Bronsted-Lowry Concept
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Brønsted-Lowry Concept

In this section we will consider the Brønsted-Lowry concept. This concept focuses on what an acid or base does.

Acids

With the Brønsted-Lowry concept we usually refer to a hydrogen ion as a proton. That is because a proton is all that is left when a hydrogen atom loses an electron to become an ion.

Brønsted and Lowry independently came up with the idea that an acid is an acid because it provides or donates a proton to something else. When an acid reacts, the proton is transferred from one chemical to another. As will be noted later, the chemical which accepts the proton is a base.

When an acid dissolves and dissociates in water it gives a proton to the water. Equations to represent this are shown here ( and in example 16 in your workbook). The Brønsted-Lowry view is that the acid (HCl) gives a proton to water to make two ions, one of which is H3O+. H3O+ is called hydronium ion. (By the way a hydronium ion is sometimes called an oxonium ion.)

HCl + H2O rtarrow.gif (850 bytes) H3O+ + Cl-

These equations show a different acid (H2SO4) giving a proton to water. In this case, the product HSO4- still has a proton that can be donated to another water molecule.
H2SO4 + H2O rtarrow.gif (850 bytes)  H3O+ +  HSO4-
HSO4- + H2O rtarrow.gif (850 bytes) H3O+ + SO42-
This equation shows HCl giving a proton to a hydroxide ion (OH-) rather than water.

HCl + OH- rtarrow.gif (850 bytes) H2O + Cl-

The first chemical in each of these equations is an acid because they are each giving a proton to something else.

Bases

Note that in order for an acid to act like an acid, there needs to be something for it to react with. There needs to be something to take the proton. There needs to be a base. A base is a proton acceptor. Compare this to the definition that an acid is a proton donor.

Bases are the opposite of acids. Bases are basic because they take or accept protons. Hydroxide ion, for example can accept a proton to form water. Brønsted and Lowry realized that not all bases had to have a hydroxide ion. As long as something can accept a proton it is a base.

OH- + H+ rtarrow.gif (850 bytes) H2O

So anything, hydroxide or not, that can accept a proton is a base under the Brønsted-Lowry definition. The water molecules that accept protons when HCl dissolves in water are acting as bases.

H2O + HCl rtarrow.gif (850 bytes) H3O+ + Cl-

 

Some additional examples of Brønsted-Lowry bases are shown accepting protons in these equations (which are also shown in example 17 in your workbook). These examples do not show the acids which are providing the protons.

OH- + H+ rtarrow.gif (850 bytes) H2O

Ammonia can accept or react with hydrogen ion to give ammonium ion NH4+.

NH3 + H+ rtarrow.gif (850 bytes) NH4+

Carbonate ion can accept a hydrogen ion, or accept a proton, to become bicarbonate ion.

CO32- + H+ rtarrow.gif (850 bytes) HCO3-

Also, water molecules, as mentioned before, can act as a base by accepting protons.

H2O + H+ rtarrow.gif (850 bytes) H3O+

Hydroxide, ammonia, carbonate and water are all Brønsted-Lowry bases.

Be sure to note the distinction between ammonia and ammonium. NH3 is ammonia and NH4+ is ammonium. They sound very much the same and their formulas are very similar, but their chemical properties are quite different. They are different because one has one more proton than the other. Ammonia is a base and ammonium is an acid. We'll take up another aspect of their relationship when we consider conjugate pairs.

Some phenomena that are readily explained using the Brønsted-Lowry concept are acid-base reactions (explained as proton transfer reactions), conjugate pair relationships, and amphoterism.

 

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