Dissociation

Dissociation

Dissociation of water

Dissociation of acid and bases

Dissociation of salts

Resources

Dissociation is the process by which a molecule separates into ions. It may also be called ionization, but because there are other ways to form ions, the term dissociation is preferred. Substances dissociate to different degrees, ranging from substances that dissociate very slightly, such as water, to those that dissociate almost completely, such as strong acids and bases. The extent to which a substance dissociates is directly related to its ability to conduct an electric current. A substance that dissociates only slightly (as in the case of a weak acid like vinegar) is a weak electrolyte, as it conducts electricity poorly. A substance that is almost completely dissociated (such as table salt, NaCl, or hydrochloric acid, HCl) conducts electricity very well. The ability to conduct electricity is based on the ionic makeup of a substance. The more ions a substance contains, the better it will conduct electricity.

Dissociation of water

Pure water dissociates only slightly. About one water molecule out of every 10 million is dissociated and the rest remain in nondissociated (or molecular) form. This ionization of water (sometimes called self-or auto-ionization) can be summarized by the following formula. Pure water produces very few ions from its dissociation and so is a poor electrolyte, or conductor of electricity.

The following equation describes the process in which a water molecule ionizes (separates into ions) to form a hydrogen ion (proton) and a hydroxide ion.

H₂O → H⁺ + OH^–

Another way to describe the dissociation of water is as follows:

H₂O + H₂O → H₃O₊ + OH^–

where two water molecules form a hydronium ion (essentially a water molecule with a proton attached) and a hydroxide ion.

Dissociation of acid and bases

Acids are molecules that can donate protons (hydrogen or H⁺ ions) to other molecules. An alternate view is that an acid is a substance that will cause an increase in the concentration of hydrogen ions in a solution.

The dissociation of a strong acid (such as hydrochloric acid, HCl) is essentially 100%.

HCI → H⁺ + CI^–

In this case, nearly every HCl molecule is dissociated (separated into ions). When any substance dissociates, both positive and negative ions will be formed. In this case, the positive ion (cation) is a proton, and the negative ion (anion) is the chloride ion. A strong acid is a strong electrolyte and a good conductor of an electric current. In the case of a strong base, nearly 100% of the molecules are dissociated as well, and

strong bases (such as sodium hydroxide, NaOH) are also strong electrolytes.

NaOH → Na⁺ + OH^–

A weak acid, such as hydrofluoric acid is only slightly dissociated. Many more of the molecules exist in the molecular (undissociated or unionized) form than in the ionized form. Since it forms fewer ions, a weak acid will be a weak electrolyte.

HF→H⁺ + F^-

In the case of a weak base, such as aluminum hydroxide, Al(OH)₃, only a small percent of molecules ionize, producing few ions, and making weak bases weak electrolytes as well.

Al(OH)₃ → Al^{+ 3} + 3OH^–

In any dissociation reaction, the total charges will mathematically cancel each other out. The case above has a positive three charge on the aluminum ion and a negative one charge on each of the three hydroxide ions, for a total of zero.

Dissociation of salts

Salts are the product of the neutralization reaction between an acid and a base (the other product of this neutralization reaction being water). Salts that are soluble in water dissociate into their ions and are electrolytes. Salts that are insoluble or only slightly soluble in water form very few ions in solution and are nonelectrolytes or weak electrolytes. Sodium chloride, NaCl, is a water-soluble salt that dissociates totally in water.

NaCl→Na⁺ + Cl^-

The process by which this takes place involves the surrounding of each positive sodium ion and each negative chloride ion by water molecules. Water molecules are polar and have two distinct ends, each with a partial positive or negative charge. Since opposite charges attract, the negative end of the water molecule will face the positive sodium ion and the positive end will face the negative ion. This process, illustrated in Figure 1, is known as solvation.

Resources

PERIODICALS

Carafoli, Ernest, and John Penniston. “The Calcium Signal,” Scientific American 253 (November 1985).

Ezzell, Carol. “Salt’s Technique for Tickling the Taste Buds,” Science News 140 (November 2, 1991).

OTHER

Shodor Education Foundation, Inc. “Acid–Base Chemistry” <http://www.shodor.org/UNChem/basic/ab/> (accessed November 18, 2006).

Purdue University, College of Science, Chemical Education Division Groups “Water, Acids, and Bases” <http://chemed.chem.purdue.edu/genchem/topicreview/bp/ch17/water.php> (accessed November 1, 2006).

Louis Gotlib