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-, HSO4-
NO3-, ClO4-, HSO4-
and H2PO4-.

Certain uncharged
molecules

None

NH3

*in ionic compounds with group 1 or 2 metal cations