conjugate base, and the base forms its conjugate acid by exchange of a proton (the hydrogen cation, or H+). This theory is a generalization of the Arrhenius theory to include in the definition substances like boric acid which are acids by virtue of their reactions with bases, but do not liberate hydrogen ions by dissociation.
Brønsted and Lowry
The theory was proposed independently and simultaneously by physical chemists Johannes Nicolaus Brønsted in Denmark and Thomas Martin Lowry in England in 1923. That same year, Gilbert N. Lewis proposed an electronic theory of acid–base reactions. For aqueous solutions, most Brønsted acids are also Lewis acids and most Brønsted bases are also Lewis bases.
Properties of acids and bases
The Arrhenius theory for defining acids and bases states that acids are substances which dissociate in their aqueous solution to give H+ (hydrogen ions) while bases are substances which dissociate in their aqueous solution to give OH− (hydroxyl ions). This concept was able to explain the catalytic action of acids and reaction of acids and bases in aqueous solution but failed to explain why molecules not having the above ions were able to neutralize acids and bases, acid – base reactions occurring in gaseous phases and why it did not identify metal oxides as bases. The Brønsted–Lowry model of proton donors and proton acceptors in acid–base reactions is an improvement over the Arrhenius theory. This theory showed that substances which have one or more lone pair of electrons and are proton acceptors are bases.
In the Brønsted–Lowry theory, an acid donates a proton and the base accepts it. A molecule containing an acidic hydrogen (a hydrogen atom attached to an electonegative atom) is considered an acid. A molecule having the tendency to form a bond with proton is considered a base. 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 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 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. The conjugate acid and the conjugate base differ by only a proton.
Consider the following acid–base reaction, seen in the image to the right:
3COOH + H
Acetic acid, CH
3COOH, is an acid because it donates a proton to water (H
2O) and becomes its conjugate base: the acetate ion (CH
). In the same sense, H
2O is the base because it accepts a proton from CH
3COOH and becomes its conjugate acid: the hydronium (H
, is the conjugate acid of water because, in the reverse reaction, it donates a proton to the acetate ion, CH
, and becomes water. The acetate ion, CH
, is the conjugate base of acetic acid because, in the reverse reaction, it accepts an proton from H
to become the acid. Both of these processes demonstrate the equilibrium nature of the acid–base reaction.
- CH3COOH + H2O CH3COO− + H3O+
Water can also act as an acid, for instance when it reacts with ammonia. The equation given for this reaction is:
- H2O + NH3 OH− + NH4+
Strength of acids and bases
A strong acid, such as hydrochloric acid, dissociates completely. A weak acid, such as acetic acid is only partially dissociated; the acid dissociation constant, Ka, is a quantitative measure of the strength of the acid. The tendency of an acid to lose proton determines its acidic strength while the tendency of a base to gain protons determines its basic strength. The conjugate of a strong acid is always a weak base and the conjugate of a weak acid is always a strong base and vice versa. In the dissociation reaction of acetic acid shown above the products formed are strong acid (H3O+) and strong base (CH3COO-).
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, pyrrole, pyridine and Meldrum's acid.
Though this theory is an improvement over the Arrhenius theory but it has some limitations;
- requires the transfer of a proton and a solvent which can dissolve the substance into respective ions.
- the reactions between acidic oxides and basic oxides cannot be explained on the basis of this theory.
An example of reaction between acidic and basic oxides is this:
CaO + SO
3 = CaSO
CaO is a basic oxide while SO3 is an acidic oxide. This reaction does not involves the transfer of a proton and cannot be explained on the basis of above concept.
- KOH and KNH2 are not considered Brønsted bases, but rather salts containing the bases OH− and NH2−.
- this theory does not recognize Lewis acids like BF
3 and AlCl
3 as acids because they do not have a proton.
- Non-protonic acid base reactions cannot be explained on the basis of this theory.
Examples of non-protonic acid–base reactions are:
2 + SO
3 + H
- The Brønsted–Lowry model calls hydrogen-containing substances (like HCl) acids. Thus, some substances, which many chemists considered to be acids, such as SO3 or BCl3, are excluded from this classification due to lack of hydrogen. Gilbert N. Lewis wrote in 1938, "To restrict the group of acids to those substances that contain hydrogen interferes as seriously with the systematic understanding of chemistry as would the restriction of the term oxidizing agent to substances containing oxygen."
Brønsted concept and Lewis acid–base
The hydrogen requirement of Arrhenius and Brønsted–Lowry was removed by the Lewis definition of acid–base reactions, devised by Gilbert N. Lewis in 1923, in the same year as Brønsted–Lowry, but it was not elaborated by him until 1938. Instead of defining acid–base reactions in terms of protons or other bonded substances, the Lewis definition defines a base (referred to as a Lewis base) to be a compound that can donate an electron pair, and an acid (a Lewis acid) to be a compound that can receive this electron pair. A Lewis base, defined as an electron-pair donor acts 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 and can be applied to non aqueous solutions also. Any donor solvent S can act as a proton acceptor.
- AH + S: A− + SH+
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.
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).
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