Hydrides

HYDRIDES

Dihydrogen combines with a number of elements to form binary compounds called hydrides. Their general formula being $M{H}_x$where M represents the element and x the number of hydrogen atoms. Depending upon the physical and chemical properties, the hydrides have been divided into the following three broad categories:

1. Ionic or salt-like or saline hydrides

2. Metallic or Interstitial hydrides

3. Molecules or Covalent hydrides

Saline Hydrides or Ionic hydrides

These are binary compounds of hydrogen and elements which are more electropositive than hydrogen such as alkali metals, alkaline earth metals (except Be), etc. Saline hydrides are formed by the transference of electron from metal to hydrogen. Some common examples of this category are:$LiH,NaH,Ca{H}_2,Cr{H}_2,$ etc. The general characteristic of these hydrides are as follows:

(i) They are crystalline solids having white or greyish colour.

(ii) They have high melting and boiling points.

(iii) They have high density and high heat of formation.

(iv) They conduct electricity in molten state liberating dihydrogen gas at anode which confirm the presence of hydride $\left(H^{-}\right)$ in then.

$Ca{H}_2\left(melt\right)\to C{a}^{2+}+2H^{-}$

At anode: $2H^{-}\to H_2+2e^{-};AtanodeC{a}^{2+}+2e^{-}\to Ca$

(v) They react vigorously with water and other protonic solvents such as ethanol and ammonia to liberate dihydrogen gas. Thus they act as strong bases.

$NaH+H_{2}O\to NaOH+H_2$

$LiH+C{H}_{3}OH\to \underset{Lithiummethoxide}{LiOC{H}_3}+H_2$

$NaH+N{H}_3\to \underset{Sodamide}{NaN{H}_2}+H_2$

$Ca{H}_2\left(s\right)+2H_{2}O\left(l\right)\to Ca{\left(OH\right)}_2+2H_2\left(g\right)$

Illustration 1. Ionic hydrides are frequently used to remove traces of water from organic compounds. What is the underlying basis of this process?

Solution: $H^{-}$ is a strong Bronsted base and thus it reacts with water easily.

Covalent Hydrides or Molecular Hydrides

These are binary compounds of hydrogen and elements of comparatively high electronegativity such as p- block elements. In these hydrides, H atoms are bonded to the other atoms by covalent bonds. Some examples of covalent hydrides are, HCl, $H_{2}O$, $P{H}_3$, $N{H}_3$etc. The general formula of covalent hydrides can be written as $X{H}_{\left(8-n\right)}$ where n is the number of outershell electrons of X atom. However, elements of group 13 are exception to this formula. The elements of group

13 such as B, Ga form polynuclear hydrides which are electron deficient compounds. B₂H₆, Ga₃H₂, etc., are some examples. Some of the general characteristic of covalent hydrides are as follows:

(i) These hydrides consist of individual covalent molecules with relatively weak interparticle forces (Vander waal's force of attraction). Hence they generally soft, with low melting and boiling points.

(ii) They are poor conductors of electricity.

(iii) Being covalent in nature, they are more soluble in organic solvents.

(iv) They undergo thermal decomposition into their respective elements.

$\underset{\left(Silane\right)}{Si{H}_4}\stackrel{Above575K}{\to }Si+H_2$

(v) They are covalent in nature, & are more soluble in organic solvents.

(vi) Some of them react with water to liberate$H_2$.

$B_2{H}_6+6H_{2}O\to \underset{Boricacid}{2B{\left(OH\right)}_3}+6H_2$

(vii) Along any given row of periodic table, the covalent hydrides become increasing acidic in moving from left to right.

\begin{array}{*{20}{c}}{N{H_3}}{{H_2}O}{HF}\\{Basic}{Amphoteric}{Acidic}\end{array}

Interstitial Hydride or Metallic Hydrides

These are binary compounds of hydrogen and transition elements.

These hydrides are generally formed by the

(a) transition metals of group 3, 4, 5 of d- block;

(b) Cr metal of group 6 and

(c) f – block elements.

It may be noted that elements of group 7, 8, 9 of d – block do not form hydrides at all. This inability of metal, of group 7, 8, 9 of periodic table to form hydrides is referred to as hydride gap of d – block.

In these compounds H atoms are supposed to occupy interstitial position in the metal lattices. Some scientists consider these compounds as simply solid solutions of hydrogen. The composition of these hydrides may not correspond to simple whole number ratio and therefore, they are also called non-stoichimotric hydrides. Their composition is also found to vary with the conditions of temperature and pressure. Some examples of interestial hydrides of elements of group 3 to 5 are$Sc{H}_2$, $Y{H}_2$, $Y{H}_3$, $Si{H}_4$, CrH, $Ti{H}_2$, $Zr{H}_2$, $Hf{H}_2$, VH, NbH, $Nb{H}_2$, TaH etc.

Some examples of non-stoichimetric hydrides are $Pd{H}_{0.6},T{i}_{0.6},$$Vb{H}_{0.05},V{H}_{0.05},$$Zr{H}_x(X=1.30-1.75),Ti{H}_x\left(X=1.8-2\right)$, etc.

Some general characteristics are as follows:

(i) They are generally powders or brittle solids having dark or metallic appearances.

(ii) They are good conductors of electricity. The conductivity, however, decreases with increase in temperature.

(iii) They have high thermal conductivity.

(iv) Most of these hydrides are harder than parent metals.

(v) They generally undergo reversible decomposition into H₂ gas and metal.

Besides three main categories of hydrides some other types of hydrides are also known. Two of these are described as follows:

(a) Polymeric Hydrides

They are formed by the elements having electronegativity range between 1.4 and 2.0. They consist of molecules held together in two or three dimensions by hydrogen bridges. Some common examples are

${\left(Be{H}_2\right)}_n,{\left(Al{H}_3\right)}_n,{\left(Si{H}_4\right)}_4,etc$

They are amorphous solids and stable up to 525 K. Above this temperature they begin to evolve hydrogen gas.

(b) Complex Hydrides

These are the compounds which contain hydride ions $\left(\stackrel{\_\_}{H}\right)$ co-ordinated to metal atom ions. Some common examples are $LiAl{H}_4$(lithium aliminium hydride), $NaB{H}_4$(sodium borohydride) etc. They are generally very good reducing agents.

(iv) An automobile engine burning hydrogen is about 25 to 50% more efficient than an automobile engine burning gasoline.

(v) Heat of combustion per gram of hydrogen is more than twice that of jet fuel.

(vi) Hydrogen – oxygen fuel cells provide other possibilities of powering motor vehicles.

(vii) Hydrogen is excellent reducing agent and can replace coal in many industrial processes involving reduction because it produces less atmospheric pollution than carbon.

The changes in out way of life by adopting widespread uses of hydrogen listed above refer to hydrogen economy.

Obstacles of Hydrogen Economy

Although hydrogen looks as very good future fuel but some of the tough problems must be solved before we adopt hydrogen economy. The problems are as described below:

(i) Availability.

Hydrogen is not available as much. It does not occur in free state is nature. Therefore, cheap production of Hydrogen is basic requirement of hydrogen economy. The most likely future source of hydrogen is water. Hydrogen might be generated at an appropriate site by using solar energy and then transporting it as fuel.

(ii) Storage and Transportation.

Hydrogen gas has explosive flammability which causes problem to its storage and transportation. Hydrogen can be stored in vacuum insulated cryogenic tanks (already in use for space programmes in USA) Liquid hydrogen can be transported by road and rail tankers. It can also be stored in underground tanks and transported pipelines. Another promising solution to this problem is the use of Fe-Ti alloy which act like a sponge to absorb hydrogen and results in the formation of the silvery power. Heating the power safely releases hydrogen gas. The other small storage units are alloys like$NaN{i}_5$, $Mg-Mg{H}_2$, $Ti-Ti{H}_2$ etc. Such storage systems are safer than storage of hydrogen as gas or liquid.

(iii) Platinum Scarcity

In oxygen-hydrogen fuel cells, a lot of platinum is required as catalyst. In each succeeding year the demand of platinum exceeds the supply. This will cause problems for fuel cells which are highly energy source for automobiles.

Uses of Liquid Hydrogen As Fuel

Liquid hydrogen has already been used as rocket fuel. The chemical reaction involved is:

$H_{2}O\left(g\right)+\frac{1}{2}{O}_2\left(g\right)\to H_{2}O\left(\ell \right)+286kJ$

Both reactions H₂ and O₂ are stored as liquid in separated tanks. The tank hold $1.5\times {10}^{6}L$ of liquid hydrogen. The oxygen tank carries $5.4\times {10}^{5}L$ of liquid oxygen. During the "lift off" operations, these properties power shuttle's main engine for about 8.5 min. Here, liquid hydrogen is consumed at the rate of nearly 3000L/sec.

Illustration 2. Activated hydrogen is obtained by

(A) electrolysis of heavy water

(B) reaction of water with heavy metals

(C) thermal decomposition of water

(D) passing silent electric discharge through hydrogen at low pressure

Solution: $H_2\underset{Lowpressure}{\overset{electricdisch\arg e}{\to }}\underset{Activatedhydrogen}{2H}$

Hence (D) is correct answer.