Why solutions of aluminum chloride are acidic

It has been used for decades in industry, most notably as a catalyst in Friedel—Crafts alkylation and acylation reactions, as well as in dozens of other processes. Its hexahydrate is the active ingredient in many antiperspirants. Learn more about this molecule from CAS , the most authoritative and comprehensive source for chemical information. If your favorite molecule is not in our archive , please send an email to motw acs. The molecule can be notable for its current or historical importance or for any quirky reason.

Thank you! Find Out More. Careers Launch and grow your career with career services and resources. As with the other covalent chlorides, if there is enough water present, these dissolve to give a hydrochloric acid solution. The reaction happens in two stages. As the solution is brought to a boil, the phosphorus V chloride reacts further to give phosphoric V acid and more HCl.

Phosphoric V acid is also known as phosphoric acid or as orthophosphoric acid:. Disulfur dichloride is one of three sulfur chlorides and is the species formed when chlorine reacts with hot sulfur. Disulfur dichloride is an orange, unpleasant-smelling covalent liquid. Its rather unusual structure is given below:. Disulfur dichloride reacts slowly with water to produce a complex mixture of hydrochloric acid, sulfur, hydrogen sulfide and various sulfur-containing acids and anions.

Jim Clark Chemguide. The structures : Sodium chloride and magnesium chloride are ionic and consist of large ionic lattices at room temperature. Aluminum chloride and phosphorus V chloride are more complicated. They change their structures from ionic to covalent as their solids transition to liquids or vapors.

This is discussed in greater detail below. The other chlorides are simple covalent molecules. Melting and boiling points: Sodium and magnesium chlorides are solids with high melting and boiling points because of the large amount of heat which is needed to break the strong ionic attractions. The rest are liquids or low melting point solids. Leaving aside the aluminum chloride and phosphorus V chloride cases where the situation is quite complicated, the attractions in the others will be much weaker intermolecular forces such as van der Waals dispersion forces.

These vary depending on the size and shape of the molecule, but will always be far weaker than ionic bonds. Electrical conductivity: Sodium and magnesium chlorides are ionic and so will undergo electrolysis when they are molten. Electricity is carried by the movement of the ions and their discharge at the electrodes not electrons. In the aluminum chloride and phosphorus V chloride cases, the solid does not conduct electricity because the ions aren't free to move. In the liquid where it exists - both of these sublime at ordinary pressures , they have converted into a covalent form, and so don't conduct either.

The rest of the chlorides do not conduct electricity either solid or molten because they don't have any ions or any mobile electrons. Reactions with water : As an approximation, the simple ionic chlorides sodium and magnesium chloride just dissolve in water. Although other chlorides all react with water in a variety of ways described below for each individual chloride. The reaction with water is known as hydrolysis. Sodium chloride NaCl Sodium chloride is an ionic compound consisting of a giant array of sodium and chloride ions.

A small representative portion of a sodium chloride lattice looks like this: This is normally drawn in an exploded form as: The strong attractions between the positive and negative ions require a large amount of heat energy to break, so sodium chloride has high melting and boiling points. Magnesium chloride MgCl 2 Like sodium chloride, magnesium chloride also forms an ionic solid, but with a more complicated crystal structure of the ions to accommodate twice as many chloride ions as magnesium ions.

Aluminum chloride AlCl 3 Electronegativity increases across the period; aluminum and chlorine do not differ enough in electronegativity to form a simple ionic bond. The structure is shown below: In the conversion, all ionic character is lost, causing the aluminum chloride to vaporize or melt depending on the pressure. This causes it to melt or vaporize due to comparatively weak intermolecular attractions. As the temperature increases further, more AlCl 3 molecules are formed.

Silicon tetrachloride SiCl 4 Silicon tetrachloride is a simple no-messing-about covalent chloride. Phosphorus III chloride PCl 3 This simple covalent chloride exists as a fuming liquid at room temperature because there are only van der Waals dispersion forces and dipole-dipole attractions between the molecules.

The products are a solid and a gas - no acidic solution is formed. Aluminium chloride in aqueous solution. If aluminium chloride is dissolved in a large amount of water the solution is acidic, but this has nothing to do with formation of hydrochloric acid. The solution contains hydrated aluminium ions and chloride ions:. The hexaqua complex ion behaves exactly like ions of similar type formed from transition metals; the small, highly charged metal ion polarises withdraws electron density from the water molecules that are attached to the aluminium ion through dative covalent bonds.