Dhristi JEE 2022-24

What are Hydrides?

Dihydrogen, which is better known as H2, is the most abundant element in the world and it can combine with almost anything and form carbohydrates, proteins, hydrides, hydrocarbons and many other compounds. Dihydrogen or H2 combines with almost every element on earth, except noble gasses, and forms binary compounds called hydrides under certain conditions. MgH2, B2H6 are two examples of hydrides. These hydrides are broadly classified into three categories:

  • Ionic or saline or salt like hydrides

  • Covalent or molecular hydrides

  • Metallic or non-stoichiometric hydrides

Ionic hydrides

These compounds of H2 are mostly formed with s-block elements which are highly electropositive in nature. Although lighter metal hydrides such as LiH, BeH2 have significant covalent character.

LiH is unreactive at a moderate temperature with O2 or Cl2, so it is used in the synthesis of other useful hydrides.

Covalent hydrides

H2 forms molecular bonds with most of the p-block elements and the most common compounds are H2O, NH3, CH4 and HF.

Covalent hydrides are further divided into three categories according to the relative numbers of electrons and bonds in their Lewis structure:

(i) electron-deficient - all the group 13 elements form electron deficient compounds with hydrogen and act as electron acceptor (Lewis acids), eg, B2H6,

(ii) electron-precise - all group 14 elements form electron precise compounds and see tetrahedral in geometry, eg, CH4,

(iii) electron-rich hydrides - elements from groups 15 through 17 form such bonds that have excess electrons which are present as lone pairs, eg, NH3 where 1 lone electron is present.

Metallic hydrides

Hydrides formed by many elements of d and f-block with H2 are called metallic hydrides. Elements from groups 7,8 and 9 do not form bonds with H2, and only Cr from group 6 forms a bond. Some of the metals can accommodate a large number of hydrogen making them highly potential for hydrogen storage, eg, Pt, Pd. In this type of hydride, hydrogen occupies the metal lattices in the interstitial spaces.

FAQs on Covalent Hydrides

1. What are Hydrides?

Hydride is a compound in which one or more hydrogen centers have nucleophilic, reducing, or basic properties. In compounds that are regarded as hydrides, the hydrogen atom is bonded to a more electropositive element or group. Compounds containing hydrogen-bonded metals or metalloids may also be referred to as hydrides. Almost all of the elements form binary compounds with hydrogen, the exceptions being He, Ne, Ar, Kr, Pm, Os, Ir, Rn, Fr, and Ra.

2. What is the Type of Bond Present in the Hydrides?

The covalent bonds between hydrogen and the other elements vary from strong to weak. Some hydrides, such as boron hydrides, do not follow traditional electron-counting laws, and their bonding is characterised using multi-centered bonds, while interstitial hydrides often use metallic bonding. Discrete compounds, oligomers or polymers, ionic solids, chemisorbed monolayers, bulk metals (interstitial), and other materials may all be hydrides. Although most metal hydrides react as Lewis bases or reducing agents, some act as hydrogen-atom donors and thus as acids.

3. Which is the least acidic and the most stable hydride among the hydrides of halogens?

Fluorine is the most electronegative element and has a small size, due to which it forms a very strong bond with hydrogen, therefore, HF doesn't give H+ ions easily hence, HF is the least acidic hydride among the other hydrides of halogens. It is also due to this very property of electrophile affinity of fluorine that makes it form a highly stable bond with hydrogen and H+ ions that are not dispersed easily.

4. What are the three types of covalent hydrides?

The three types of covalent hydrides are electron-deficient hydrides, electron-precise hydrides and electron-rich hydrides which are named according to the availability of free electrons after the elements form compounds with hydrogen. LiH is an electron-deficient hydride, CH4 is an electron-precise hydride and H2O is an electron-rich hydride.

5. What are the uses of hydrides?

Hydrides are used as reducing agents in the chemical industry, in battery storage such as nickel hydride batteries and for conducting electricity, producing hydrogen. Hydrides are also used as drying agents, heat and hydrogen storage as well as strong bases for organic synthesis.

6. Do N, O and F element hydrides have lower boiling points than their subsequent group members?

From the point of view of molecular masses, N, O and F element hydrides, their boiling points are supposed to be lower than the other members of the subsequent groups but due to their high electronegativity, there is a high affinity for an electrophile like H+ which makes their boiling points higher than expected and definitely higher than the hydrides of the subsequent group members.

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