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Hydride

physical science Maturity 11-13 Next article →

In the world of chemistry, a hydride is a substance that involves hydrogen bonded to other elements. Formally, a hydride is defined as the anion of hydrogen, written as H−. This is a hydrogen ion that carries two electrons.

NaH
NaH

While this specific anion is rarely seen in a free state, the term is often used more broadly. In modern chemistry, it usually refers to compounds with ionic bonds. However, it is also used to describe compounds where hydrogen is covalently bound to other atoms. In this older, broader sense, water is considered a hydride of oxygen, and ammonia is a hydride of nitrogen.

To understand how hydrides work, we must look at how they bond. Bonding can range from being highly ionic to somewhat covalent. In ionic hydrides, the bond is formed between hydrogen and an electropositive metal, such as an alkali metal. These are often called saline hydrides because they behave like salts. They are typically insoluble in common solvents. In covalent hydrides, the hydrogen atoms share electrons with other atoms. This group includes many non-metals and some specific metals like aluminum or gallium. There are also interstitial hydrides, where hydrogen enters the spaces within a metal lattice.

Metal Hydride for Hydrogen Storage-Ovonic
Metal Hydride for Hydrogen Storage-Ovonic

Scientists classify these substances into three distinct types based on their bonding nature. The first type is ionic hydrides, which consist of a hydride bound to metals like lithium or sodium. These are often used as strong bases in organic synthesis. The second type is covalent hydrides, which include molecules like hydrocarbons or complex metal hydrides. Some of these, such as lithium aluminium hydride, are powerful reducing agents used to build new molecules. The third type is interstitial hydrides. These exist within metals or alloys, where hydrogen atoms occupy the tiny gaps between metal atoms. In these systems, the hydrogen can exist as single atoms or as diatomic molecules.

Research into these materials has led to many important discoveries. For example, scientists have studied how hydrogen moves into metal lattices through different mechanisms. One way is through the adsorption of dihydrogen, where the H−H bond breaks and the protons diffuse into the metal. Another way is through the electrolytic reduction of ionized hydrogen on the metal surface. This process can cause a volume expansion in certain electrodes. However, this movement can also lead to hydrogen embrittlement, a condition where the metal becomes brittle and prone to cracking. Understanding these processes is vital for materials engineering.

Many specific numbers illustrate the power and scale of hydride chemistry. Palladium is a remarkable example of an interstitial hydride former. At room temperature, palladium can absorb up to 900 times its own volume of hydrogen. This makes it a subject of intense study for hydrogen storage. In a common palladium hydride structure, about 70% of the octahedral holes in the metal lattice are occupied. When discussing the energy of these particles, the hydride anion is known to be a powerful Lewis base. It reacts exothermically with protons with a heat change of −1676 kJ/mol. These precise measurements help chemists predict how hydrides will react in different environments.

Hydrides serve many practical roles in both industry and nature. In the laboratory, chemicals like sodium borohydride and DIBAL are used as reducing agents in chemical synthesis. Other hydrides, such as calcium hydride, act as desiccants to remove trace water from solvents. In technology, hydrides are essential components in nickel-metal hydride batteries.

TTMSS
TTMSS

They are also being examined as a way to store hydrogen for fuel cell-powered electric cars. Even in biology, certain enzymes called hydrogenases operate using hydride intermediates. This shows that these chemical processes are not just for labs, but are part of life itself.

Finally, the study of hydrides connects to many broader scientific fields. The different isotopes of hydrogen create unique types of hydrides. Hydrides containing protium are called protides, those with deuterium are deuterides, and those with tritium are tritides. Some deuterides, like lithium deuteride, are used as fuels in nuclear reactors and fusion technology. This connection links basic molecular chemistry to the massive field of nuclear physics. Whether they are helping a battery power a device or helping an enzyme function, hydrides are central to how the physical world operates.

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File:NaH.jpg
NaH.jpg
File:TTMSS.png
TTMSS.png
File:Katalysezyklus-Wilkinson.png
Katalysezyklus-Wilkinson.png
File:Metal Hydride for Hydrogen Storage-Ovonic.jpg
Metal Hydride for Hydrogen Storage-Ovonic.jpg
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