What Is a Hydride?
A hydride is a binary compound formed by hydrogen and other elements. However, in general scientific and technological work, binary compounds of hydrogen and metal are always referred to as hydrides, and binary compounds of hydrogen and non-metals are referred to as certain hydrogen hydrides. In the periodic table, almost all elements other than rare gases can form hydrides with hydrogen, which are roughly divided into three types: ionic, covalent and transitional. They have different properties.
- Chinese name
- Hydride
- Category
- Binary compound
- Species
- Ionic hydride / covalent hydride
- Function
- Reducing agents, initiators and catalysts
- A hydride is a binary compound formed by hydrogen and other elements. However, in general scientific and technological work, binary compounds of hydrogen and metal are always referred to as hydrides, and binary compounds of hydrogen and non-metals are referred to as certain hydrogen hydrides. In the periodic table, almost all elements other than rare gases can form hydrides with hydrogen, which are roughly divided into three types: ionic, covalent and transitional. They have different properties.
Hydride salt
- Ionic hydride is also called salt hydride. It is a binary compound formed by hydrogen, calcium, strontium, barium and radium in alkali metals and alkaline earth metals. Its solid is ionic crystal, such as NaH, BaH2 and so on. The electronegativity of these elements is smaller than that of hydrogen. In this type of hydride, hydrogen exists in the form of H-, which can conduct electricity in the molten state, and releases hydrogen at the anode during electrolysis. Therefore, this method is also called a metal hydrogen storage method. Ionic hydrides are colorless or white crystals, often grayed out due to the inclusion of metallic impurities, and blue-violet in excess of metal. The oxidation number of hydrogen in ionic hydrides is -1, which has a strong tendency to lose electrons. It is a strong reducing agent. It reacts strongly with water in aqueous solutions to emit hydrogen, making the solution strongly alkaline, such as:
- CaH2 + 2H2O Ca (OH) 2 + 2H2
- Stronger reducibility at high temperatures, such as:
- NaH + 2CO HCOONa + C
- 2CaH2 + PbSO4 PbS + 2Ca (OH) 2
- 2LiH + TiO2 Ti + 2LiOH
- Ionic hydrides are unstable to air and water, and some even spontaneously ignite.
- Ionic hydrides can be prepared by direct synthesis of metals and hydrogen under different conditions. The reaction temperature is 300-700 ° C. In order to prevent the reaction from generating hydrides on the metal surface to prevent further reactions, the dispersoid of the metal in mineral oil is usually used, or a surfactant is added.
- In addition to being used as a reducing agent, it is also used as a desiccant, dehydrating agent, and hydrogen generating agent. Under standard conditions, 1 kg of lithium hydride can react with water to produce 2.8 m3 of hydrogen. In non-aqueous solvents, it can form complex hydrides, such as lithium aluminum hydride, which are widely used in organic and inorganic synthesis with B (), Al (), etc. in the oxidation state of + :
- 4LiH + AlCl3 LiAlH4 + 3LiCl
- Composite hydrides are mainly used as reducing agents, initiators and catalysts.
Hydride covalent
- Covalent hydrides are also called molecular hydrides. It is formed by hydrogen and group IIIA AA elements. Among them, the hydrides formed with group IIIA elements are electron-deficient compounds and polymeric hydrides, such as diborane B2H6, aluminum hydride (AlH3) n, and the like. The thermal stability of each covalent hydride is very different. Lead hydride PbH4 and bismuth hydride BiH3 strongly decompose at room temperature. Hydrogen fluoride and water hardly decompose when heated to 1000 ° C. Covalent hydrides also have reducing properties. Because the oxidation number of hydrogen is +1, its reducing ability depends on the electron loss ability of another element, Rn. Generally speaking, the reductivity of the same family increases from top to bottom, and the reductivity decreases from left to right in the same cycle, for example:
- 4NH3 + 5O2 4NO + 6H2O
- 2PH3 + 4O2 P2O5 + 3H2O
- 2H2S + 3O2 2SO2 + 2H2O
- Covalent hydrides behave more complexly in water. Commonly:
- Strong acid forming: HCl, HBr, HI;
- Formation of weak acids: HF, H2S, H2Se, H2Te;
- Alkali-forming: NH3;
- Water liberated from hydrogen: B2H6, SiH4;
- Does not interact with water: CH4, PH3, AsH3, GeH4, SnH4, SbH3.
- The ability of hydride RHn to give protons is generally related to the electronegativity and radius of R. From the same period, the acidity from left to right increases with the increase of R's electronegativity; from the same family, from the top to the bottom, the acidity enhancement is mainly determined by the corresponding increase of the radius of R. The strength of acidity and alkalinity is determined by the total energy effect during the thermochemical cycle of ionization of H + protons in water
Hydride transition
- Transition hydrides are also called metal hydrides. In addition to the above two types, the remaining elements are binary compounds formed with hydrogen. The composition of this type of hydride does not conform to the normal valence rule, such as lanthanum hydride LaH2.76, cerium hydride CeH2.69, palladium hydride Pd2H, etc. The arrangement of metal atoms in their lattices remains essentially the same, except that the distance between adjacent atoms has increased slightly. Because the hydrogen atom occupies the position of the void in the metal lattice, it is also called a mesofilled hydride. The formation of transition hydrides is related to the nature of the metal, temperature, and hydrogen partial pressure. Their properties are very similar to those of the parent metal, and they have obvious strong reducing properties. Generally, the thermal stability is poor, and it is easy to emit hydrogen gas when heated. As a promising energy source in the future, the central problem to be solved is how to store hydrogen. Some metals or alloys are good materials for hydrogen storage. Palladium, palladium alloys, and uranium are all strongly hydrogen-absorbing materials, but they are expensive. The most noticeable thing is lanthanum nickel-5LaNi5 (LaNi5H6 after hydrogen absorption), which is a good material for hydrogen storage. [1] The small cylinder with a capacity of 7L contains hydrogen (304kPa) that can be held by lanthanum nickel-5, which is equivalent to the hydrogen contained in a 15000kPa high-pressure hydrogen cylinder with a capacity of 40L (equivalent weight). As long as it is slightly heated, LaNi5H6 can put The entire stored hydrogen is released. In addition to lanthanum nickel-5, La-Ni-Cu, Zr-Al-Ni, Ti-Fe and other hydrogen absorbing materials are also being studied. It is of greater significance to study the hydrogen-absorbing effects of high-yield elements in China, especially rare earth metals and their alloys.
- Both alkali metal hydrides. When the alkali metal reacts with hydrogen, hydrides of the alkali metal are formed. They are all ionic compounds, and hydrogen exists in the form of anion H-, such as sodium hydride (NaH), potassium hydride (KH), and the like.
- Distribution of hydrides in the periodic system
Hydride boiling point
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