Strong Acids and Bases
The most straight-forward examples involving acids and bases deal with strong acids and bases. Strong acids, like HCl or HNO3, are such good protonThe positively charged particle in an atomic nucleus; its mass is similar to the mass of a hydrogen atom. donors that none of their own molecules can remain in aqueous solution. All HCl molecules, for example, transfer their protons to H2O molecules, and so the solution contains only H3O+(aq) and Cl–(aq) ions. Similarly, the ions of strong bases, like BaO or NaH, are such good proton acceptors that they cannot remain in aqueous solution. All O2– ions, for example, are converted to OH– ions by accepting protons from H2O molecules, and the H2O molecules are also converted to OH–. Therefore a solution of BaO contains only Ba2+(aq) and OH–(aq) ions.
Table 1 lists molecules and ions which act as strong acids and bases in aqueous solution. In addition to those which react completely with H2O to form H3O+ and OH–, any compoundA substance made up of two or more elements and having those elements present in definite proportions; a compound can be decomposed into two or more different substances. which itself contains these ions will serve as a strong acid or base. Note that the strength of an acid refers only to its ability to donate protons to H2O molecules and the strength of a base to its ability to accept protons from H2O molecules. The acidity or basicity of a solution, on the other hand, depends on the 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. as well as the strength of the dissolved acid or base.
TABLE 1 Species Which Are Strong Acids and Bases in Aqueous Solution.
|Strong Acids||Strong Bases|
| H3O+ (Only a few compounds like H3OCl
and H3ONO3 are known to contain hydronium ions.)
| OH– [Only LiOH, NaOH, KOH, RbOH,
CsOH, Ca(OH)2, Sr(OH)2 and Ba(OH)2 are sufficiently soluble to produce large concentrations of OH–(aq).]
| HCl, HBr, HI
| O2– (Li2O, Na2O, K2O, Rb2O, Cs2O, CaO,
SrO, and BaO are soluble
|HNO3, H2SO4, HClO3||H–, S2–, NH2–, N3–, P3–|
As a general rule, strong proton donors are molecules in which a hydrogen is attached to a rather electronegative atom, such as oxygen or a halogenOne of the elements in the same column of the periodic table as fluorine.. Considerable electron densityThe ratio of the mass of a sample of a material to its volume. is shifted away from hydrogen in such a molecule, making it possible for hydrogen ions to depart without taking along any electrons. The strong acids in Table 1 fit this rule nicely. They are either hydrogen halides (HCl, HBr, HI) or oxyacids (whose general formula is HnXOm). Below are a set of lewis structures for some of the strong oxyacids. For each lewis structure, there is also a 3D Jmol model which shows the electrostatic potential of along the Van der Waals surface of the molecule. It can be seen that the acidic hydrogen 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. all show a distinct blue coloration, denoting partial positive charge, and that the oxygen connected to the acidic hydrogen shows an orange color which denotes a negative charge. This gives a sense of the polarization of the bond to the acidic hydrogen.
The Lewis structures indicate a proton bonded to oxygen in each of the oxyacids, hence their general name. Note that for a strong oxyacid the number of oxygens is always larger by two or more than the number of hydrogens. That is, in the general formula HnXOm, m ≥ n + 2.
The strength of a base depends on its ability to attract and hold a proton. Therefore bases often have negative charges, and they invariably have at least one lone pairIn a covalently bonded molecule or ion, a pair of electrons not shared between two atoms and hence not involved in a bond. of electrons which can form a coordinate covalent bondA bond between two atoms in which the shared electrons are considered to be contributed by only one of the atoms. to a proton. The strong bases in Table 1 might be thought of as being derived from neutral molecules by successive removal of protons. For example, OH– can be obtained by removing H+ from H2O, and O2– can be obtained by removing H+ from OH–. When the strong bases are considered this way, it is not surprising that they are good proton acceptors.