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Water is an amphiprotic solvent
Acid and base chemistry is closely related to the chemistry of water iself. Perhaps the starting point should be to consider the fact that H2 dissociates into hydronium ion (H3O+) and hydroxide ion (OH-). In the absence of any other chemical species the equilibium concentration of each of these charged species is 10-7. That is quite low, but still high enough to conduct electricity. However, the aspect of water chemsitry that makes it so versatile is that other compounds can shift the equilibrium between hydronium and hydroxide so that the concnetration of one or the other is signifinantly greater than 10-7. These compounds are known as acids and bases, respecitvely. In order to introduce the concept of acid-base chemsitry we begin with some basic definitions of a logarithmic scale of conecentarion known as the p-scale. The p-scale presents the concentration of a species as the negative log10 of that species. In this segment we consider how to define acidity, basicity and water dissociation constants using the p-scale.
Strong acids and bases
There are only a few strong acids and bases. The strong acids are HCl, HBr, HI, H2SO4, HNO3 and NClO4. The strong bases are NaOH, KOH and CsOH. The alkaline earh hydroxides Ca(OH)2 and Mb(OH)2 are strong as well, but they are necessarily soluble enough that they can produce a strongly basic soluion. The meaning of the terms strong acid and strong base is that the compounds are 100% dissociated. The equilibrium for solution to form the ions lies far to the right. As a consequence we can assume that the [H+] concentration is equal to the initial [acid} concentration. The same is true for the bases.The [OH-] concentration is equal to the initial [base] concentration. This assumption makes it easy to calculate pH or pOH for these respective solutions. This is important in studies of acid ancd behavior since we may assume that a strong acid reacts quantitatively with the acid to produce its conjugate base. Thi sis extremely important since without this assumption the calculation of concentrations in a titration would be very tedious.
Weak acids and bases
ALl of the acids and bases that are not in the category of strong acids and bases are two considered
In this chapter we will
- Introduce the equilibrium for water dissociation, Kw
- Introduce the calculation of pH = -log10([H+\)
- Introduce the calculation of pOH = -log10([OH-\)
- Introduce the concept of a strong acid or base
- Introduce the concept of a weak acid or base
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