Hydride – Definition, Formula and Types

Hydrides are binary compounds that form when hydrogen reacts with other elements in the periodic table. Except for a few noble gases, almost any element can produce hydride compounds. Trends and attributes are influenced by the type of intermolecular force that holds the components together, as well as temperature, molecular masses, and other factors.

What is Hydride in Chemistry?

Hydride is a hydrogen anion with the formula \({{\rm{H}}^ – }\). Hydrides may be defined as binary compounds of hydrogen with other elements. Hydrides are created when dihydrogen reacts with a significant number of other elements, such as metals or non-metals, under the right reaction conditions. Hydrogen is covalently bonded to a less electronegative element in hydride. The hydrides may be represented by the general formula \({\rm{E}}{{\rm{H}}_{\rm{x}}}\) or \({{\rm{E}}_{\rm{m}}}{{\rm{H}}_{\rm{n}}}\). Where \({\rm{E}}\) is the symbol of elements.

Water, methane, and ammonia are all examples of hydrides.

Hydride Ion

hydride anions exist only in extreme conditions and are not used in homogeneous solutions. Many compounds, on the other hand, have hydrogen centres with hydridic properties.  The simplest anion is the hydride ion, which consists of two electrons and a proton. Therefore, the electronic configuration of hydride ion is \({\rm{1}}{{\rm{S}}^2}\).

Types of Hydride

The main types of Hydrides are;

Ionic Hydride: Salt-like hydrides are another name for ionic hydrides. The transfer of an electron from a metal atom to a hydrogen atom results in the formation of saline hydrides. These are binary and stoichiometric hydrogen compounds with more electropositive elements than hydrogen, such as alkali metals and alkaline earth metals (except Be). Some common examples of this category are; \({\rm{LiH,NaH,Ca}}{{\rm{H}}_{\rm{2}}}{\rm{,Sr}}{{\rm{H}}_{\rm{2}}}\) etc.

1. They are crystalline solids with white or greyish colour and a high melting and boiling point. They act as a powerful reducing agent.
Common examples to show Ionic hydrides are powerful reducing agents are
i. Sodium hydride reduces carbon monoxide to elemental carbon.

\(\mathop {{\rm{2CO}}}\limits_{{\rm{Carbon}}\,{\rm{monoxide}}} {\rm{ + }}\mathop {{\rm{NaH}}}\limits_{{\rm{Sodium}}\,{\rm{hydride}}} \to \mathop {{\rm{HCOO}}{\rm{Na}}\,}\limits_{{\rm{Sodium}}\,{\rm{formate}}} {\rm{ + }}\,{\rm{C}}\)

ii. Calcium hydride reduces lead sulphate to lead sulphide.

2. Except, \({\rm{LiH}}\) metal hydrides generally undergo combustion when heated strongly in the air. This is due to their decomposition into highly inflammable \({{\rm{H}}_{\rm{2}}}\)

3. They are densely packed. In general, they have a greater melting point than the metals from which they are made.

4. They produce dihydrogen gas when they react aggressively with water and other protonic solvents like ethanol and ammonia.

\({\rm{NaH + }}{{\rm{H}}_2}{\rm{O}} \to {\rm{NaOH + }}{{\rm{H}}_{\rm{2}}}\)

\({\rm{NaH + N}}{{\rm{H}}_{\rm{3}}} \to {\rm{NaN}}{{\rm{H}}_{\rm{2}}}{\rm{ + }}\mathop {{{\rm{H}}_{\rm{2}}}}\limits_{{\rm{Dihydrogen}}} \)

5. They conduct electricity in a molten state and release hydrogen gas at the anode, indicating that hydride is present  in them.

2. Covalent or Molecular Hydrides: These are binary compounds of hydrogen and the elements of hydrogen, as well as elements with relatively high electronegativity, such as p-block elements. In this hydride, the \({\rm{H}}\) atoms are bonded to other atoms by covalent bonds. \({\rm{HCl,}}{{\rm{H}}_{\rm{2}}}{\rm{O,N}}{{\rm{H}}_{\rm{3}}}{\rm{,P}}{{\rm{H}}_{\rm{3}}}{\rm{,C}}{{\rm{H}}_{\rm{4}}}\), etc, are good examples of covalent hydride.

Classification of Covalent hydrides: Covalent hydrides are classified based on the number of electrons and bonds present in their Lewis structures.

1. Electron Deficient Hydrides: The quantity of electrons in these hydrides is adequate to establish a conventional covalent bond.
   Examples are hydrides of group \(13\) elements such as \({\rm{B}}{{\rm{H}}_{\rm{3}}}{\rm{,Al}}{{\rm{H}}_{\rm{3}}}\), etc. They generally exist in polymeric forms like \({{\rm{B}}_{\rm{2}}}{{\rm{H}}_{\rm{6}}}{\rm{,}}\,{{\rm{B}}_{\rm{4}}}{{\rm{H}}_{{\rm{10}}}},\) etc.
2. Electron Precise Hydrides: These hydrides have the exact number of electrons needed to form covalent bonds. Group \(14\) elements include Electron-precise hydrides. For example, \({\rm{C}}{{\rm{H}}_{\rm{4}}}{\rm{,Si}}{{\rm{H}}_{\rm{4}}}\), etc.
3. Electron Rich Hydrides: These hydrides have more electrons than are required for a conventional covalent bond to form. Electron-rich hydrides are formed by elements in groups \(15,16\), and \(17\) The excess electrons are present in the form of lone pairs. For example, ammonia, \({\rm{HCl,}}\) etc.

General Characteristics of Covalent Hydrides

1. They are poor electrical conductors.

2. They are a powerful reducing agent.

3. Because they are covalent, they are more soluble in organic solvents.

4. They undergo thermal decomposition into their respective elements.

5. Some of the covalent hydrides react with water and release hydrogen gas.

\({{\rm{B}}_{\rm{2}}}{{\rm{H}}_{\rm{6}}}{\rm{ + 6}}{{\rm{H}}_{\rm{2}}}{\rm{O}} \to \mathop {{\rm{B}}{{\left( {{\rm{OH}}} \right)}_{\rm{3}}}}\limits_{{\rm{Boric}}\,\,{\rm{acid}}} {\rm{ + 6}}{{\rm{H}}_{\rm{2}}}\)

6. The acidic nature of covalent hydride increases from left to right.

\(\begin{array}{*{20}{c}}{{\rm{N}}{{\rm{H}}_{\rm{3}}}}&{{{\rm{H}}_{\rm{2}}}{\rm{O}}}&{{\rm{HF}}}\end{array}\)

3. Metallic Hydrides or Interstitial Hydrides: These are hydrogen-transition-element binary compounds. These hydrides are typically formed by transition metals from groups \(3,4,5,\)and \(6\) of the d-block and \({\rm{Cr}}\) metal of group \(6\) and f-block elements.

For example \({\rm{Sc}}{{\rm{H}}_{\rm{2}}}{\rm{,Y}}{{\rm{H}}_{\rm{2}}}{\rm{,Hf}}{{\rm{H}}_{\rm{2}}}{\rm{,TaH}}\)

Non-stoichiometric Hydrides

Recent research has revealed that except hydrides of \({\rm{Ni,Pd,Ce}}\), and \({\rm{Ac}}\), the other hydrides of this type have lattices that differ from the parent metal. These compounds are solid hydrogen solutions. Because the composition of these hydrides may not correspond to a simple whole-number ratio, they are also known as non-stoichiometric hydrides.

For example, \({\rm{Pd}}{{\rm{H}}_{\left( {{\rm{0}}{\rm{.6 – 0}}{\rm{.8}}} \right)}}{\rm{Ti}}{{\rm{H}}_{\left( {{\rm{1}}{\rm{.8 – 2}}} \right)}}{\rm{V}}{{\rm{H}}_{\left( {{\rm{0}}{\rm{.6}}} \right)}}\)

General Properties of Metallic Hydrides

1. They have high thermal conductivity.
2. The majority of these hydrides are harder than their parent metals.
3. Generally, they are powders or brittle solids with a dark or metallic appearance.
4. They are good conductors of electricity. However, the conductivity decreases with an increase in temperature.
5. They undergo reversible decomposition and form metal and hydrogen gas.

Sodium Hydride

Sodium hydride is the chemical compound with the empirical formula \(24.0\;{\rm{g}}/{\rm{mol}}\), and its molar mass is . It is a saline hydride composed of \({\rm{N}}{{\rm{a}}^ + }\) and \({{\rm{H}}^{\rm{ – }}}\) ions. It is an insoluble ionic material in organic solvents, consistent with \({{\rm{H}}^{\rm{ – }}}\)  ions that do not exist in the solution.

Preparation of Sodium Hydride

At \({\rm{700 – 750^\circ C,}}\) dry and air-hydrogen is passed over molten sodium metal to produce sodium hydride.

\({\text{2Na + }}{{\text{H}}_{\text{2}}}\xrightarrow{{{\text{700 – 75}}{{\text{0}}^{\text{o}}}{\text{C}}}}{\text{2NaH}}\)

Properties of Sodium Hydride

1. It is a colourless solid, but it generally occurred as a grey solid due to its reactivity.
2. It has a high melting point.
3. It is a strong base.

Uses of Sodium Hydride

1. It is a strong reducing agent that reduces a variety of compounds.
2. \({\rm{NaH}}\) is a base capable of deprotonating a wide range of even weak Bronsted acids to yield sodium derivatives.
3. \({\rm{NaH}}\) pellet, when crushed in the presence of water releases \({{\rm{H}}_2}\). Therefore, \({\rm{NaH}}\) is proposed for \({{\rm{H}}_2}\) storage for use in fuel cell vehicles. 
4. Because of its quick and irreversible reaction with water, it can be used to dry some organic solvents.

Lithium Aluminium Hydride

\({\rm{LAH}}\), or lithium aluminium hydride, is a chemical compound with the formula \({\rm{LiAl}}{{\rm{H}}_4}.\) It is a solid dark grey colour. The molar mass of it is \(37.95\;{\rm{g}}/{\rm{mol}}\). It is a monoclinic crystalline solid composed of lithium cation \(({\rm{L}}{{\rm{i}}^ + })\) and aluminium hydride anion \(({\rm{AlH}}_4^ – ),\) with \({\rm{Al}}\) covalently bonded to the four H atoms.

Preparation of Lithium Aluminium Hydride

1. \({\rm{LiAl}}{{\rm{H}}_{\rm{4}}}\) was first synthesised by reacting lithium hydride \(\left( {{\rm{LiH}}} \right)\) with aluminium chloride.

\({\rm{4LiH + AlC}}{{\rm{l}}_{\rm{3}}} \to {\rm{LiAl}}{{\rm{H}}_{\rm{4}}}{\rm{ + 3LiCl}}\)

2. There are two steps involved in the industrial preparation of lithium aluminium hydride.
In the first step, sodium and aluminium metals react with hydrogen at high pressure and temperature to form sodium aluminium hydride.

\({\rm{Na + Al + 2}}{{\rm{H}}_{\rm{2}}} \to {\rm{NaAl}}{{\rm{H}}_{\rm{4}}}\)

In the second step, the formed sodium aluminium hydride is reacted with lithium chloride, resulting in the formation of lithium aluminium hydride as a product.

\({\rm{NaAl}}{{\rm{H}}_{\rm{4}}}{\rm{ + LiCl}} \to {\rm{LiAl}}{{\rm{H}}_{\rm{4}}}{\rm{ + NaCl}}\)

Properties of Lithium Aluminium Hydride

1. It is an odourless, odourless solid that appears grey due to contamination.
2. Its melting point is \({\rm{15}}{{\rm{0}}^{\rm{^\circ }}}{\rm{C}}\) and its density is \(0.97\;{\rm{g}}/{\rm{mL}}\).
3. \({\rm{LAH}}\) reacts violently with water, releasing hydrogen gas. \({\rm{LiAl}}{{\rm{H}}_{\rm{4}}}{\rm{ + 4}}{{\rm{H}}_{\rm{2}}}{\rm{O}} \to {\rm{LiOH + Al}}{\left( {{\rm{OH}}} \right)_{\rm{3}}}{\rm{ + 4}}{{\rm{H}}_{\rm{2}}}\)
4. \({\rm{LAH}}\) decomposes over time due to instability, producing \({\rm{L}}{{\rm{i}}_{\rm{3}}}{\rm{Al}}{{\rm{H}}_{\rm{6}}}\).
5. It is a powerful reducing agent and a strong base.
6. It is soluble in ether and tetrahydrofuran but only sparingly in other organic solvents.

Uses of Lithium Aluminium Hydride

1. Lithium aluminium hydride \(\left( {{\rm{LiAl}}{{\rm{H}}_{\rm{4}}}} \right)\) is a common reducing agent in organic chemistry.
2. \({\rm{LAH}}\) is widely used to synthesise the main group and transition metal hydrides from metal halides.
3. It is also used as a polymerisation catalyst and a propellant.
4. It is considered a potential hydrogen storage agent in fuel cells.

Hydride Gap in the Periodic Table

Hydride is a binary compound of hydrogen with other elements. But some elements of group \({\rm{7,8,9}}\) of the d-block do not form hydrides. So, this inability of metals of group \({\rm{7,8,9}}\) of the periodic table to form hydrides is referred to as the hydride gap or hydride gap of d-block.

From this article, we can conclude that Hydride is an anion of hydrogen. Because of this negative charge, hydrides have reducing or basic properties. This property of Hydride has many applications in the industrial field.

Frequently Asked Questions on Hydrides

Q.1. What is the symbol of hydride?
Ans: Symbol of hydride is \({{\rm{H}}^ – }\).

Q.2. Why is water called a hydride of oxygen?
Ans: Water is called a Hydride of oxygen because it consists of an oxygen atom covalently bonded to two hydrogen atoms.

Q.3. What is a hydride in chemistry?
Ans: In chemistry, a hydride is formally the anion of hydrogen, \({{\rm{H}}^ – }\). Hydrides may be defined as binary compounds of hydrogen with other elements.

Q.4. What is the hydride formula?
Ans: Hydride is an anion of hydrogen with the formula \({{\rm{H}}^ – }\)

Q.5. What are hydrides explained with example?
Ans: Hydrides are hydrogen compounds with fewer electronegative elements. For example, water is a hydride of oxygen, ammonia is a hydride of nitrogen, etc.

Q.6. Which hydride is the strongest base?
Ans: Ammonia is the strongest base hydride because it has a lone pair on the central atom, and the size of N is small.

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