NEET Chemistry Notes The d- and f- Block Elements -Some Important Compounds of Transition Elements
Some Important Compounds of Transition Elements
Some Important Compounds of Transition Elements
The transition elements are those elements having a partially filled d- or f- subshell in any common oxidation state. The term transition elements most commonly refers to the d-block transition elements.
Some important compounds of transition element are as follow:
Potassium Dichromate (K2Cr2o7) Ore Ferrochrome or chromite (FeO • Cr203) or (FeCr204)
Sodium dichromate is more soluble than potassium dichromate. Chromates and dichromates are interconvertible in aqueous solution depending upon pH of the solutions.
Properties
Sodium and potassium dichromates are strong oxidising agents, thus, acidified K2Cr2o7 will oxidise iodides to iodine, sulphides to sulphur, tin (II) to tin (IV) and iron (II) salts to iron (III).
Uses
- K2Cr2o7 is used as oxidising agent in volumetric analysis.
- It is used in mordant dyes, leather industry, photography (for hardening of film).
- It is used in chromyl chloride test.
- It is used in cleaning glassware. ’
- It is used in the preparation of chrome alum K2S04 Cr2(S04)3 24H2O.
Oxidation States
Ce+4 or some elements are favoured by its noble gas configuration, but it is a strong oxidant reverting to the common +3 state. The E° value for Ce4+/Ce3+ is +1.74 V, the reaction rate is very slow and hence, Ce(IV) is a good analytical reagent. Pr, Nd, Tb and Dy also exhibit +4 state but only in oxides. Eu2+ is formed by losing the two s-electrons and its f7 configuration accounts for the formation of this ion. However, Eu2+ is a strong reducing agent changing to the common +3 state.
Similarly, Yb2+ which has f14
configuration is a reductant, Tb+4 has half-filled f-orbitals and is an oxidant.
Reactivity
Lanthanoid are very reactive metals like alkaline earth metals.
The hydroxides are ionic and basic. The base strength decreases from Ce(OH)3 to Lu(OH)3.
Alloy Misch metals, contain lanthanoids about 90-95% (Ce 40.5%. Lanthanum and neodymium 44%) iron 4.5%, calcium, carbon and silicon about 10.5%, are used in cigarette and gas lighters, toys, tank and tracer bullets. It is a pyrophoric alloy, i.e. takes up fire easily.
Actinoids
Actinoids exhibit a range of oxidation states due to comparable energies of 5f,6d and 7s orbitals. The general oxidation state of actinoids is +3.
All the actinoids are strong reducing agents and very reactive. Actinoids also react with oxygen, halogen, hydrogen and sulphur, etc., like lanthanoids. Actinoids are radioactive in nature and therefore, it is difficult to study their chemical nature.
The atomic and ionic radii of actinoids decreases regularly across the series due to poor shielding effect of 5f-electrons. This is known as actinoid contraction.
The allocation of electrons to the 5f- or 6d-orbitals in the actinoid atoms is more uncertain than the allocation to the 4f- or 5d-orbitals in the lanthanoid atoms, because the external shielding of the 5/ electrons is less than that of the 4f-electrons.
The magnetic properties of f-block elements are due to both the orbital motion as well as spin magnetic moment for d-block elements.
Magnetic moment for f-block elements,
where, S is the sum of spin‘quantum numbers and L is the angular momentum Quantum numbers.