The calcium chloride (CaCl 2) is an inorganic salt composed of calcium, alkaline earth metal and halogen chlorine. In this compound there are several electrostatic interactions that define the external appearance of its crystals and the rest of its physical properties.
Likewise, it is always accompanied by water molecules, forming hydrates of general formulas CaCl 2 · xH 2 O, with x = 0, 1, 2, 4 and 6. When x = 0, the salt lacks water and is anhydrous, as indicates its chemical formula mentioned above.
Solid portions of CaCl 2 are illustrated in the upper image . In low humidity conditions, it is possible to keep anhydrous salt free of water, although its natural tendency is to absorb it until it dissolves (deliquescence).
Its chemical formula is CaCl 2 : it expresses that for each Ca 2+ ion there are two Cl ions – which neutralize the positive charge. The calcium metal – from group 2 of the periodic table (Mr. Becambara) – gives up its two electrons to each chlorine atom, an element of group 17.
Structure of calcium chloride
The structure of CaCl 2 anhydride is illustrated in the upper image . The green spheres correspond to the Cl – ions , while the white ones to the Ca 2+ ions . These spheres are arranged in a parallelepiped, which is nothing more than the orthorhombic unit cell of the crystals.
The structure can give the false idea that calcium predominates; however, if more repeats of the unit cell were reproduced, the greater abundance of the green spheres would be clear: Cl – ions .
On the other hand, Ca 2+ ions have smaller ionic radii than Cl – ions . This is because, when they lose electrons, the nuclei of the atoms exert a greater attractive force on the outer electronic shells, which reduces the ionic radius.
In the case of Cl – , it has an additional electron that cannot be attracted with the same force, consequently increasing its ionic radius.
Molecular geometry and aqueous calcium complexes
In the center of the parallelepiped the Ca 2+ is surrounded by six Cl – . Four of these lie on a square plane and the other two are located perpendicularly (the green spheres farthest from the white sphere).
As a result of the arrangement of these ions, an octahedron is “assembled” around Ca 2+ , thus assigning it an octahedral molecular geometry.
Considering how the green spheres are arranged, a water molecule can substitute for one of these, which happens with CaCl 2 · H 2 O, possibly in the square plane. This fact modifies the crystalline structure and, as the water replaces the green spheres, the more the arrangement of the ions changes.
When all the Cl – ions are replaced by water molecules, the hydrate CaCl 2 · 6H 2 O is formed. At this point the octahedron is “aqueous” and the molecules are now able to interact with each other by hydrogen bonds (Ca 2+ OH-H-OH 2 ).
Consecutively, calcium can accept even more water molecules without altering the set ratio. This means that CaCl 2 · 6H 2 O can adopt other complex structures, to the point of being considered crystalline polymers of calcium and water.
However, these structures are less stable than that created by electrostatic interactions (Ca 2+ and Cl – ) of the anhydrous salt.
Uses / applications of calcium chloride
The main applications of calcium chloride are:
- Avoid freezing the water during winter. Calcium chloride generates a lot of heat as it dissolves and then, as the temperature increases, the ice melts. For this reason it is used to reduce the risk of the movement of people and vehicles during the cold season.
- Helps control dust on unpaved roads.
- Accelerates the drying rate of concrete after pouring.
- CaCl 2 liquids increase drilling efficiency for extracting gas from its underground reservoirs, as well as for oil.
- It is added to pools to reduce the erosion suffered by the concrete on their walls. The settled calcium fulfills this function.
- Because it is a hygroscopic salt, calcium chloride can be used as a desiccant, being able to lower the humidity of the surrounding air and, therefore, of the substances in contact with that air.
- It is used as a preservative in some foods, as well as an additive in several of these, such as energy drinks used by athletes, cheeses, beers, etc.
- In medical practice it is also useful in treating depression caused by magnesium sulfate overdose, as well as lead poisoning.
Synthesis of calcium chloride
The natural sources of this compound are the brines extracted from the seas or lakes.
However, its main source comes from the Solvay process, in which limestone (CaCO 3 ) undergoes a series of transformations until it becomes the by-product calcium chloride:
2NaCl (aq) + CaCO 3 (s) <=> Na 2 CO 3 (s) + CaCl 2 (aq)
The product of interest from this process is actually sodium carbonate, Na 2 CO 3 .
Properties of calcium chloride
Physical properties and deliquescence
It is a white, odorless and hygroscopic solid. This tendency to absorb humidity from the environment is due to the basicity of Ca 2+ ions .
Basicity of what kind: Lewis or Bronsted? From Lewis, due to the fact that the positive species is capable of accepting electrons. These electrons are donated, for example, by oxygen atoms in water molecules.
The solid absorbs moisture to the point of dissolving in the same water that wets its crystals. This property is known as deliquescence.
Its density is 2.15 g / mL. As it incorporates water into its structure, the crystal “expands”, increasing its volume and, consequently, decreasing its density. Only CaCl 2 · H 2 O breaks with this trend, showing a higher density (2.24 g / mL).
The molecular weight of the anhydride salt is approximately 111 g / mol, and for each molecule of water in its structure this weight increases by 18 units.
CaCl 2 is very soluble in water and in some polar solvents, such as ethanol, acetic acid, methanol, and other alcohols.
Heat of dissolution
When dissolved in water the process is exothermic and therefore heats the solution and its surroundings.
This is because the aqueous complex stabilizes Ca 2+ ions in solution to a better degree than electrostatic interactions with Cl – ions . Since the product is more stable, the solid releases energy in the form of heat.
Molten CaCl 2 can be subjected to electrolysis, a physical process that consists in the separation of a compound into its elements from the action of an electric current. In the case of this salt, the products are metallic calcium and gaseous chlorine:
CaCl 2 (l) → Ca (s) + Cl 2 (g)
Ca 2+ ions are reduced at the cathode, while Cl – ions are oxidized at the anode.