Weak Acids and Bases
From Chemwiki Weak Acids and Chemwiki Weak Bases
Weak Acids
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 density 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 atoms like oxygen or fluorine, a hydrogen atom is not always strongly acidic.
EXAMPLE 7 Acetic acid has the projection formula
Write an equation for transfer of a proton from acetic acid to water.
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. The strength of carboxylic acids is dependent on the electronegative strength of the atoms in the "R" group. Consider the compounds F3COOH and H3COOH. Fluorine is the most electronegative element, 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 bond, 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, m ? n + 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. 2. 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(H_2O)_6^(3+) + H_2 O ? Cr(H_2O)_5 (OH)^(2+) + H_3 O^(+)`
Figure 2 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+:
`NH_4^(+) + H_2 O ? NH_3 + H_3 O^(+)`
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:
`H_2SO_4 + H_2O -> H_3O^(+) + HSO_4^(-)`
`HSO_4^(-) + H_2O ? H_3O^(+) + SO_4^(2-)`
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 diprotic because they can donate two protons. Phosphoric acid, H3PO4, is triprotic—it can donate three protons.
Weak Bases
By analogy with weak acids, weak bases are not strong enough proton acceptors to react completely with water. A typical example is ammonia, which reacts only to a limited extent:
`NH_3 + H_2O ? NH_4^(+) + OH^(-)`
As in the case of acetic acid, the current conducted by 0.001 M NH3 is slightly above 10 percent that of 0.001 M NaOH or KOH, indicating that somewhat more than one-tenth of the NH3 molecules have been converted to NH4+ and OH–ions.
Weak bases fall into two main categories.
Ammonia and amines Amines may be derived from ammonia by replacing one or more hydrogens with alkyl groups. They react with water in the same way as ammonia. Trimethyl amine behaves as follows:
Anions of weak acids The fact that the molecules of a weak acid are not entirely converted into hydronium ions and anions implies that those anions must have considerable affinity for protons. For example, the anion of acetic acid, acetate ion, can accept protons as follows:
`C_2 H_3 O_2^(-) + H_2O ? HC_2 H_3 O_2 + OH^(-)`
Thus when a compound like sodium acetate,dissolves, some hydroxide ions are produced and the solution becomes slightly basic. (Sodium ions do not react with water at all, and so they have no effect on acidity or basicity.) Other examples of weak bases which are anions of weak acids are CN–, CO32–, PO43–.