Definitions of acids and bases

Acids and bases are substances that are commonly found in our everyday lives.

For example, acids give fruits like oranges and lemons a sour taste.

There are several definitions of acids and bases that are used in chemistry.

Our first definition of acids and bases is from a chemist named Arrhenius.

An Arrhenius acid is a substance that increases the concentration of hydrogen ions in solution when it is dissolved in water.

An example of an Arrhenius acid is hydrogen chloride gas, which has the chemical formula HCl.

When gaseous HCl dissolves in water, it forms hydrogen ions,

H plus, and chloride anions, Cl minus in aqueous solution.

Since dissolving HCl in water increases the concentration of hydrogen ions in solution, HCl is an Arrhenius acid.

An aqueous solution of HCl is called hydrochloric acid.

An Arrhenius base is a substance that increases the concentration of hydroxide ions when it is dissolved in water.

The hydroxide ion has the formula OH minus.

An example of an Arrhenius base is sodium hydroxide, which is a white solid and has the chemical formula NaOH.

When NaOH dissolves in water, it dissociates into Na plus and OH minus.

Since dissolving NaOH in water increases the concentration of hydroxide ions in solution, sodium hydroxide is an Arrhenius base.

Our second definition of acids and bases is from two chemists, Bronsted and Lowry.

A Bronsted-Lowry acid is a “proton donor,” and a Bronsted-Lowry base is a “proton acceptor.”

Before we look at examples of Bronsted-Lowry acids and bases, we need to understand what is meant by the word “proton.”

A neutral hydrogen atom has a proton in the nucleus and an electron outside of the nucleus.

We represent a neutral hydrogen atom with an “H” and a single dot next to the H to represent the one valence electron.

If we take this electron away from our neutral hydrogen atom, we are left with a proton.

So, in acid-base chemistry, whenever we are talking about a proton, we are talking about the nucleus of a hydrogen atom.

Since a proton has a positive charge and is the nucleus of a hydrogen atom, we symbolize a proton by writing H+.

We can also refer to a proton as a hydrogen ion.

While it is convenient to write H+, a proton interacts strongly with a source of electron density such as a lone pair of electrons on a water molecule.

If a water molecule picks up a proton, a hydronium ion is formed.

A hydronium ion has the formula H3O plus.

Chemists use the notations of H+ and H3O+ interchangeably, even if it is more accurate to write H3O+.

The Bronsted-Lowry definition for acids and bases can be used whenever a reaction involves the transfer of a proton from one substance to another.

Let’s look at the reaction between HCl and H2O to form the chloride anion, Cl minus, and the hydronium ion, H3O plus.

If we look closely at the reactants and products in the balanced equation, this reaction involves the transfer of a proton from HCl to H2O.

A lone pair of electrons on the water molecule takes a proton from HCl to form the hydronium ion, H3O plus.

The two electrons that used to be in the bond between H and Cl end up on the Cl to form the chloride anion, Cl minus.

Remember that a Bronsted-Lowry acid is a “proton donor” and a Bronsted-Lowry base is a “proton acceptor.”

Since HCl donates a proton, HCl is a Bronsted-Lowry acid.

Since water accepts a proton, water is a Bronsted-Lowry base.

When HCl donates a proton, HCl turns into Cl-.

When water accepts a proton, H2O turns into H3O+.

Let's look at another example.

In this acid-base reaction, ammonia reacts with water to form the ammonium ion and the hydroxide ion.

A lone pair of electrons on the nitrogen in the ammonia molecule take a proton from a water molecule.

Adding a proton to NH3 forms NH4 plus.

The two electrons that used to be in the bond between oxygen and hydrogen end up on the oxygen which forms the hydroxide ion, OH minus.

Since water is donating a proton, in this reaction, water is a Bronsted-Lowry acid.

Since ammonia is accepting a proton, ammonia is a Bronsted-Lowry base.

The reverse reaction is also possible.

NH4 plus can react with OH minus to form NH3, and H2O.

If NH4 plus donates a proton, it turns into NH3.

Since NH4 plus is donating a proton, the ammonium ion is a Bronsted-Lowry acid.

If OH minus accepts a proton, it turns into H2O.

Since OH minus accepts the proton, the hydroxide ion a Bronsted-Lowry base.