General Characteristics of Acids and Bases

Learning Goal 1
List the characteristics of acidic and basic solutions, and identify the ions responsible for these properties.

Acids are generally a class of substances that taste sour, such as vinegar, which is a dilute solution of acetic acid. Bases, or alkaline substances, are characterized by their bitter taste and slippery feel. The first precise definition of an an acid and base was given by Svante Arrhenius, and is referred to as Arrhenius Theory.

The Proton in Water: Arrhenius Theory

Swedish chemist Arrhenius defined an acid as a substance that ionizes in water to give hydrogen ions, and a base as a substance that ionizes in water to give hydroxide ions.

Hydrochloric acid,

HCl

, is a strong acid, and is very soluble in water. It dissociates into its component ions in the following manner:

HCl (g)

H⁺(aq) + Cl^-(aq)

The hydrogen ion interacts strongly with a lone pair of electrons on the oxygen of a water molecule. The resulting ion, H₃O⁺ is called the hydronium ion.

ACIDIC solutions are formed when an acid transfers a proton to water.

The reaction of HCl with water can be written in either of the following ways:

HCl (aq)  +  H

₂O (l)

H₃O⁺ (aq) + Cl^-(aq)

$HCl (aq)$ H⁺ (aq) + Cl^- (aq)

The Brønsted-Lowry Concept of Acids and Bases

Learning Goal 2
Define Brønsted acid and Brønsted base and conjugate acid-base pair. Identify the conjugate base associated with a Brønsted acid, and the conjugate acid associated with a given Brønsted base.

Acids are substances that are capable of donating a proton, and bases are substances capable of accepting a proton.

So, in the example above, HCl acts as a Brønsted Acid by donating a proton in water, and water in turn acts as a Brønsted Base by accepting a proton from HCl. (as shown in this animation).

Water can act as an acid or a base. Here is another example:

NH

₃(aq) + H₂O(l)

NH₄⁺(aq) + OH^-(aq)

Here, $H$ ₂O acts as a Brønsted acid by donating a proton to $NH$ ₃ which acts as a Brønsted base.
Using the Arrhenius definition, we say that the resulting solution is basic because it contains $OH$ ^- ions, thus we say that the $NH$ ₃ molecule is basic (a proton acceptor).

All Arrhenius acids are also Brønsted acids.
All Arrhenius bases are also Brønsted bases.

Conjugate Acid-Base Pairs

Let's look at the reaction of

NH

₃ and

H

₂O again:

(1)

NH

₃ + H₂O

NH₄⁺ + OH^-

The reverse of this reaction is:

(2)

NH

₄ + OH^-

NH₃ + H₂O

In this case, $NH$ ₄⁺ acts as an acid which donates a proton to $OH$ ^-. $OH$ ^- acts as a base.

An acid and a base that are related by the gain and loss of a proton are called a conjugate acid-base pair. For example, $NH$ ₄⁺ $is the conjugate acid of NH$ ₃, and $NH$ ₃ is the conjugate base of $NH$ ₄⁺.

Every acid has associated with it a conjugate base.
Likewise, every base has associated with it a conjugate acid.

Acids and bases can be either neutral or charged species: $H$ ₂O (acid or base, neutral), $O$ ₂^- (base, charged), $C$ ₂H₃O₂^- (acid, charged)

For any reaction:

HA + H

₂O

H₃O⁺ + A^-

If HA is a strong acid because it gives up its proton readily, then $A$ ^- is a weak base because it has little affinity for the proton.

If HA is a weak acid because it donates very few protons to the water, then $A$ ^- has a high affinity for a proton, and $A$ ^- is a stronger base than water.