The Development and Theories of Acids and Bases

What are Acids and Bases?

The terms acid and base are often used to classify substances used in chemical reactions. The specific definition of each term depends on which acid-base theory we choose to use. The three main acid-base definitions we should be familiar with are those credited to Arrhenius, Brønsted-Lowry, and Lewis.

Arrhenius Concept

The Arrhenius Concept defines acids as substances that produce H⁺ (hydrogen ions) in solution.

HCl(aq) → H¹⁺(aq) + Cl^1–(aq)

H₃PO₄(aq) → 3H¹⁺(aq) + PO₄^3–(aq)

According to this concept, hydrochloric and phosphoric acids are acids because they dissociate to make hydrogen ions in solution.

It defines bases as substances that produce OH^– (hydroxide ions) in solution.

NaOH(aq) → Na¹⁺(aq) + OH^1–(aq)

Ca(OH)₂(aq) → Ca²⁺(aq) + 2OH^1–(aq)

According to this concept, sodium and calcium hydroxides are bases because they dissociate to make hydroxide ions in solution.

Brønsted-Lowry Model

The Brønsted-Lowry Model defines acids as proton donors. In other words, acids donate H⁺ (hydrogen ions) in solution.

HCl(aq) + H₂O(l) → H₃O¹⁺(aq) + Cl^1–(aq)

H₃PO₄(aq) + H₂O(l) → H₃O¹⁺(aq) + H₂PO₄^1–(aq)

According to this model, hydrochloric and phosphoric acids are acids because they donate a hydrogen ion to the water forming the hydronium ion.

It defines bases as proton acceptors. In other words, bases accept H⁺ (hydrogen ions) in solution.

NH₃(aq) + H₂O(l) → NH₄¹⁺(aq) + OH^1–(aq)

According to this model, NH₃(aq) is a base because it accepts a hydrogen ion to form an ammonium ion.

The first equation sets up two interesting relationships. Not only does the ammonia accept a hydrogen ion to become the ammonium ion, the hydroxide ion could accept a hydrogen ion from the ammonium and become water. These two relationships make the ammonia/ammonium ion and the water/hydroxide ion conjugate acid/base pairs.

Lewis’ Theory

In Lewis’ Theory, acids are electron pair acceptors in solution, while bases are electron pair donors in solution.

BF₃(aq) + :NH₃(aq) → F₃B:NH₃

According to this theory, BF₃(aq) is an acid because it accepts the electron pair from NH₃(aq) which is the base because it donates the electron pair to BF₃(aq).

The pH Scale

pH represents the relative acidity of a solution based on hydrogen ion or hydronium ion concentration and can be found using either of the following equations:

The amphoteric nature of pure water contributes to its neutral nature. This means that water can act as an acid or a base due to the concept known as autoionization, which is shown in the following two examples.

H₂O(l) → H¹⁺(aq) + OH^1–(aq)

According to the Arrhenius Concept, water (in the equation above) is acting as an acid by producing hydrogen ions as well as a base by producing hydroxide ions.

2H₂O(l) → H₃O¹⁺(aq) + OH^1–(aq)

According to the Brønsted-Lowry Model one water molecule (in the equation above) is acting as an acid by donating a hydrogen ion to the other molecule of water forming a hydronium ion. The water molecule that is accepting the hydrogen ion is acting as the base.

If we write the equilibrium expression for this we get:

K_w = [H₃O¹⁺ ][OH^1– ] = 1.0×10^–14 at 25 °C

We can solve for the [H₃O¹⁺ ] or [OH^1– ] based on the previous equation.

For every H₃O¹⁺ produced, one OH^1– is made so:
[H₃O¹⁺ ][OH^1– ] = 1.0×10^–14 becomes
[x][x] = 1.0×10^–14
x² = 1.0×10^–14
x = 1.0×10^–7
Therefore, [H₃O¹⁺ ] = 1.0×10^–7 = [OH^1–]
If you are given either the [H₃O¹⁺ ] or [OH^1– ], you can solve for the other.

Sample Problem

Given [OH^1– ] = 1.0×10^-5 M at 25°C, solve for [H₃O¹⁺ ].

K_w = [H₃O¹⁺ ][OH^1– ] = 1.0×10^–14 M²
[H₃O¹⁺ ][1.0×10^-5 M] = 1.0×10^–14 M²

Substances other than water can also be amphoteric. Let’s look at a hydrated aluminum hydroxide ion as an example.

[Al(H₂O)₄(OH)₂]¹⁺+ NaOH ↔ Al(H₂O)₃(OH)₃ + OH^1–

Then a water of hydration from [Al(H₂O)₄(OH)₂]¹⁺ donates the hydrogen ion to become the third hydroxide ion it is acting as a Brønsted-Lowry acid.

[Al(H₂O)₄(OH)₂]¹⁺ + HCl ↔ [Al(H₂O)₅OH]²⁺ + Cl^1–

When a hydroxide from [Al(H₂O)₄(OH)₂]¹⁺ accepts the hydrogen ion to create the fifth water of hydration it is acting as a Brønsted-Lowry base.

Relative Strengths of Acids and Bases

Acids and bases can be concentrated or dilute, weak or strong. Do not confuse the first two which describe the solution concentration, with the last two which describe the amount of dissociation.

• A weak acid or base is one where only a small portion of the particles dissociate or one that only partially ionizes.
• A strong acid or base is one that completely dissociates in water, or has 100% ionization. Strong acids include: HCl, HBr, HI, HNO₃, H₂SO₄, and HClO₄. All others are considered weak. Strong bases include: LiOH, NaOH, KOH, Mg(OH)₂, Ca(OH)₂, Sr(OH)₂ and Ba(OH)₂. All others are considered weak.