Why Is Lithium Aluminium Hydride A Better Reducing Agent Than Sodium Boro Hydride?

Both Lithium Aluminium Hydride(Li[AlH4]) and Sodium Boro Hydride(Na[BH4]) are co-ordination complexes.  They serve as reducing agents by releasing the hydride(H-) ions from their respective co-ordination spheres.

 The central atoms in both the cases, namely, Al & B, belong to Group - IIIA of the periodic table.  As we know that electro positive character increases upon going down a group in the periodic table, aluminium(Al) is more electro positive than boron(B).  Hence, by virtue of greater electro positive character, Al easily releases the H- ions that are bonded to it than B. 

Thus, Li[AlH4] serves as a better reducing agent for organic compounds than Na[BH4]

Dissolution of Gold in Aqua Regia

Date: 31.12.2007

Here I wish to explain how the so called noble metal gold dissolves in Aqua regia. Aqua regia is a mixture of 3 parts of concentrated HCl and one part of concentrated HNO3. Aqua regia is supposed to be the best and also the only solvent for gold.

The dissolution of the noble metal gold occurs by the formation of a square planar complex between gold and chloride ions. The name of the complex is Hydrogen tetrachloroaurate(III). The stoichiometry of the reaction is as under:

3AuCl3 + HNO3 +3HCl -----------> 3H[Au(Cl)4] + 2H2O + NO

Aqua regia also solublises other metals like silver, nickel, cobalt etc.

Did you know?

Date: 28.11.2007

As a student of Chemistry, I wish to share some interesting facts on the elements in the periodic table. Also, here I wish to cover only uncommon elements. By uncommon elements, I mean elements which people, who don't possess Chemistry back ground, may not be knowing.

Lithium(Li):
It is the smallest metal in the periodic table. Some of its ores are Lepidolite, Spodumene etc. Lithium Carbonate is used as an anti-tussive agent.

Germanium(Ge):
The atomic number of Germanium is 32. It is also known as Eka-Silicon. It is a metalloid and it is widely used as a semi-conductor. It was discovered by Winkler and it is named after his mother country, which is Germany.

Tungsten(W):
The atomic number of tungsten is 74. The element owes it's symbol to it's another name Wolfram, which in Swedish language means "Heavy Stone". Tungsten finds its applications in electrical filament lamps.

Chlorine(Cl):
The atomic number is 17. It is an important member of halogen family. In Greek, "Chlor" means "Green" and chlorine is a greenish-yellow gas. It is an effective disinfectant.

Polonium(Po):
The atomic number is 84. It is radioactive. It is named after the mother country of its discoverer Marie Curie. As we all know, the mother country of the first woman Nobel Laurete, Marie Curie, is Poland. Apart from Polonium, she is also the discoverer of another radioactive element Radium(Ra). The atomic number of radium is 88. Also, one of the trans-uranic elements is named after the spouse of Marie Curie, which is, Curium(Cm) of atomic number 96.

Gallium(Ga):
The atomic number is 31. Many people know that mercury is a liquid metal but mercury is not the sole liquid metal in the periodic table. Even Gallium is a liquid metal. There are totally five liquid elements in the periodic table. Of those five, Bromine(Br) is the only liquid non-metal.

Technitium(Tc):
The atomic number is 43. One thing peculiar to technitium is that it is the only radioactive element amongst d-block elements.

Tin(Sn):
The atomic number is 50. It owes its symbol to its Latin name "Stannum". There are two allotropic forms of tin, namely, white tin and grey tin. When one of the allotropic forms transforms into another, a crackling sound would be heard, which is referred to as "Tin Cry".

Cadmium(Cd):
The atomic number is 48. It is a heavy metal poison. The disease caused by cadmium poisoning is known as "Itai itai".

Indium(In):
The atomic number is 49. Like the people of Germany and Poland, we Indians cannot take pride that this element has been named after our India, since it owes its name only to the indigo coloured lines observed in its atomic spectrum.

(P. S.: Of all the alphabets of English, "J" is the only alphabet that doesn't occur in the periodic table)

How does starch act as an indictor?

Date: 31.10.2007

All Chemistry lovers would know that starch (a bio-polymer) serves as an indicator in iodometric and iodimetric titrations, which are epitomes of redox titrations.

As a student of Chemistry, I wish to explain the intricate mechanism by which the bio-polymer serves as an indicator in the aforesaid redox titrations.

The bio-polymer starch comprises of 20% water soluble amylose and 80% water insoluble amylopectin. The amylose component of starch possesses helical structure by virtue of which it serves as an indicator in iodometric and iodimetric titrations.

In both the titrations, the system will possess a mixture of molecular iodine(I2) and iodide ions(I-). Molecular iodine combines with iodide ions in a reversible manner, thereby giving rise to a complex called tri-iodide ions(I3-). The equilibrium of the reaction of formation of tri-iodide(I3-) ions is always backwards. So, I3- ions are actually considered as molecular iodine(I2) in a solution of iodide ions(I-).

It is hypothesised that when an aqueous solution of starch is added into a system comprising a mixture of I2 and I- ions, the I3- ions(that will be present all the time) and I2 will enter into the cage formed by the helical structure of starch, where they combine together to form I5- ions. The I5- ions complex with starch through weak Van-der-Waals force. This complex of I5- ions and starch is blue in colour. Thus as soon as starch is added into the system, violet colour is observed. With subsequent addition of the titrant, usually sodium thiosulphate(Na2S2O3), the violet colour gradually disappers and the end-point is complete disappearnce of the violet colour.

A necessary precaution that has to be taken while making use of starch as an indicator is that, it should be added only after the colour of the solution in the conical flask turns straw yellow in colour. If starch is added in prior, then excess of iodine would get into the helical structure of starch and get coagulated. Thus, one would end up getting non-reproducible end-points.

Why do silver articles/utensils turn black?

Date: 18.09.2007

Moist silver is highly prone to attack by hydrogen sulphide or any other potential source of sulphide(S2-). Silver reacts with sulphide forming black silver sulphide, which is an insoluble salt of silver. Consequently silver utensils/articles turn black.

Hence, it is advisable to wipe off water immediately from silver articles/utensils post wash to prolong its attack by sulphide.

Why is lithium aluminium hydride a better reducing agent than sodium borohydride?

Date: 18.09.2007

When I started learning organic chemistry in college I used to think quite often, as to what makes lithium aluminium hydride (Li[AlH4]) a better redcuing agent than sodium borohydride (Na[BH4]). As a student of Chemistry, I feel that it should be because of the following reasons:

Both lithium aluminium hydride and sodium borohydride are hydride complexes, in which, the former contains aluminium as the central metal ion and the latter contains boron as the central ion. They both serve as reducing agents by donating the hydride ions. Since the hydride ions (that are of interest to us now) are present in the coordination sphere, let us solely consider the constituents in the coordination sphere.

In sodium borohydride the 1s orbital of the hydride ions has to overlap with the 2p orbitals of boron whereas in the case of lithium aluminium hydride the 1s orbital of the hydride ions has to overlap with the 3p orbitals of aluminium. Overlap of an 1s orbital with 3p orbitals is obviously less favourable than the overlap of an 1s orbital with 2p orbitals. Consequently, the Al-H bonds are cleaved easily as compared to the B-H bonds. This easy cleavage of Al-H bonds actually facilitates the reducing power of the reagent.

One more thing is, both boron and aluminium belong to the same group in the periodic table, which is group-13A. Boron, the predecessor of aluminium in the group, is more electronegative. Thus boron, being more electronegative than aluminium, will have a tendency to retain more negative charge than does aluminium, which in this case is acquired from hydride ions. Thus, the difference in the electronegativities of the central atoms of the two complexes can also serve as one of the factors for the difference in reducing power between the two.