5.3: Strong Acids and Bases
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- 284488
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The most straight-forward examples involving acids and bases deal with strong acids and bases. Strong acids, like HCl or HNO3, are strong electrolytes. All HCl molecules, for example, transfer their protons to H2O molecules in an aqueous solution. The solution contains no HCl molecules, only H3O+(aq) and Cl–(aq) ions. Similarly, strong bases are strong electrolytes and dissociate completely into hydroxide ions and the appropriate cation. All soluble hydroxides are strong bases. In an aqueous solution of barium hydroxide, for example, the base has completely dissociated into Ba2+(aq) and OH–(aq) ions.
Table \(\PageIndex{1}\) lists molecules and ions which act as strong acids and bases in aqueous solution. 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 concentration as well as the strength of the dissolved acid or base.
| Strong Acids | Strong Bases |
|---|---|
|
HCl, HBr, HI HClO4, HNO3, H2SO4 |
Soluble Hydroxides: LiOH, NaOH, KOH, RbOH, CsOH, Sr(OH)2 and Ba(OH)2 |
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 halogen. Considerable electron density 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 \(\PageIndex{1}\) fit this rule nicely. They are either hydrogen halides (HCl, HBr, HI) or oxyacids (whose general formula is HnXOm).
Below are the resonance structures for oxoacids after they donate a proton. The reason for the strength of the following acids is the stability of the anion, which is shown by the number of resonance structures and the distribution of the negative charge amongst all of the oxygen atoms. The distribution of the negative charge can be visualized in the 3D structure, with red being representing negative charge and blue representing positive charge. The 3D structure represents the average of the resonance structures shown to the left.
| Name | Lewis Structure (Resonance) | 3D Structure |
|---|---|---|
|
Perchloric Acid |
|
|
|
Nitric Acid |
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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.
Contributors
Ed Vitz (Kutztown University), John W. Moore (UW-Madison), Justin Shorb (Hope College), Xavier Prat-Resina (University of Minnesota Rochester), Tim Wendorff, and Adam Hahn.