Acts dry HCl as an acid

Hydrogen chloride - an acid

The starting material for hydrochloric acid: table salt

With the action of conc. Sulfuric acid on sodium chloride gives off the colorless, pungent smelling gas hydrogen chloride. It is heavier than air and does not give a clear reaction to litmus paper (dry). Aqueous solutions of hydrogen chloride are called hydrochloric acid or hydrochloric acid. Hydrogen chloride is a very strong acid. The molecular formula is HCl.


Hydrogen chloride eagerly dissolves in water. In one liter of water, 520 l dissolve at 0 ° C when heated, which corresponds to 850 g of HCl gas. At 20 ° C, 442 liters of hydrogen chloride dissolve in one liter of water. In humid air, HCl gas forms a mist of fine hydrochloric acid droplets.



The fountain-like penetration of water into the flask observed in experiment 3 can easily be explained: First, the entire cylinder is filled with hydrogen chloride. If a small part of this gas dissolves in water, a negative pressure is created in the cylinder and the external air pressure pushes the water in the tub into the cylinder. The red color of litmus shows that hydrogen chloride forms an acid solution with water. Because it is made from the most famous of all salts, table salt, it is called hydrochloric acid.




The explanation of the reactions of hydrogen chloride with water

When hydrogen chloride is introduced into water (experiment 5), heat is generated, the indicator reacts and the electrical conductivity increases. So there has to be a chemical reaction. Hydrogen chloride is a gas, so it cannot be an ionic compound. Rather, in the molecules of hydrogen chloride, the hydrogen atoms and chlorine atoms are linked to one another via a polarized atomic bond (Chapter 28). However, ions must form when hydrogen chloride is introduced into water, as the increase in conductivity shows. How can this be explained? If hydrogen chloride and water molecules come together, they attract each other because both molecules represent dipoles (Chapter 28).

The attraction of the oxygen atom is so strong that the hydrogen ion (= proton), which is loosely bound due to the polarization, is completely separated from the chlorine atom and is bound by a free electron pair of the oxygen atom. The binding electron pair between chlorine and hydrogen retains the chlorine; a chloride ion, Cl. The water molecule receives a positive charge through the absorption of a hydrogen ion; it becomes a molecular ion called the oxonium ion. The transition of a proton from one particle to another is called protolysis.


$ \ mathrm {{\ underbrace {H: {\ overset H {\ overset {\ Large {\ cdotp \ \ cdotp}} {\ underset {\ Large {\ cdotp \ \ cdotp}} {O}}}:}} _ {Water} + {\ underbrace {H: {\ overset {\ Large {\ cdotp \ \ cdotp}} {\ underset {\ Large {\ cdotp \ \ cdotp}} {Cl:}}}} _ {Hydrogen chloride} }} \ longrightarrow \ underbrace {\ Biggl [: {\ overset H {\ overset {\ Large {\ cdotp \ \ cdotp}} {\ underset H {\ underset {\ Large {\ cdotp \ \ cdotp}} {O} }}}}: H \ Biggr] ^ +} _ {Oxonium-Ion} + \ underbrace {\ Biggl [: {\ overset {\ Large {\ cdotp \ \ cdotp}} {\ underset {\ Large {\ cdotp \ \ cdotp}} {Cl}}}: \ Biggr] ^ -} _ {Chloride-Ion}} $


The resulting ions are free to move (conductivity!). The oxonium ion can be detected with indicators, the chloride ion with silver ions. In water, these ions are immediately enveloped by other water molecules, they are hydrated. See hydration, Chapter 33. The heat development observed during the reaction is mainly due to the hydration (exothermic process).



The formation of ions during the protolysis of HCl in H2O is formulated in a simplified way like the solution process of a salt:


$ \ mathrm {HCl \; \ xrightarrow {H_ {2} O} \; H ^ {+} _ {(aq)} + Cl ^ {-} _ {(aq)}} $


It should be noted, however, that in salts the ions are already present in the crystal (Chapter 31), while in protolysis they are in H.2O can only be formed through a reaction with water molecules.



Commercial forms of hydrochloric acid

The "Concentrated hydrochloric acid" is a 35-40% (weight percent) aqueous solution of hydrogen chloride in water. Because the hydrogen chloride vapors escaping at room temperature form a hydrochloric acid mist with the water vapor in the air, it also becomes "Fuming hydrochloric acid" called. "Crude hydrochloric acid" is a concentrated acid in which some ferric chloride is dissolved, which is why it has a yellow color.