Lexico Unisciel english version

The Arrhenius theory of acids and bases defines an acid as a substance that releases H+ ions in solution, and a base as a substance that releases OH- ions in solution.

We know that ammonia, NH3, can act as a base and exhibits alkaline properties, but contains no OH- ions. How is this possible?

In this lesson, we will learn about the Bronsted-Lowry theory of acids and bases, which fills in the gaps in the Arrhenius theory.

You have learned that acids are substances with a pH lower than 7.

We can test for acids using universal indicator solution or paper, which turns red or orange, all depending on the strength of the acid.

Blue litmus paper also turns red in the presence of an acid.

An example of an acid that you are likely familiar with is hydrochloric acid.

Hydrogen chloride dissolves in water to give hydrochloric acid.

Hydrogen chloride will dissociate to give a H+ ion and a Cl- ion.

HCl  H+ and Cl-

The H+ ion is not just “released”, but it is transferred to another substance.

HCl(g) + H2O(l)  H3O+(aq) + Cl-(aq)

For our example, water accepts the H+ ion to form a hydronium ion, H3O+.

It is also known as an oxonium ion.

A Bronsted-Lowry acid is a substance that releases or donates H+ ions in solution, in our case, HCl.

A Bronsted-Lowry base is a substance that accepts H+ ions in solution, in our case, H2O.

Notice that the definition of a Bronsted-Lowry acid or base is not a function of pH.

So an acid-base reaction, according to the Bronsted-Lowry theory, involves the transfer of a H+ ion from one substance to another.

In the introduction, we mentioned that ammonia acts as a base even though it does not release OH- ions, which is stipulated in the Arrhenius theory.

When ammonia dissolves in water, it accepts a H+ ion from water.

NH3(g) + H2O(l)  NH4+(aq) + OH-(aq)

Notice that this reaction also involves a H+ ion transfer.

Can you figure out which substance is acting as an acid in this example? Which is acting as a base?

Please pause the lesson to think about this, and resume when you are ready.

In this example, water acts as a Bronsted-Lowry acid because it releases a H+ ion.

Ammonia acts as a Bronsted-Lowry base because it accepts a H+ ion.

A base, according to the Bronsted-Lowry theory, does not need to have a pH greater than 7 or turn red litmus paper blue, or turn universal indicator solution or paper blue or purple.

The only requirement is that it is able to accept H+ ions.

Have you noticed that H2O acted as a base in our first example but as an acid in the second example?

Substances that can act as either an acid or a base are amphoteric.

An easy way to remember this term is to recall that frogs are amphibians – they live on land and water.

In conclusion, according to the Bronsted-Lowry theory, an acid is a substance that dissociates to release or donate H+ ions.

A Bronsted-Lowry base is a substance that accepts H+ ions.

Therefore, an acid-base reaction, according to this theory, involves the transfer of a proton.