Identification of Bases

Identification of Strong and Weak Bases

Certain ionic compounds with anions other than hydroxide are weak Arrhenius bases. Sodium acetate, NaC₂H₃O₂, is an example. When it dissolves, it forms sodium ions, Na⁺, and acetate ions, C₂H₃O₂⁻. The latter react with water in a reversible fashion to form acetic acid molecules, HC₂H₃O₂, and hydroxide ions, OH⁻. In this reaction, an H⁺ ion is transferred from each water molecule to an acetate ion.

NaC₂H₃O₂(s)         Na⁺(aq) + C₂H₃O₂⁻(aq)

C₂H₃O₂⁻(aq) + H₂O(l)    HC₂H₃O₂(aq) + OH⁻(aq)

Both the C₂H₃O₂⁻ ions and the OH⁻ ions attract H⁺ ions strongly enough to be constantly passing them back and forth. In a very short time, the rate of the forward reaction becomes equal to the rate of the reverse reaction, yielding a constant amount of C₂H₃O₂⁻, water, HC₂H₃O₂, and OH⁻. The hydroxide ion attracts the H⁺ ions more strongly than the acetate ion does, so the reaction leads to more C₂H₃O₂⁻ in the final solution than OH⁻. In a typical solution of sodium acetate, for each 100,000 acetate ions added, there are 99,998 acetate ions, C₂H₃O₂⁻, 2 acetic acid molecules, HC₂H₃O₂, and 2 hydroxide ions, OH⁻.

We can expect the anions in other water-soluble ionic compounds that contain group 1 and 2 metal cations to react with water in a way that is similar to the reaction in the sodium acetate solution.

NaA(s)         Na⁺(aq) + A⁻(aq)                A⁻ = anion

A⁻(aq) + H₂O(l)        HA(aq) + OH⁻(aq)

Some anions do not react with water. These are the anions formed from the strong monoprotic acids: Cl⁻, Br⁻, I⁻, NO₃⁻, and ClO₄⁻. They are very stable in water solution, so they have almost no tendency to attract H⁺ ions. Ionic compounds that contain these ions are not acids or bases in the Arrhenius acid-base sense. They do not react with water to form either H₃O⁺ or OH⁻. We call them neutral in the Arrhenius acid-base sense.

NaCl(s)         Na⁺(aq) + Cl⁻(aq)

Cl⁻(aq) + H₂O(l)         no reaction

Anions created from the incomplete loss of hydrogen ions by polyprotic acids can be acidic or basic. It is useful to remember that the following two ions are acidic: hydrogen sulfate, (HSO₄⁻) and dihydrogen phosphate (H₂PO₄⁻). Sodium hydrogen sulfate is used as a disinfectant and in the manufacture of paper, soap, perfumes, foods, and industrial cleaners. Boiler water is treated with sodium dihydrogen phosphate to minimize the buildup of scale on the walls of the boiler, and NaH₂PO₄ is also used as a food additive and in paint removers and cleaners.

NaHSO₄ (s)         Na⁺(aq) + HSO₄⁻(aq)

HSO₄⁻(aq) + H₂O(l)        H₃O⁺(aq) + SO₄²⁻(aq)

NaH₂PO₄ (s)         Na⁺(aq) + H₂PO₄⁻(aq)

H₂PO₄⁻(aq) + H₂O(l)      H₃O⁺(aq) + HPO₄²⁻(aq)

We will use the following criterion to identify ionic compounds that are basic.

We expect water-soluble ionic compounds that contain group 1 or 2 metal cations to be basic unless they contain one of the neutral anions (Cl⁻, Br⁻, I⁻, NO₃⁻, and ClO₄⁻) or one of the acidic ions (HSO₄⁻ and H₂PO₄⁻).

We will use the following criteria to identify strong and weak bases.

Water-soluble ionic compounds that contain hydroxide ions are strong bases.

We will expect all other bases to be weak.

For example, using these guidelines, we would predict that sodium hydrogen carbonate, NaHCO₃, is a weak base. We know it is a base because it is a water-soluble ionic compound that contains a group 1 metal ion and an anion that is not on our list of neutral or acidic anions. We know it is weak because it is not a water-soluble hydroxide. Knowing this, we can expect that when the NaHCO₃ dissolves in water, the hydrogen carbonate ions, HCO₃⁻, react with water in a reversible way to yield hydroxide ions.

NaHCO₃ (s)        Na⁺(aq) + HCO₃⁻(aq)

HCO₃⁻(aq) + H₂O(l)        H₂CO₃(aq) + OH⁻(aq)

Sodium carbonate is found in fire extinguishers, baking powders, soaps and detergents, antacids, and mouthwashes.

The table below summarizes the characteristics of Arrhenius bases. It shows how you can recognize substances as bases and how you can classify them as strong or weak bases.

Table: Arrhenius Bases (There are others in each category. You should recognize the examples given.)

Strong Weak

Anions* OH^- Anions in ionic compounds except OH⁻, Cl⁻, Br⁻, I⁻, NO₃⁻, ClO₄⁻, HSO₄⁻, and H₂PO₄⁻

Certain uncharged molecules None NH₃

*in ionic compounds with group 1 or 2 metal cations