Nature of Hydrides - Hydrides of s-Block Elements, Types, Uses, Practice Problems and FAQ

You must have wondered about the glowing stars at some point in your life when looking up at the sky, especially when the blazing sun has hidden its face behind the night's veil! The stars then flash their 'little light' and sing 'Twinkle twinkle, all night!' But did you know that everything we study in our books is as fascinating as the stars themselves?

Because when we study and comprehend hydrides, we are simply attempting to learn a little more about one of the major components present in the atmospheres of stars, including the Sun– 'Hydrides'!

Indeed, 'hydride' is one of the universe's oldest formed species. The helium hydride ion (HeH+) was the first molecule to form when the young universe's lower temperatures allowed the light elements created in the Big Bang to recombine 14 billion years ago.

Let’s understand ‘Hydrides’ in detail here.

TABLE OF CONTENTS

• What are Hydrides?
• Types of Hydrides
• Covalent Hydrides (or) Molecular Hydrides
• Metallic Hydrides (or) Non-Stoichiometric Hydrides (or) Interstitial Hydrides
• Hydrides of s-Block Elements - Ionic Hydrides
• Nature of Hydrides of s-block Elements
• Uses of Hydrides
• Practice Problems
• Frequently Asked Questions - FAQ

What are Hydrides?

The hydrogen anion H^- is a negative hydrogen ion or a hydrogen atom with an additional electron captured. The hydrogen anion is a key component of stars' atmospheres, such as that of the Sun. This ion is known as a ‘hydride’ in chemistry. The electromagnetic force binds two electrons to a nucleus containing one proton in the ion.

These ions are very reactive and readily form hydride molecules with positively charged ions. 

Under certain reaction conditions, hydrogen combines with almost all elements, except noble gases, to form binary compounds known as hydrides. If ‘M’ is the symbol of an element, then its hydride can be expressed as MH_x.

Examples: MgH₂, B₂H₆, CaH₂, etc.

Types of Hydrides

There are specifically four types of hydrides:

1. Ionic Hydrides (or) Saline Hydrides (or) Salt-like Hydrides (Formed by s-block elements)
2. Covalent Hydrides (or) Molecular Hydrides
3. Metallic Hydrides (or) Non-Stoichiometric Hydrides (or) Interstitial Hydrides
4. Polymeric Hydrides (BeH₂ and MgH₂ are polymeric in nature)

Covalent Hydrides (or) Molecular Hydrides

• Covalent hydrides involve the formation of covalent bonds between the hydrogen atoms and other atoms by sharing electrons.
• Dihydrogen forms compounds with most of the p-block elements to produce covalent or molecular hydrides. The most familiar examples are CH4, NH3, H₂O, and HF. Few more examples include B₂H₆,SiH4, PH3, H₂S, HCl, GeH4, AsH3, H₂Se, HBr.
• The stability of these hydrides decreases down the group because of the increase in the bond length which eventually leads to the decrease in bond strength.

NH3 > PH3 > AsH3> SbH3 > BiH3

• In a period, the stability increases with increasing electronegativity because with an increase in the electronegativity difference, the ionic character increases.

Example: CH4< NH3 < H₂O < HF

• According to the relative numbers of electrons and bonds in their Lewis structure, the molecular hydrides are further classified into the following:

1. Electron-rich hydrides
2. Electron-precise hydrides
3. Electron-deficient hydrides

Types of Molecular Hydrides

Electron-deficient hydrides	Electron-precise hydrides	Electron-rich hydrides
Have a lesser number of electrons than that required for writing the conventional Lewis structure. They act as Lewis acids.	Have the required number of electrons to write their conventional Lewis structure.	Have an excess of electrons which are present as lone pairs around the central highly electronegative atom. They act as Lewis bases.
B2H6 and hydrides of group 13 elements. E.g. BH3	Elements of group 14 form such hydrides. E.g. CH4	Elements of groups 15-17 form such compounds. E.g. NH3 (one lone pair), H2O(two lone pairs), and HF (three lone pairs)

Metallic Hydrides (or) Non-Stoichiometric Hydrides (or) Interstitial Hydrides

• Interstitial or non-stoichiometric hydrides are formed by the elements of d and f-block except the metals of groups 7, 8, and 9.

• Since they are deficient in hydrogen, they are almost non-stoichiometric.

• Examples: LaH_2.87, PdH_0.6-0.8, ZrH_1.3-1.75, VH_0.56,YbH_2.55, TiH_1.5-1.8, NiH_0.6-0.7, etc.

• Group 7, 8, and 9 metals do not form hydrides. This region of the periodic table is known as the hydride gap.

• Recent studies have shown that except for the hydrides of Ni, Pd, Ce, and Ac, others have lattices different from that of the parent metals.

• A position between the regular positions in an array of atoms or ions that can be occupied by other atoms or ions is known as an interstitial site.

Applications of Metallic Hydrides

• Due to the property of absorption of hydrogen on transition metals, it is widely used in catalytic reduction/hydrogenation reactions.
• Metallic hydrides are used to provide hydrogen in fuel cells to produce electricity.
• Metallic hydrides are used to provide hydrogen in the formation of many compounds on an industrial level.
• Some metals (For example, Pd, Pt) can accommodate a very large volume of hydrogen. Therefore, they can be used as their storage media. This property has a high potential for hydrogen storage and is a source of energy

Hydrides of s-Block Elements - Ionic Hydrides

• They are binary, stoichiometric compounds of hydrogen with most of the s-block elements that are highly electropositive in nature.
• All the s-block elements except Be and Mg (due to high ionisation enthalpy) form ionic hydrides.
• In solid state, ionic hydrides are crystalline, non-volatile and non-conducting.
• On electrolysis, the molten saline hydrides liberate H2 gas at the anode due to the oxidation of the hydride ion. 

2H^- (molten) --> H₂(g) + 2e^- [At Anode]

• The liberation of H₂ gas at the anode during electrolysis confirms the existence of H^-.
• Saline hydrides react violently with water and liberate H₂.

NaH (s) +H₂O (l) --> NaOH (aq)+ H₂(g) |

• Significant covalent character is found in lighter s-block metal hydrides.

Examples: LiH, BeH₂ and MgH₂

• The order of thermal stabilities of the first and second group hydrides is

LiH > NaH > KH > RbH > CsH and CaH₂ > SrH₂ >BaH₂

• The ionisation of metals with low first ionisation potentials, such as alkali metals and alkaline earth metals, produces the majority of the electrons that hydrogen can accept and form negative ions (H^-).
• Labile hydride anions are commonly associated with alkali metal hydrides (E.g. NaH). The degree of reactivity is related to the metal's electronegativity.
• Ionic hydrides of s-block elements are used for making important industrial reagents like LiAlH4 and NaBH4.
• They are also used for the removal of the last traces of water from organic compounds.

Nature of Hydrides of s-Block Elements

Hydrides of Alkali Metals

Alkali metals react with H₂ to form ionic hydrides, MH (M = An alkali metal).The reaction of alkali metals with H₂ to form hydrides is given as follows:

2M + H₂⟶ 2M⁺H^- ; M = an alkali metal

The thermal stability of these hydrides decreases down the group from Li to Cs. As the size of the alkali metal cation increases down the group, the MH bond becomes weak. Hence, the thermal stability of alkali metal hydrides decreases down the group.

• The value of electron affinity of hydrogen is −74.5 kJ mol^-1.
• All the alkali metal hydrides have the sodium chloride crystal structure, and the ionic hydrides are white solids with high melting temperatures.
• The ionic hydrides are sometimes referred to as saline or salt like hydrides because they resemble the salts of alkali and alkaline earth metals.
• The Li–H bond is thought to have a high degree of covalent nature. The higher covalency of this compound compared to other alkali metal hydrides is most likely due to Li having higher ionisation potential and the fact that the 1s of hydrogen and and 2s orbitals of lithium are of equal size.
• Due to the huge difference between the size of 1s orbital of H and the 3s or 4s orbital of Na or K, the effectiveness of overlap is reduced. Also, due to lower ionisation potentials of sodium and potassium than lithium, it tends to make NaH and KH more ionic than LiH.

Hydrides of Alkaline Earth Metals 

Except Be and Mg (due to high electronegativity and ionisation energy), all the alkaline earth metals form ionic hydrides (MH₂) on heating directly with H₂. BeH₂ can be prepared by the action of LiAlH4 on BeCl₂.

2BeCl₂ +LiAlH4 → 2BeH₂ + LiCl + AlCl3

Where (M = Ca, Sr , Ba, and Ra), CaH₂, SrH₂, and BaH₂ are ionic, and contain the hydride ion H^-. CaH₂ is known as hydrolith. Hydrolith means a stone removing hydrogen. Both BeH₂ ,and MgH₂ are covalent and polymeric. 

Beryllium and magnesium form covalent hydrides where each hydrogen is connected to two metal atoms. This is an example of molecules with three centers sharing only two electrons called “banana Bond”.

Calcium, strontium and barium react with hydrogen to form metallic hydrides. Metallic hydrides give hydrides ions.

M + H₂ → 2MH₂ → M²⁺+2 H^-

• Hydrides react violently with water to release hydrogen. Calcium hydride also known as “Hydrolith” , is used to produce hydrogen.

CaH₂ + 2H₂O → Ca(OH)₂ + H₂

• The very basic nature of the H^- ion, is the most prominent property of the chemistry of ionic hydrides. All ionic hydrides quickly react with protonic solvents to form hydrogen gas and a weaker base than H. Hence, s-block hydrides are also good reducing agents (Alkali metals being better).

H^- + H₂O → OH^- + H₂
H^- + ROH → RO^- + H₂

• The ionisation enthalpy of alkali metals decreases as one moves down the group, making electrons more readily available to form H^- ions. As a result, the hydrides act as a reducing agent. Their reducing nature increases down the group for both Group I and Group II due to increase in ionic radii of metal ions, which reduces the metal-hydrogen bond strength and hydride ion is readily available for reducing other species.

• Since the hydride ion has two electrons, it can also be used as a Lewis base. Many coordination compounds with the hydride ion as a ligand are known. 

Example: LiAlH4 and NaBH4 are used as reducing or hydrogenating agents in several organic reactions. 

• The electron cloud of H^- is easily polarisable due to its enormous size, and as a result, H^- is a soft base. As a result, complexes containing transition metals are frequently generated with mild Lewis acids as the metals. Because the metals are usually in low oxidation states, they are milder acids than metals with higher oxidation states.

Example: FeCO₄H₂, MnCO₅H

Uses of Hydrides

• To remove trace water from organic solvents, desiccants, or drying agents, such as calcium hydride, are utilised. When hydride reacts with the dissolved water, corresponding metal hydroxide is formed and hydrogen is liberated. The dry solvent can then be distilled or vacuum transferred from the solvent pot.
• In storage battery systems such as the nickel-metal hydride battery, hydrides are crucial. The use of various metal hydrides as hydrogen storage for fuel cell-powered electric vehicles and other components of a hydrogen economy has been researched.
• Strong bases, such as sodium hydride (NaH) and potassium hydride (KH), are used in organic synthesis. The hydride produces H2 when it reacts with a mild Bronsted acid.
• Some of the metallic hydrides of metals (E.g. Pd, Pt) can accommodate a very large volume of hydrogen. Therefore, they can be used as storage media. This property is used for the transportation of hydrogen.
• Hydrides like sodium borohydride, DIBAL, and super hydride are frequently used as reducing agents in chemical synthesis. An electrophilic core, usually unsaturated carbon, interacts with the hydride.
• In a variety of homogeneous and heterogeneous catalytic cycles, hydride complexes operate as catalysts and intermediates. Hydrogenation catalysts, hydroformylation catalysts, hydrosilylation catalysts, and hydrodesulphurisation catalysts are all examples where hydrides are used.

Practice Problems

Q1. What is observed when an electric current passes through an ionic hydride in its molten state?

1. No reaction 
2. H^- ions migrate towards the cathode
3. H₂ gas is obtained at cathode
4. H₂ gas is obtained at anode

Answer: When an electric current is made to pass through an ionic hydride, i.e., on electrolysis, the molten saline hydride liberates H₂ gas at anode due to the oxidation of hydride ions. 

2H^- (molten) --> H₂(g) + 2e^- [At Anode]

So, option D) is the correct answer.

Q2. How does hydrogen combine with other elements?

1. By loss of electron
2. By gain of electron
3. Sharing of Electron
4. Losing/Gaining/Sharing Electron

Answer: 

Ionic bonds are formed by gaining or losing electrons. Hydrogen can gain an electron like in the formation of calcium hydride. Hydrogen can lose an electron like in the formation of hydrochloric acid. Covalent bonds are formed by the sharing of electrons like in methane. In methane, hydrogen and carbon share electrons to complete the octet of carbon and the duplet of hydrogen.

So, option D) is the correct answer.

Q3. Ionic Hydrides are formed by:

1. s-Block Elements
2. p-Block Elements
3. d-block Elements
4. Lanthanides 

Answer: The value of electron affinity of hydrogen is −74.5 kJ mol^-1. Hence, H^- forms an ionic bond readily with unipositive or dispositive ions that has low ionisation enthalpies. The ionisation of metals with low first ionisation potentials, such as alkali metals and alkali earths, produces the majority of the electrons that hydrogen can accept and form negative ions (H^-). 

So,option A) is the correct answer.

Q4. The reducing nature of hydrides of alkali metals:

1. Decreases down the group
2. Increases down the group
3. Remains constant
4. Unpredictable

Answer: Alkali metal hydrides are commonly thought of as labile/unstable hydride anions. The degree of liability is related to the metal's electronegativity. As a result, alkali metal hydrides with less electronegative metals (going down the group) have more labile hydride anions and a more "reducing" character.

So down the group, reducing nature of hydrides of alkali metals will increase. For both Group I and Group II, due to the increase in ionic radii of metal ions, the metal-hydrogen bond strength decreases and the hydride ion becomes readily available for reducing other species.

So, option B) is the correct answer.

Frequently Asked Questions - FAQ

Q1. Why LiH has considerable covalent character?
Answer: 

Q2. What is the nature of non-metallic hydrides?
Answer: Most of the non-metallic hydrides are volatile in nature as they are held together in condensed form by weak van der Waals intermolecular forces. Covalent hydrides are liquids or gases with low melting and boiling points unless hydrogen bonding changes their properties.

Q3. Can alkali metals form non-stoichiometric hydrides?
Answer: Stoichiometric hydrides are formed by alkali metals. The hydrides in question are ionic in nature. Alkali metal ions and H^- ions have equivalent diameters (208 pm). As a result, the forming metal and the hydride ion have high binding forces. Stoichiometric hydrides are generated as a result. Non-stoichiometric hydrides are unable to be produced by alkali metals.

Q4. Which alkaline earth metals do not combine directly with hydrogen?
Answer: Except Be and Mg (due to high electronegativity and ionisation energy), all the alkaline earth metals form ionic hydrides (MH₂) on heating directly with H₂. BeH₂ can be prepared by the action of LiAlH4 on BeCl₂.

2BeCl₂ +LiAlH4 → 2BeH₂ + LiCl + AlCl3

Related Topics