7.3. Acids and Bases http://www.ck12.org
Acids
There are multiple ways to define what an acid is, but for the purposes of this book, we will define anacidas any
compound that produces hydrogen ions (H+) when dissolved in water. Based on this definition, all acids contain at
least one hydrogen atom, but not all hydrogen-containing compounds are acids. When isolated as a pure material,
most acids exist as molecular substances. However, when dissolved in water, one or more of the hydrogen atoms
acts as an H+ion that transfers to water, leaving behind the remainder of the molecule as an anion. This reaction
with water can be represented by the following generic equation, where HA represents an acid:
HA(aq)+H 2 O(l)→A−(aq)+H 3 O+(aq)
As you can see, the acid reacts with a molecule of water to produce a hydronium ion (H 3 O+) and the A−anion.
(Note: The H+ion is sometimes referred to as a proton. This makes sense when you consider that the most common
form of the hydrogen atom consists of one proton and one electron. When the single electron is removed to make a
cation, only a proton is left behind. As a result, the above reaction is sometimes referred to as a proton transfer.) A
specific example of this process can be seen in the following animation:
http://group.chem.iastate.edu/Greenbowe/sections/projectfolder/animations/HCl(aq).html
In this animation, hydrochloric acid (HCl) reacts with water to produce the hydronium ion and the chloride ion.
Even though HCl exists as a molecular gas in the absence of water, it produces ions when water is present. Acids
have some unique properties and reactivity patterns that we will discuss in future chapters. For now, we will focus
on the ways in which they are named.
Binary Acids
Binary acidsare acids in which one or more acidic hydrogen atoms are bound directly to a single atom. As a result,
the anion left behind when a binary acid is dissolved in water is a monatomic anion. Examples include hydrogen
chloride (HCl(g)and hydrogen sulfide (H 2 S(g)). Both of these substances are molecular gases in their pure form, but
change their properties and their names when dissolved in water. To name a binary acid, start with the name of the
anion left behind after the acidic hydrogens have been removed. Then, add the prefixhydro- and replace the suffix
- idewith -ic acid. For example, HCl produces Cl−ions when dissolved in water, so it would therefore be named
hydrochloric acid. Some other common binary acids are shown in the followingTable7.9.
TABLE7.9: Common Binary Acids
Formula Name Anion Name
HF(aq) hydrofluoric acid F− fluoride
HCl(aq) hydrochloric acid Cl− chloride
HBr(aq) hydrobromic acid Br− bromide
HI(aq) hydroiodic acid I− iodide
H 2 S(aq) hydrosulfuric acid S^2 − sulfideMost of the binary acids listed here are monoprotic, because they have only one acidic hydrogen. Hydrosulfuric acid,
on the other hand is diprotic. Its hydrogen ions are transferred to two water molecules in two subsequent reactions.
Oxoacids
So far we have looked at acids that leave behind monatomic anions. However, many strong acids leave behind
polyatomic anions as well. In particular, many of the oxoanions we looked at earlier can combine with one or more
hydrogen ions (enough to make a neutral molecule) to produce strong acids calledoxoacids. A common example