Hydrochloric acid: Encyclopedia II - Hydrochloric acid - Chemistry

Hydrochloric acid - Chemistry

Hydrogen chloride (HCl) is a monoprotic acid, which can dissociate (i.e., ionize) only once to give up one H⁺ ion (a single proton). In aqueous hydrochloric acid, the H⁺ joins a water molecule to form a hydronium ion, H₃O⁺:

The other ion formed is Cl^- or chloride ion. Hydrochloric acid can therefore be used to prepare salts called chlorides, such as sodium chloride. Hydrochloric acid is considered a strong acid, since it is practically fully dissociated in water.

Monoprotic acids have one acid dissociation constant, K_a, which indicates the level of dissociation in water. For a strong acid like HCl, K_a is large. Theoretical attempts to assign a K_a to HCl have been made; see [2]. When chloride salts such as NaCl are added to aqueous HCl they have practically no effect on pH, indicating that Cl^- is an exceedingly weak conjugate base and that HCl is fully dissociated in aqueous solution. For intermediate to strong solutions of hydrochloric acid, the assumption that H⁺ molarity (a unit of concentration) equals HCl molarity is excellent, agreeing to four significant digits.

Of the seven common strong acids in chemistry, all of them inorganic, hydrochloric acid is the monoprotic acid least likely to undergo an interfering oxidation-reduction reaction. It is one of the least-hazardous strong acids to handle; despite its acidity, it produces the less-reactive and non-toxic chloride ion. Intermediate-strength hydrochloric acid solutions are quite stable, maintaining their concentrations over time. These attributes, plus the fact that it is available as a pure reagent, means that hydrochloric acid makes an excellent acidifying reagent and acid titrant (for determining the amount of an unknown quantity of base in titration). Strong acid titrants are useful because they give more distinct endpoints in a titration, making the titration more precise. Hydrochloric acid is frequently used in chemical analysis and to digest samples for analysis. Concentrated hydrochloric acid will dissolve some metals to form oxidized metal chlorides and hydrogen gas. It will produce metal chlorides from basic compounds such as calcium carbonate or copper(II) oxide. It is also used as a simple acid catalyst for some chemical reactions.

The physical properties of hydrochloric acid, such as boiling and melting points, density, and pH depend on the concentration or molarity of HCl in the acid solution. They can range from those of water at 0% HCl to values for fuming hydrochloric acid at over 40% HCl.

The reference temperature and pressure for the above table are 20°C and 1 atmosphere (101 kPa).

Hydrochloric acid as the binary (two-component) mixture of HCl and H₂O has a constant-boiling azeotrope at 20.2% HCl and 108.6 °C (227 °F). There are four constant-crystallization eutectic points for hydrochloric acid, between the crystal form of HCl·H₂O (68% HCl), HCl·2H₂O (51% HCl), HCl·3H₂O (41% HCl), HCl·6H₂O (25% HCl), and of course ice (0% HCl). There is also a metastable eutectic at 24.8% between ice and the HCl·3H₂O crystallization.

Adapted from the Wikipedia article "Chemistry", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki