What happens during Carbide Measurements?
Every chemical reaction runs until it has reached its endpoint. This endpoint has been reached, when no more water can come into contact with the carbide. Water and carbide have to be able to touch each other during the reaction. The transformation to acetylene, the actual reaction, happens very quickly (in a few micro seconds).
How quickly the reaction can reach its endpoint, depends on how intensively the two reaction partners are in contact with each other.
When the waters path gets interrupted and it can’t reach the carbide, for example by not shaking the bottle, the chemical reaction can take up to a few days. In a carbide measurement water is the only mobile component. The slower the water moves and the longer its path is, the slower the reaction. If the pressure bottle gets shaken the carbide also turns into a mobile component.
If there is no water on the surface of the sample (sand filling), but in the pore systems, meaning on the inner surface, the water can only move very slowly (diffusion, effusion) so the reaction time correspondingly slow. Screeds, especially cementitious systems, exhibit because of their porous structures a large inner surface. A maze or the canals of Venice are the best forms of descriptions for the porous structures of cementitious screeds.
If you leave a pressurized bottle standing around unshaken, it will take a few weeks up to the end of reaction. If the bottle however gets shaken a lot, the reaction can be concluded in 2 days at room temperature.
For Chemistry Fans!
Fundamentally, a chemical reaction always runs to the very end, until it has reached its chemical equilibrium. The chemical equilibrium has been reached, when the forward reaction creates the same amount of products and through the backwards reaction the output products are re-created. Each reaction has a backwards reaction, which then again gives us the output products.
Where the point of equilibrium exactly is depends on the places of the equilibrium constants (law of mass action). These lie, in the case of the chemical reaction of calcium chloride with water clearly on the side of the product, so, on the gaseous acetylene.
The amount of rest water of calcium carbide can be calculated and lies at partial pressure of ca. 2 x 10 – 10 mbar.
If we make a small comparison: The steam pressure of water is at 20˚C with around 23 mbar at 100 billion times higher.
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