Calcium hydride

Template:Chembox new Calcium hydride is the chemical compound with the formula CaH₂. This grey powder (white if pure, which is rare) reacts vigorously with water liberating hydrogen gas. CaH₂ is thus used as a drying agent, i.e. a desiccant.

CaH₂ is a saline hydride, meaning that its structure is salt-like. The alkali metals and the alkaline earth metals all form saline hydrides. A well-known example is sodium hydride, which crystallizes in the NaCl motif. These species are insoluble in all solvents with which they do not react because they have extended structures. CaH₂ crystallizes in the PbCl₂ motif.

Use as a desiccant

The reaction of CaH₂ with water can be represented as follows:

CaH₂ + 2 H₂O → Ca(OH)₂ + 2 H₂

The two hydrolysis products, H₂, a gas, and Ca(OH)₂, a solid, are readily separated from the solvent by distillation, filtration, or decantation.

As calcium hydride is a relatively mild desiccant, it is safe compared with more reactive agents such as sodium metal or sodium-potassium alloy. Calcium hydride is widely used as a desiccant for basic solvents such as amines and pyridine. It is also used to pre-dry solvents prior to the use of a more reactive desiccant.

Drawbacks

Although CaH₂ is indeed convenient and often the drying agent of choice, it has a few drawbacks:

• it is an insoluble in all solvents, in contrast to LiAlH₄, thus the speed of its drying action can be slow.
• it is incompatible with some solvents, and can in fact explosively react with chlorocarbons (LiAlH₄ also suffers from this disadvantage).
• Because CaH₂ and Ca(OH)₂ are almost indistinguishable in appearance, the quality of a sample of CaH₂ is not obvious visually.
• Since calcium hydride does not remove dissolved oxygen, it is not useful for deoxygenating solvents.