# Bronsted Acids and Bases

## A Brønsted acid is a molecule or ion that is able to donate a proton, and a Brønsted base is a species with the ability to accept a proton.

#### Key Points

• The Brønsted-Lowry theory can be defined by the following reaction: acid + base <=> conjugate base + conjugate acid. After the acid loses a proton, a conjugate base is formed, while the conjugate acid forms when the base accepts the proton. The reaction can proceed in either direction.

• Brønsted-Lowry acid-base theory has several advantages over Arrhenius theory. Only Bronsted theory can define the reaction between acetic acid and ammonia, which does not produce hydronium. It can also be used to describe molecular compounds, whereas Arrhenius acids must be ionic compounds.

• Some Lewis acids, which are electron-pair acceptors, also act as Brønsted-Lowry acids. In such cases, an ion accepts an electron pair from a water molecule, forming an aqua ion that acts as a weak Brønsted acid. But if the proton exchange is too weak, a Lewis acid cannot generate a Brønsted acid.

#### Terms

• any chemical species that acts as an acceptor of protons

• any chemical species that acts as a donor of protons

#### Examples

• In the reaction HNO3 + H2O <=> H3O+ + NO3-, NO3- is the conjugate base; this is easy to find because it doesn't have one proton, while the HNO3 acid does. H3O+ is the conjugate acid and has one more proton than its base, H2O.

#### Figures

1. ##### Water Acts As an Acid and a Base

One water molecule acts as a base and gains a proton to become a hydronium ion; the other water molecule acts as an acid and loses a proton to become a hydroxide ion.

In chemistry, the Brønsted-Lowry theory is an acid-base theory, proposed independently by Johannes Nicolaus Brønsted and Thomas Martin Lowry in 1923. In this system, Brønsted-Lowry acids and Brønsted-Lowry bases are defined as follows: an acid is a molecule or ion that is able to lose, or "donate," a hydrogen cation (H+, a proton); a base is a species with the ability to gain, or "accept," a hydrogen cation (a proton). It follows that, if a compound is to behave as an acid by donating a proton, there must be a base to accept the proton. So the Brønsted-Lowry concept can be defined by the reaction:

acid + base $\rightleftharpoons$ conjugate base + conjugate acid

The conjugate base is the ion or molecule remaining after the acid has lost a proton, and the conjugate acid is the species created when the base accepts the proton. The reaction can proceed in either the forward or backward direction; in each case, the acid donates a proton to the base.

Brønsted-Lowry acid-base theory has several advantages over Arrhenius theory. Consider the following reactions of acetic acid (CH3COOH), the organic acid that gives vinegar its characteristic taste:

1. $CH_3COOH + H_2O \rightleftharpoons CH_3COO^− + H_3O^+$
2. $CH_3COOH + NH_3 \rightleftharpoons CH_3COO^− + NH_4^+$

Both theories easily describe the first reaction: CH3COOH acts as an Arrhenius acid because it acts as a source of H3O+ when dissolved in water, and it acts as a Brønsted acid by donating a proton to water. In the second example CH3COOH undergoes the same transformation, in this case donating a proton to ammonia (NH3), but this cannot be described using the Arrhenius definition of an acid because the reaction does not produce hydronium.

Water is amphoteric, which means it can act as an acid or as a base. In the reaction between acetic acid, CH3CO2H, and water, H2O, water acts as a base. The acetate ion, CH3CO2-, is the conjugate base of acetic acid, and the hydronium ion, H3O+, is the conjugate acid of the base, water.

Figure 1

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

$H_2O + NH_3 \rightleftharpoons OH^− + NH_4^+$

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

Brønsted-Lowry theory can also be used to describe molecular compounds, whereas Arrhenius acids must be ionic compounds. Hydrogen chloride (HCl) and ammonia combine under several different conditions to form ammonium chloride, NH4Cl. In aqueous solution, HCl behaves as hydrochloric acid and exists as hydronium and chloride ions. The following reactions illustrate the limitations of Arrhenius's definition:

1. $H_3O^+ (aq) + Cl^− (aq) + NH_3 \rightarrow Cl^− (aq) + NH_4^+ (aq)$
2. $HCl (benzene) + NH_3 (benzene) \rightarrow NH_4Cl (s)$
3. $HCl (g) + NH_3 (g) \rightarrow NH_4Cl (s)$

As for the acetic acid reactions, both definitions work for the first example, where water is the solvent and a hydronium ion is formed. The next two reactions do not involve the formation of ions but are still proton transfer reactions. In the second reaction, hydrogen chloride and ammonia (dissolved in benzene) react to form solid ammonium chloride in a benzene solvent, and in the third, gaseous HCl and NH3 combine to form the solid.

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

A Lewis base, defined as an electron-pair donor, can act as a Brønsted-Lowry base since 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.

$AH + {S:} \rightleftharpoons A^− + SH^+$

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 following reaction:

$Al^{3+} + 6H_2O \rightarrow Al(H_2O)_6^{3+}$

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

$Al(H_2O)_6^{3+} + H_2O \rightleftharpoons Al(H_2O)_5OH^{2+} + H_3O^+$ ... $Ka = 1.2 × 10^{−5}$

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:

$Mg^{2+} + 6H_2O → Mg(H_2O)_6^{2+}$

but here, very few protons are exchanged since the Brønsted-Lowry acidity of the aqua ion is negligible (Ka = 3.0 × 10-12).

#### Key Term Glossary

acid
an electron pair acceptor; generally capable of donating hydrogen ions
##### Appears in these related concepts:
acidity
The quality or state of being acid.
##### Appears in these related concepts:
amine
organic compounds or functional group that contains a basic nitrogen atom with a lone pair
##### Appears in these related concepts:
amphoteric
Having the characteristics of both an acid and a base, and capable of reacting as either; amphiprotic.
##### Appears in these related concepts:
aqueous
Consisting mostly of water.
##### Appears in these related concepts:
atom
the smallest possible amount of matter that still retains its identity as a chemical element, now known to consist of a nucleus surrounded by electrons
##### Appears in these related concepts:
base
A proton acceptor, or an electron pair donor.
##### Appears in these related concepts:
bond
a link or force between neighboring atoms in a molecule
##### Appears in these related concepts:
carboxylic acid
Any of a class of organic compounds containing a carboxyl functional group (or a carbon with one double bond to an oxygen and another single bond to another oxygen, which is in turn bonded to a hydrogen).
##### Appears in these related concepts:
cation
A positively charged ion, as opposed to an anion.
##### Appears in these related concepts:
chemistry
The branch of natural science that deals with the composition and constitution of substances and the changes that they undergo as a consequence of alterations in the constitution of their molecules.
##### Appears in these related concepts:
compound
A substance made from any combination elements.
##### Appears in these related concepts:
conjugate acid
The species created when a base accepts a proton.
##### Appears in these related concepts:
conjugate base
The species that is created after the donation of a proton.
##### Appears in these related concepts:
electron
The subatomic particle having a negative charge and orbiting the nucleus; the flow of electrons in a conductor constitutes electricity.
##### Appears in these related concepts:
hydrolysis
A chemical process of decomposition involving the splitting of a bond and the addition of the hydrogen cation and the hydroxide anion of water.
##### Appears in these related concepts:
hydronium
The hydrated hydrogen ion, $H_3O^+$.
##### Appears in these related concepts:
hydroxide
An univalent anion (OH-1) based on the hydroxyl functional group.
##### Appears in these related concepts:
ion
An atom or group of atoms bearing an electrical charge, such as the sodium and chlorine atoms in a salt solution.
##### Appears in these related concepts:
ionic
of, relating to, or containing ions
##### Appears in these related concepts:
ionic compound
named by its cation followed by its anion
##### Appears in these related concepts:
Lewis acid
Any electrophylic compound that can accept a pair of electrons and form a coordinate covalent bond.
##### Appears in these related concepts:
Lewis base
Any nucleophylic compound that can donate a pair of electrons and form a coordinate covalent bond.
##### Appears in these related concepts:
liquid
A substance that flows and keeps no definite shape, such as water. A substance whose molecules, while not tending to separate from one another like those of a gas, readily change their relative position, and which therefore retains no definite shape, except that determined by the containing receptacle; an inelastic fluid.
##### Appears in these related concepts:
lone pair
a valence set of two electrons that exist without bonding or sharing with other atoms
##### Appears in these related concepts:
molecular compounds
named with a prefix before each element. The more electronegative element is written last and with an -ide suffix.
##### Appears in these related concepts:
molecule
the smallest particle of a specific element or compound that retains the chemical properties of that element or compound; two or more atoms held together by chemical bonds
##### Appears in these related concepts:
nitrogen
a chemical element (symbol N) with an atomic number of 7 and atomic weight of 14.0067
##### Appears in these related concepts:
oxygen
a chemical element (symbol O) with an atomic number of 8 and relative atomic mass of 15.9994
##### Appears in these related concepts:
proton
A positively charged subatomic particle forming part of the nucleus of an atom and determining the atomic number of an element; the nucleus of the most common isotope of hydrogen; composed of two up quarks and a down quark
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solid
A substance in the fundamental state of matter that retains its size and shape without need of a container (as opposed to a liquid or gas).
##### Appears in these related concepts:
solution
A homogeneous mixture, which may be liquid, gas or solid, formed by dissolving one or more substances.
##### Appears in these related concepts:
Solution
A homogeneous mixture, which may be liquid, gas or solid, formed by dissolving one or more substances.
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solvent
a substance that dissolves a solute, resulting in a solution
##### Appears in these related concepts:
system
the part of the universe being studied, arbitrarily defined to any size desired
##### Appears in these related concepts:
work
In thermodynamics, work performed by a closed system is the energy transferred to another system that is measured by the external generalized mechanical constraints on the system.