Which chloride may exist as a dimer

It is known as the anhydrous salt hexagonal; r. Aluminium chloride may be prepared by passing hydrogen chloride or chlorine over hot aluminium or industrially by passing chlorine over heated aluminium oxide and carbon. The chloride ion is polarized by the small positive aluminium ion and the bonding in the solid is intermediate between covalent and ionic. In the liquid and vapour phases dimer molecules exist, Al 2 Cl 6 , in which there are chlorine bridges making coordinate bonds to aluminium atoms see formula.

The AlCl 3 molecule can also form compounds with other molecules that donate pairs of electrons e. At high temperatures the Al 2 Cl 6 molecules in the vapour dissociate to planar AlCl 3 molecules. Aluminium chloride is used commercially as a catalyst in the cracking of oils. It is also a catalyst in certain other organic reactions, especially the Friedel-Crafts reaction. From: aluminium chloride in A Dictionary of Chemistry ». Subjects: Science and technology — Chemistry. View all related items in Oxford Reference ».

A small representative portion of a sodium chloride lattice looks like this:. 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.

The compound does not conduct electricity in the solid state because it has no mobile electrons, and the ions are constrained by the crystal lattice. However, when it melts it undergoes electrolysis. Sodium chloride dissolves in water to give a neutral solution. 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.

As with sodium chloride, large amounts of heat energy are needed to overcome the attractions between the ions because of the high lattice enthalpy of the compound , so the melting and boiling points are also high. Solid magnesium chloride is a non-conductor of electricity because the ions are constrained.

However, upon melting, the compound undergoes electrolysis. Magnesium chloride dissolves in water to give a slightly acidic solution with a pH of approximately 6. Many complex ions are acidic, the degree of acidity depending on the attraction between the electrons in the water molecules and the metal at the center of the ion. The hydrogen atoms carry less electron density in this state, and are thus more easily removed by a base.

For magnesium, the amount of distortion is quite small, and only a small proportion of the hydrogen atoms are removed, in this case by water molecules in the solution:. The hydronium ions make the solution acidic.

Few are formed the equilibrium lies well to the left because the solution is only weakly acidic. The previous equation can be simplified as follows:. Electronegativity increases across the period; aluminum and chlorine do not differ enough in electronegativity to form a simple ionic bond.

The structure of aluminum chloride changes with temperature. At room temperature, the aluminum is 6-coordinated i. The structure is an ionic lattice , but it has a lot of covalent character. Both of these temperatures are far below the expected range for an ionic compound. They suggest comparatively weak attractions between molecules instead of strong attractions between ions. This is because the coordination of the aluminum changes at these temperatures.

It becomes 4-coordinated—each aluminum is surrounded by 4 chlorine atoms rather than 6. The original lattice converts into an Al 2 Cl 6 arrangement of molecules. 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. These dimers and simple AlCl 3 molecules exist in equilibrium.

As the temperature increases further, the position of equilibrium shifts more and more to the right of the following system:. Solid aluminum chloride does not conduct electricity at room temperature because the ions are not free to move. Molten aluminum chloride only possible at increased pressures is also nonconductive, because it has lost its ionic character. Aluminum chloride reacts dramatically with water. A drop of water placed onto solid aluminum chloride produces steamy clouds of hydrogen chloride gas.

Solid aluminum chloride in an excess of water still splutters, but instead an acidic solution is formed. A solution of aluminum chloride of ordinary concentrations around 1 mol dm -3 , for example has a pH around Strohmann, Chem. To request permission to reproduce material from this article, please go to the Copyright Clearance Center request page.

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