Calcium carbide, CP | C2H2Ca | CID 124202440

Chemical Characteristics of Calcium Carbide and Its Reaction

Calcium carbide is a chemical compound composed of calcium and carbon, appearing as a white crystalline substance in its pure form. It is produced through the reaction:

Ca + 2C → CaC₂

Calcium carbide holds significant practical importance and is also referred to as calcium acetylide.

Chemical Characteristics of Calcium Carbide

This compound is non-volatile and insoluble in any known solvent. When it reacts with water, it produces acetylene gas and calcium hydroxide. Its density is 2.22 g/cm³, and it has a melting point of 2160 °C and a boiling point of 2300 °C. Due to the flammability of acetylene gas that forms upon contact with water, it is classified as a hazardous material in class 4.3.

Calcium acetylide was first synthesized by German chemist Friedrich Wöhler when he heated an alloy of zinc and calcium with coal. He described the vigorous reaction of calcium carbide with water. This compound reacts energetically with minute amounts of water, releasing a substantial amount of heat. In situations where water is scarce, the resulting acetylide can ignite spontaneously. Additionally, calcium acetylide reacts violently with aqueous solutions of alkalis and diluted inorganic acids, releasing acetylene. Thanks to its powerful reducing properties, CaC₂ can reduce all metal oxides into pure metals or convert them into carbides.

Obtaining calcium carbide from its oxide is easier than from elemental calcium itself, as the oxide reduces at high temperatures (above 2000 °C). The reaction proceeds as follows:

CaO + 3C → CO + CaC₂

This reaction occurs in an electric arc furnace where a mixture of unslaked lime and coke or anthracite is heated. The resulting product is typically gray due to the presence of free carbon, calcium oxide, phosphide, sulfide, and other compounds. Calcium carbide constitutes approximately 80-85% of the product by mass.

Uses of Calcium Carbide

In the past, calcium carbide was utilized in carbide lamps, which provided a source of acetylene flame. Nowadays, these lamps are still used to power lighthouses and beacons, as well as for cave exploration. CaC₂ also serves as a raw material in developing chemical technologies, especially in the production of synthetic rubber. Moreover, calcium carbide is essential in manufacturing vinyl chloride, acetylene black, acrylonitrile, acetic acid, acetone, ethylene, styrene, and synthetic resins.

In metallurgy, calcium carbide is employed to deoxidize metals and reduce their oxygen and sulfur content (desulfurization). It is also used to produce powdered carbide, which serves as a plant growth regulator. The energy requirement to produce one ton of calcium carbide is about 3,000 kWh of electricity. Consequently, the manufacturing of this compound is only economically viable when electricity costs are low. Nevertheless, global production of calcium carbide continues to rise.

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 odor caused by impurities such as NH₃, H₂S, and PH₃. In its pure state, acetylene is a colorless gas that emits a faint characteristic smell and is soluble in water.

A simple experiment to demonstrate the reaction between calcium carbide and water involves pouring water into a 1.5 L bottle, quickly adding several pieces of calcium carbide, and sealing the bottle with a stopper. The ensuing reaction produces acetylene, building pressure in the bottle. After the reaction ceases, inserting a burning piece of paper into the bottle will likely trigger an explosion accompanied by a fireball. Due to the possibility of the bottle bursting, this experiment is hazardous and should only be conducted with strict safety precautions.

Warning! Do not attempt these experiments without professional supervision! For safe experiments involving flames, click here.

To further demonstrate the reaction with water, the experiment can be modified using a six-liter bottle. In this case, precise weighing of the components is essential, as a larger bottle has a reduced capacity to withstand high pressure (assuming identical materials and thickness). A larger bottle's radius increases while the wall thickness remains relatively unchanged, making it less resilient to pressure. It is crucial to calculate the amount of calcium carbide beforehand, considering that calcium has a molar mass of 40 g/mol, while carbon has a molar mass of 12 g/mol. Therefore, the molar mass of calcium carbide is around 64 g/mol. Consequently, 64 g of carbide yields approximately 22.4 liters of acetylene. As the bottle's volume is 6 liters, the pressure could increase up to roughly 4 atmospheres.

The bottle must endure five atmospheres of pressure: for conducting the experiment, around 64 g of calcium carbide and about 0.5 L of water are necessary. Calcium carbide should be placed inside a small bag, pushed into the bottle, and then sealed with a stopper. The reaction between calcium carbide and water lasts several minutes, swelling the bottle and producing loud bangs, but the bottle should remain intact.

After the complete release of acetylene, placing a hot rag soaked in a flammable liquid on the stopper and retreating to a safe distance will result in a bright yellow flash and a flame fountain reaching up to 4 meters high. The stopper will burn, and the bottle may warp, yet it should remain intact. This experiment must be performed outdoors, far from flammable or explosive materials. Always ensure observance of safety precautions.

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