Weak Acids

Submitted by ChemPRIME Staff on Thu, 12/16/2010 - 14:20

Not all molecules which contain hydrogen are capable of donating protons. For example, methane (CH4) and other hydrocarbons show no acidic properties at all. Carbon is not highly electronegative, and so electron densityThe ratio of the mass of a sample of a material to its volume. is fairly evenly shared in a C―H bond, and the hydrogen atom is unlikely to depart without at least one electron. Even when it is bonded to highly electronegative atomsThe smallest particle of an element that can be involved in chemical combination with another element; an atom consists of protons and neutrons in a tiny, very dense nucleus, surrounded by electrons, which occupy most of its volume. like oxygen or fluorine, a hydrogen atom is not always strongly acidic.

EXAMPLE 1 Acetic acid has the projection formula

Write an equation for transfer of a proton from acetic acid to water.

Solution By the electronegativity rule given above, only the hydrogen attached to oxygen should be acidic. The equation is

CH3COOH + H2O $\rightleftharpoons$ H3O+ + CH3COO      (1)

Note: To emphasize that only one hydrogen atom is acidic, the formula acetic acid is often written HC2H3O2. You may also find the formulas HAc or HOAc for acetic acid, where Ac or OAc represents the acetate ion, CH3COO.

The equation for the proton transfer between acetic acid and water is written with a double arrow because it occurs to only a limited extent. Like HgCl2, acetic acid is a weak electrolyte. According to Table 1 in Ions in Solution (Electrolytes), 0.001 M HC2H3O2 conducts slightly more than one-tenth as much current as the same concentrationA measure of the ratio of the quantity of a substance to the quantity of solvent, solution, or ore. Also, the process of making something more concentrated. of the strong acids HCl or HNO3. Therefore we can conclude that at a given instant only a little over 10 percent of the acetic acid molecules have donated protons to water molecules according to Eq. (1). Nearly 90 percent are in molecular form as CH3COOH (or HC2H3O2) and make no contribution to the current. Because acetic acid is not a strong enough proton donor to be entirely converted to hydronium ions in aqueous solution, it is called a weak acidAn acid that ionizes only partially in a given solvent.. A given concentration of a weak acid produces fewer hydronium ions per unitA particular measure of a physical quantity that is used to express the magnitude of the physical quantity; for example, the meter is the unit of the physical quantity, length. volume and therefore less acidity than the same concentration of a strong acid.

There are a large number of weak acids, but fortunately they fall into a few well-defined categories.

Carboxylic acids      These compounds have the general formula RCOOH. All react with water in the same way as acetic acid [Eq. (1)]. The strength of carboxylic acids is dependent on the electronegative strength of the atoms in the "R" groupThose elements that comprise a single column of the periodic table. Also called family.. Consider the compounds F3COOH and H3COOH. Fluorine is the most electronegative elementA substance containing only one kind of atom and that therefore cannot be broken down into component substances by chemical means., while hydrogen is comparable to carbon in electronegativity. Thus, the fluorines pull electron density away from the carboxyl group. This removes electron density from the acidic oxygen-hydrogen bondAn attractive force, either intramolecular or intermolecular, between an electronegative atom and a hydrogen atom attached to another electronegative atom., which weakens it. This weaker bond means that the hydrogen can be removed more easily, which creates a stronger acid. This concept can be applied to any R group. The more electronegative the R group, the stronger the carboxylic acid will be.

Weak oxyacids      These have the same general formula HnXOm as strong oxyacids, but the number of hydrogens is equal to or one less than the number of oxygens. For a weak oxyacid, in other words, mn + 1. Some examples are

Some of the weak oxyacids, H2CO3 for example, are very unstable and cannot be separated in pure form from aqueous solution.

Other molecules containing acidic hydrogen atoms      Hydrogen fluoride (HF) has a very strong bond and does not donate its proton as readily as other hydrogen halides. Other molecules in this category are hydrogen sulfide (H2S) and hydrogen cyanide (HCN). In the latter case, even though H is bonded to C, the electronegative N atom pulls some electron density away, and the HCN molecule is a very weak proton donor.

Hydrated cations      Cations, especially those of charge +3 or more or of the transition metals, are surrounded closely by four to six water molecules in aqueous solution. An example is Cr(H2O)63+, shown in Fig. 1. The positive charge of the metal ion pulls electron density away from the surrounding water molecules, weakening the hold of the oxygen atoms for the hydrogen atoms. The latter can consequently be more easily donated as protons:

Cr(H2O)63+ + H2O $\rightleftharpoons$ Cr(H2O)5(OH)2+ + H3O+

Figure 1 Space-filling and ball-and-stick of the hydrated chromium ion, Cr(H2O)63+. The Cr atom is linked in an identical way to the six oxygen arranged octahedrally around it.

Ions having acidic protons      Certain other ions can donate protons. One example is the ammonium ion, NH4+:

NH4+ + H2O $\rightleftharpoons$ NH3 + H3O+

Anions formed when some acids donate protons can lose yet another H+. An example of this is the hydrogen sulfate ion formed when sulfuric acid donates a proton:

H2SO4 + H2O → H3O+ + HSO4

HSO4 + H2O $\rightleftharpoons$ H3O+ + SO42–

Although sulfuric acid is strong, the negative charge on the hydrogen sulfate ion holds the proton tighter, and so the ion is a considerably weaker acid. Acids such as H2SO4, H2S, H2SO3, and H2CO3 are called diproticDescribes an acid that can donate two hydrogen ions (protons) to a base. because they can donate two protons. Phosphoric acid, H3PO4, is triprotic—it can donate three protons.