Hydrogen is a pretty reactive element. It reacts with every element on the periodic table except non-metals of group VIIIA. These compounds that hydrogen forms are known as hydrides. So every element that hydrogen forms, like water, hydrochloric acid etc are all actually hydrides! Let us learn more.
Very simply put a hydride is the product when hydrogen reacts with any other element, except of course the noble gases. So hydrides are compounds where one atom is hydrogen bonded to another more electropositive element. A hydride is an anion of hydrogen (H-).
The bonding between hydrogen and other elements is always covalent. It just varies in its degrees of covalency. The general chemical formula of a hydride is MHx. Here M is the other element with which hydrogen makes a binary compound. And the x denotes the number of hydrogen atoms. There are three possible types of hydrides based on their structure and chemical properties. These are
These are hydrides that form when hydrogen reacts with s-Block elements. These are elements found in Group 1 and Group 2. These alkali metals and alkaline earth metals are more electropositive than hydrogen. The only two exceptions are beryllium hydride and magnesium hydride. Due to the negatively charged ion (H-) they are also known as Ionic Hydride.
Some examples are Lithium Hydride (LiH), Sodium Hydride (NaH) etc. Let us look at some of their chemical properties
- These saline hydrides are in solid form. They are actually crystals and usually have a whitish hue
- Ionic hydride has both high melting point and the boiling point as all metal compounds usually do
- They have high density since these are stable molecules.
- They conduct electricity in molten state liberating a diatomic molecule of hydrogen gas at the anode. This is also a way to confirm the presence of hydrogen in the compound. Let us take a look at the reaction of Calcium Hydride electrolysis
CaH2 (melt) → Ca2+ + 2H–
At anode: 2H- → H2 + 2e-
At cathode: Ca2+ + 2e– → Ca
- One very unique chemical property of Saline hydrides is that they react very vigorously with water and other solvents such as ethanol and ammonia. In the process, they release pure hydrogen gas. This is why they must be kept in moisture free environments. Here is a reaction of Sodium Hydride reacting with water and ammonia
NaH + H2O → NaOH + H2
NaH + NH3 → NaNH2 + H2
These are hydrides that form when hydrogen reacts with p-block elements. These are highly electronegative elements, far more than hydrogen. Hydrogen atoms form a covalent bond with these p-block atoms. The general chemical formula for Covalent Hydrides is XH(8-n), where “n” is the number of electrons the element has in its outermost shell.
Some examples of covalent or molecular hydrides are HCl, HFl, H2O, NH3 etc. Let us take a look at some chemical characteristics
- These hydrides consist of individual covalent molecules. These covalent bonds are weak and have a weak interparticle force
- Molecular hydrides due to their weak covalent bonding have very low melting and boiling points
- Like most non-metal compounds covalent hydrides are poor conductors of electricity
- They can be in a liquid state if their properties are adapted due to hydrogen bonding. Like in the case of water there is a hydrogen bond, and it slightly alters the chemical properties of these compounds.
- As we move across the periodic table the hydrogen compounds with these non-metals become more acidic
- Hydrogen forms the most number of hydrides with Carbon. These are all covalent hydrides, formed by covalent bonding between hydrogen and carbon.
These are compounds that Hydrogen forms by reacting with transition elements. These transition elements are:
- Transitional Metals in group 3, 4 and 5 (which are in the d-block)
- All the f-block elements
- One metal of group 6 – Chromium
There are three groups of the d-block Groups 7,8, and 9 that do not form compounds with hydrogen. This phenomenon is known as the Hydride Gap of the d-bock. Let us see some characteristics of metal hydrides are:
- They are found in a solid state. They have the dark hues of metals
- Again like all metal compounds, they are good conductors of electricity since they have a high thermal capacity.
- They can decompose into metal and hydrogen gas. but this reaction is easily reversible.
Solved Question for You
Q: Which of the following statements about a Saline Hydride is correct?
- They are dehydrating hydrides
- Form by transition metals
- Are very poor reducing agents
- Are stable towards water and Alcohol
Sol: The correct answer is option “A”. Saline hydrides are formed by alkali metals and alkaline earth metals (not transition metals). They are good reducing agents and are readily decomposed by water and alcohol to liberate hydrogen. They also act as powerful dehydrating agents. Therefore the first statement is the correct answer. The saline hydride is a good dehydrating hydride.