Calcium hydride is widely used as a desiccant for alkaline solvents

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What is calcium hydride? Calcium hydride is a compound with the chemical formula CaH2, so it is an alkaline earth metal hydride. This gray powder (pure white, sometimes white) reacts violently with water and releases hydrogen. Therefore, CaH2 is used as a desiccant.

CaH2 is a brine hydride, which means that its structure is similar to salt. During the Battle of the Atlantic, German submarines used calcium hydride as a sonar decoy called bold. Alkali metals and alkaline earth metals are heavier than beryllium, and both form hydrogen halides. A well-known example is sodium hydride, which crystallizes in the NaCl master mold. These substances are insoluble in all solvents that do not react with them. CaH2 crystals have a PbCl2 (perovskite) structure.
Why is calcium hydride called Hydrolith?
Calcium hydride is called Hydrolith because CaH2 is brine hydride, which means that its structure is similar to salt. Both alkali metals and alkaline earth metals form sodium hydride.

What is the role of calcium hydride?
It is safer to use than more reactive reagents (such as sodium metal or sodium-potassium alloy). Calcium hydride is widely used as a desiccant for alkaline solvents such as amines and pyridine. It is also used to dry alcohol.

Reduced metal oxide
CaH2 is a reducing agent used to produce metals from metal oxides of Ti, V, Nb, Ta and U. It is recommended to decompose it into Ca metal for operation:
TiO2 + 2 CaH2-Ti + 2 CaO + 2H2

Hydrogen source
CaH2 has been used for hydrogen production. In the 1940s, it was used as a hydrogen source under the trade name “Hydrolith”:

The trade name of this compound is “hydrolith”. In an emergency, it can be used as portable hydrogen to fill the airship. This use is quite expensive.

The reference to “emergency” may refer to wartime use. However, this compound has been widely used for decades as a safe and convenient means of inflating weather balloons. Similarly, it is usually used in the laboratory to produce small amounts of high-purity hydrogen for experiments.
The reaction of CaH2 with water can be expressed as follows:
CaH2 + 2 H2O-Ca(OH)2 + 2 H2
The two hydrolysis products of gaseous H2 and Ca(OH)2 are easily separated from the dry solvent.

Calcium hydride is a relatively mild desiccant and may be inefficient compared to molecular sieves. It is safer to use than more reactants such as sodium metal or sodium-potassium alloy. Calcium hydride is widely used as a desiccant for alkaline solvents such as amines and pyridine. It is also used to dry alcohol.

Although convenient, CaH2 has some disadvantages:
Compared with LiAlH4, it is insoluble in all solvents that do not react violently, so its drying speed may be slow. Since the appearance of CaH2 and Ca(OH)2 is almost indistinguishable, the quality of CaH2 samples is not visually obvious.

What happens when water is added to calcium hydride?
Calcium hydride (CaH2) reacts violently with water and releases hydrogen. It is found that the energy activation (Ea = 20.03 kJ/mol) of the hydrolysis reaction of CaH2 in ethanol solution is less than other reactions.

How do you make calcium hydride?
Calcium hydride can be produced by reacting dry hydrogen with calcium metal at a temperature between 300 and 400degC.
Another method of preparing calcium hydride is to heat calcium chloride with hydrogen and sodium metal. The reaction occurs in the following way:
CaCl2 + H2 + 2 Na-CaH2 + 2 NaCl
In this reaction, calcium (Ca) and hydrogen (H) form calcium hydride molecules, and sodium atoms and chlorine form sodium chloride molecules.

Calcium hydride can also be produced by reducing calcium oxide (CaO) with magnesium (Mg). The reaction is carried out in the presence of hydrogen. This reaction also produces magnesium oxide. The following is the formula of the chemical reaction:
CaO + Mg + H2-CaH2 + MgO

What type of bond is calcium hydride?
Ionic hydride reacts violently with water to remove hydrogen (H2). Compounds with only hydrogen and one other element are called dihydrohydrides, and their form is usually MH or MH2, such as sodium hydride (NaH), lithium hydride (LiH), calcium hydride (CaH2), and magnesium hydride (MgH2).

Unstable calcium hydride as a promising high-temperature thermal battery
Calcium hydride (CaH2) is considered an ideal candidate for thermochemical energy storage (thermal batteries) due to its high energy density and low cost. Its extremely high operating temperature and poor cycle stability are the main factors hindering its development and implementation as a concentrated solar (CSP) factory thermal battery. In this work, alumina (Al2O3) was used to thermodynamically destabilize CaH2 at a molar ratio of 1:1, thereby releasing hydrogen at a lower temperature than the hydride alone.
Temperature-programmed desorption measurements show that compared with the decomposition of pure CaH2 to about 1000degC under 1 bar of hydrogen pressure, the addition of Al2O3 will lower the decomposition temperature to ~600degC, thereby making the reaction thermodynamically unstable for the release of hydrogen from CaH2. The experimental enthalpy and entropy of the system are determined by the pressure component isotherm measurement between 612 and 636degC.
Enthalpy is measured as DHdes=100+-2 kJ mol-1 for H2, and entropy is measured as DSdes=110+-2 J*K-1 mol-1 for H2. The XRD and in-situ XRD data after TPD confirmed that the main product is Ca12Al14O33. X-ray diffraction and scanning electron microscopy confirmed that the system showed a capacity loss during the hydrogen cycle at 636degC, which was caused by sintering excess Al2O3. By reducing the initial amount of Al2O3 to a molar ratio of CaH2 to Al2O3 of 2:1, the hydrogen cycle capacity has been significantly improved. This is considered a highly promising high-temperature thermal battery for the next generation of CSP plants.
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