Chemical characteristics of calcium carbide and its reaction with water
Why is it so vigorous?
Calcium carbide is a chemical compound of calcium and carbon, which in pure form is a white crystalline substance. 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 is not soluble in any known solvent, and breaks down on contact with water. The density of calcium acetylide is 2.22 g/cm³. Its melting temperature is 2,160 degrees Celsius, and its boiling temperature is 2,300 degrees Celsius. For its impact on the human body, it is classified as a highly dangerous substance (1st class of danger).
Calcium acetylide was first obtained by the German chemist Friedrich Wöhler in 1862 – by heating an alloy of zinc and calcium with coal. The scientist described the reaction of calcium carbide with water. Even with traces of Н₂O, calcium carbide reacts vigorously, releasing a large amount of heat. If there is insufficient water, the formed acetylide self-combusts. Calcium acetylide reacts violently with water solutions of alkalis and diluted non-organic acids. Acetylide is released during the reaction. With its strong reductive properties, CaC₂ reduces all metal oxides to pure metals or carbides.
It is more convenient to obtain calcium carbide from its oxide, and not from calcium itself. At a high temperature (over 2,000 degrees Celsius), the substance is reduced. 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 in color because of the presence of free carbon, oxide, calcium phosphide, sulfide and other chemical compounds. The mass content of CaC₂ in the product is from 80% to 85%.
Use of calcium carbide
In the past, calcium carbide was used in carbide lamps, where the substance served as a source of acetylene flame. Nowadays these lamps are used in caving, and also in lighthouses and beacons. CaC₂ is a raw material for the development of chemical technologies. In the manufacture of products of organic synthesis, synthetic rubber is the most important of these. Calcium carbide is used to make vinyl chloride, acetylene black, acrylonitrile, acetic acid, acetone, ethylene, styrene and synthetic resins.
In metallurgy, calcium carbide is used for deoxidation of metals and to reduce the content of oxygen and sulfur (desulfuration). Calcium carbide is used to manufacture powdered carbide reagent, a plant growth regulator. To obtain one ton of CaC₂, 3,000 kWt/hours of electricity is required. For this reason, manufacture of the substance is only justified with low electricity prices. At the same time, the world production of calcium carbide is constantly growing.
Calcium carbide – reaction with water
When calcium carbide reacts with water, acetylene is released:
2H₂O + CaC₂ → C₂H₂ + Ca(OH)₂
Acetylene is an industrial chemical with an unpleasant smell, because of the impurities it contains (NH₃, H₂S, PH₃ and others). In pure form, acetylene is a colorless gas with a characteristic faint smell, and it dissolves in water.
To understand how the reaction of calcium carbide with water takes place, we can conduct an experiment: pour water into a 1.5-liter bottle, quickly add several pieces of calcium carbide and close the bottle with a stopper. As a result, acetylene collects in the bottle under excessive pressure. As soon as the reaction of calcium carbide with water stops, place a burning piece of paper in the bottle – an explosion takes place, accompanied by a fiery cloud. The walls of the bottle explode as a result of the reaction, so this experiment is dangerous, and should only be conducted following safety rules.
To demonstrate the reaction of calcium carbide with water, the experiment can be repeated in a different way – using a bottle with a capacity of 6 liters. In this case, we must weigh the components we use precisely, because the greater radius of the bottle, the less the container can withstand high pressure (with an identical thickness of walls and with identical material). A bottle of a large capacity has a large radius, but its walls are approximately the same – accordingly, it is less resistant to pressure. To stop it from exploding, the amount of calcium carbide must be calculated beforehand. Calcium has a molar mass of 40 and carbon 12, so the molar mass of calcium carbide is around 64 m/mole. Accordingly, 64 g. of carbide will give 22.4 l of acetylene. The volume of the bottle is 6 l, and so the excess pressure is around 4 atmospheres.
The bottle must withstand four 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 withstands 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. Soon there will be a bright yellow flash, and a fountain of flame up to 4 meters high will rise out of the bottle. The stopper burns as a result, and the bottle is warped, but it remains intact. This experiment must be conducted in the open air, far away from flammable and explosive objects. Be sure to follow safety rules.