0.5 Bis2a 02.appendix ii detailed look at acid-base equilibrium  (Page 2/8)

Additional links

Below you will find additional links that are meant to serve as supplemental material if you are having difficulty with this topic. These links go to the UC Davis Chemwiki site and videos from the Khan Academy.

Khan academy links

• Acids and Bases .

Ucd chemwiki links

• acids and bases
• Bronsted acids and bases

Observation 1: strong acids and weak acids

From the definition of an acid given in the Foundation, a typical acid can be written asHA, representing the hydrogen ion which will be donated and the rest ofthe molecule which will remain as a negative ion after the donation. The typical reaction of an acid in aqueous solutionreacting with water can be written as

In this reaction, HA(aq) represents anacid molecule dissolved in aqueous solution. H ₃ O ⁺ (aq) is a notation to indicate that the donated proton has beendissolved in solution. Observations indicate that the proton is associated with several water molecules in a cluster, rather thanattached to a single molecule. H ₃ O ⁺ is a simplified notation to represent this result. Similarly, theA ^– (aq) ion is solvated by several water molecules. [link] is referred to as acid ionization .

[link] implies that a 0.1 M solution of the acid HAin water should produce H ₃ O ⁺ ions in solution with a concentration of 0.1M. In fact, the concentration ofH ₃ O ⁺ ions, [H ₃ O ⁺ ],can be measured by a variety of techniques. Chemists commonly use a measure of theH ₃ O ⁺ ion concentration called the pH, definedby:

$$\mathrm{pH}=-\lg \left[{\text{H}}_3{\text{O}}^{+}\right]$$

We now observe the concentration [H ₃ O ⁺ ] produced by dissolving a variety of acids in solution at aconcentration of 0.1 M, and the results are tabulated in [link] .

Note that there are several acids listed for which[H ₃ O ⁺ ] = 0.1 M,and pH.This shows that, for these acids, the acid ionization is complete: essentially every acid molecule is ionized in the solutionaccording to [link] . However, there are other acids listed for whichH ₃ O ⁺ is considerably less than 0.1M and the pH is considerably greater than 1. For each of these acids, therefore, not all of the acidmolecules ionize according to [link] . In fact, it is clear in [link] that in these acids the vast majority of the acid molecules do not ionize, and only a smallpercentage does ionize.

From these observations, we distinguish two classes of acids: strong acids and weak acids . Strong acids are those for which nearly 100% of the acid molecules ionize, whereas weak acids are those forwhich only a small percentage of molecules ionize. There are seven strong acids listed in [link] . From many observations, it is possible to determine that theseseven acids are the only commonly observed strong acids. The vast majority of all substances with acidic properties are weak acids.We seek to characterize weak acid ionization quantitatively and to determine what the differences in molecular properties are betweenstrong acids and weak acids.