Chemical characteristics of calcium carbide and its reaction with water
Why so vigorous?
Calcium carbide is a chemical compound of calcium and carbon, and is a white crystalline substance when pure. It is obtained from the reaction
Ca + 2C → CaC₂
Calcium carbide has great practical significance. It is also known as calcium acetylide.
The chemical characteristics of calcium carbide
Calcium carbide is not volatile and not soluble in any known solvent, and reacts with water to yield acetylene gas and calcium hydroxide. Its density is 2.22 g/cm³. Its melting point is 2160 °C, and its boiling point is 2300 °C. Since the acetylene that forms upon contact with water is flammable, the substance is listed in hazard class 4.3.
Calcium acetylide was first obtained by German chemist Friedrich Wöhler in 1862 when he heated an alloy of zinc and calcium with coal. The scientist described the reaction of calcium carbide with water. Calcium carbide reacts vigorously with even mere traces of Н₂O, releasing a large amount of heat. If there is an insufficient quantity of water, the resulting acetylide spontaneously combusts. Calcium acetylide reacts violently with aqueous solutions of alkalis and diluted non-organic acids. These reactions release acetylide. With its strong reductive properties, CaC₂ reduces all metal oxides to pure metals or turns them into carbides.
It is easier to obtain calcium carbide from its oxide than from calcium itself, as the oxide is reduced at temperatures above 2000 °C. The metal and carbon combine:
CaO + 3C → CO↑ + CaC₂
The reaction takes place in an electric arc furnace, where a mixture of unslaked lime and coke or anthracite is heated. The technical product is grey due to the presence of free carbon, calcium oxide, phosphide, sulfide, and other chemical compounds. CaC₂ comprises 80-85% of the product by mass.
Use of calcium carbide
In the past, calcium carbide was used in carbide lamps, where it served as a source of acetylene flame. Nowadays these lamps are still used to power lighthouses and beacons, and also in cave exploration. CaC₂ also serves as a raw material in the development of chemical technologies, most notably synthetic rubber. Calcium carbide is also used to make vinyl chloride, acetylene black, acrylonitrile, acetic acid, acetone, ethylene, styrene, and synthetic resins.
In metallurgy, calcium carbide is used to deoxidize metals and reduce their oxygen and sulfur content (desulfuration). Calcium carbide is used to manufacture powdered carbide, a plant growth regulator. 3,000 kW/h of electricity is required to obtain one ton of CaC₂. For this reason, the manufacture of the substance is only profitable when electricity costs are low. At the same time, the production of calcium carbide worldwide is constantly increasing.
Calcium carbide – reaction with water
When calcium carbide reacts with water, acetylene is released:
2H₂O + CaC₂ → C₂H₂↑ + Ca(OH)₂
Acetylene is an industrial substance with an unpleasant smell, which is caused by the impurities it contains (NH₃, H₂S, PH₃, and others). In its pure form, acetylene is a colorless gas with a characteristic faint smell, and it dissolves in water.
A simple experiment can be used to demonstrate the reaction of calcium carbide with water: pour water into a 1.5 L bottle, quickly add several pieces of calcium carbide, and close the bottle with a stopper. As a result of the ensuing reaction between calcium carbide and water, acetylene collects in the bottle as pressure builds. As soon as the reaction stops, place a burning piece of paper in the bottle – this should trigger an explosion accompanied by a fiery cloud. As the walls of the bottle can burst as a result of the reaction, this experiment is dangerous, and should only be conducted with strict observance of safety precautions.
To demonstrate the reaction of calcium carbide with water, the experiment can be repeated in modified form – using a six-liter bottle. In this case, the components must be weighed with precision, because the greater the radius of the bottle, the less the container can withstand high pressure (assuming identical material and wall thickness). A bottle with a large capacity has a large radius, but its walls are approximately the same – accordingly, it is less resistant to pressure. To prevent it from exploding, the amount of calcium carbide must be calculated beforehand. Calcium has a molar mass of 40 g/mol, while carbon’s is 12 g/mol, so the molar mass of calcium carbide is around 64 g/mol. Accordingly, 64 g of carbide will yield 22.4 L of acetylene. The volume of the bottle is 6 L, and the pressure has risen by approximately 4 atmospheres.
The bottle must withstand five atmospheres: to conduct the experiment, we take around 64 g of calcium carbide and about 0.5 L of water. Place a piece of carbide inside a small bag. Push the bag into the bottle, then quickly close the bottle with the stopper. The reaction of calcium carbide with water continues for several minutes, the bottle swells up and the process is accompanied by loud bangs, but the bottle should withstand this.
After the release of acetylene is complete, place a hot rag soaked in hendecane on the bottle stopper, then move away to a maximum safe distance. You will soon see a bright yellow flash, and a fountain of flame up to 4 meters high will rise out of the bottle. This will burn the stopper and warp the bottle, but the bottle should remain intact. This experiment must be conducted in the open air, far away from flammable and explosive objects. Be sure to observe all relevant safety precautions.