Difference between revisions of "Calcium carbide"

Revision as of 17:24, 13 December 2017

Calcium carbide is a solid chemical compound of calcium and carbon, with the formula CaC₂, a carbide, that reacts with water to produce acetylene.

Calcium carbide is often referred to as simply carbide.

Properties

Chemical

Calcium carbide reacts vigorously with water to produce calcium hydroxide and acetylene gas.

CaC₂ + 2 H₂O → C₂H₂ + Ca(OH)₂

If there are traces of other substances, it will also produce phosphine, ammonia or hydrogen sulfide. The presence of phosphine can lead to spontaneous ignition of the acetylene. Because it reacts with water, and releases acetylene, pure calcium carbide can be used as a drying agent. However, calcium carbide in general is a poor drying agent, as consumer grade contains lots of impurities and will also release phosphine and ammonia which may react with various reagents, spoiling the purity of the wet compound. Acetylene may also react with some chemical compounds.

Pure calcium carbide reacts slowly with anhydrous acids.

Calcium carbide will react with nitrogen at high temperature, to form calcium cyanamide, in the Frank–Caro process:

CaC₂ + N₂ → CaCN₂ + C

Calcium carbide, like all ionic carbides, is a superbase. It is capable of deprotonating not just water but also alcohols, evolving acetylene.

Physical

Pure calcium carbide is white, but traces of impurities usually gives the material grey, blue or brown aspect.

Availability

Technical grade calcium carbide can be found at certain construction stores, but this type contains traces of other ionic calcium compounds, such as calcium phosphide, calcium nitride, calcium sulfide, that when react with water release an unpleasant smell. Acetylene derived from calcium carbide provides the driving force behind many types of toy cannons, so calcium carbide can be obtained as a kind of fuel in stores or on websites that sell these. Purer samples can be found at lab suppliers.

Certain groundhog killers or repellents contain chunks of calcium carbide. These samples are not very pure, and also have certain additives, biocides and perfumes, like 2-undecanone, citriodiol, coated on the surface of the boulders and inside the pores of the material. These function as the groundhog repellents and do not limit the hydrolysis of the carbide as one might think.

Preparation

Calcium carbide is produced industrially in an electric arc furnace, from a mixture of calcium oxide and coke (or pure carbon) at approximately 2000 °C.

CaO + 3 C → CaC₂ + CO

Another method involves the reaction between finely divided calcium metal and powdered carbon (activated charcoal e.g.), at 810°C in an inert atmosphere. This reaction is highly exothermic, the calcium will not require further heating once melted.^[1]

The latter method has the advantage of obtaining a very pure product, which upon hydrolysis no longer releases the typical phosphine smell associated with the technical grade carbide.

Projects

• Make acetylene
• Metal acetylides, which can be produced by bubbling acetylene through a salt solution of a metal.
• Calcium cyanamide synthesis

Handling

Safety

Calcium carbide must be handled with gloves, as while it is not as caustic as alkali metals e.g., it can still irritate the skin by reacting to the sweat and moisture. Reaction with water gives off toxic fumes, so it should be performed outside or in a fume hood; it also poses a fire hazard because of potential phosphine and diphosphane generated due to phosphorus impurities in commercial samples. In powdered form can severely irritate the eyes and mucous membrane.

Storage

Calcium carbide must be stored in dry containers and away from any moisture. As the technical grade gives off unpleasant smells, it's best to keep it in a closed container or in a well ventilated area.

Avoid storing the carbide in the basement! In case of a flood or water leak, any carbide that enters contact with water will release acetylene which, if it builds up may lead to explosion. Try to store it in the highest place from the room or in the attic if possible. Make sure rainwater doesn't leak in the storage area.

A good storage container is the steel paint can. The can offers excellent protection against air and water. However, normal steel cans are prone to rusting, which sometimes can be a good thing, as it is an indicator that the environment where the CaC₂ is stored is too wet to be safe. Airtight stainless steel cans are even better and offer good protection against most corrosive agents.

Disposal

Calcium carbide can be neutralized by adding various types of alcohol, such as ethanol, isopropanol, resulting in acetylene, vinyl ethers, calcium alkoxides and higher alcohols from decomposition of calcium alkoxides.^[2]

References

1. ↑ https://www.sciencemadness.org/whisper/viewthread.php?tid=2492
2. ↑ http://www.hgxb.com.cn/EN/abstract/abstract14646.shtml

Relevant Sciencemadness threads

• Calcium Carbide CaC2 - Acquisition, Uses and Synthesis
• Synthesis of calcium carbide with electric arc
• Calcium Carbide impurities
• Calcium Carbide and Methylene Chloride ?
• Nice carbide making experiment