Explain Chemical properties of f-block element, Chemistry

Q. Chemical Properties of f-block element?

The lanthanides are silvery-white, highly electropositive and reactive metals. They all react slowly with cold water and quickly on heating to liberate hydrogen:

2M + 6H20 ---------------------> 2M (OH)3 + 3H2

The hydroxides are ionic and basic. They are less basic than Ca (OH)2 but more basic than amphoteric Al(OH)3. The base strength decreases from Ce(OH)3 to Lu(OH)3 as the ionic  radius decreases from ce3+ to LU3+.

The lanthanide metals dissolve in dilute acids, even in the cold, to liberate hydrogen gas:

2Ln + 6HC1 ------------------> 2LnCL3 + 3H2

The metals tarnish readily in air forming an oxide coating. On heating in oxygen, they bum simply to give M203, except for cerium which forms Ce02. The oxides are ionic and basic, the base strength decreases as the ionic radius decreases.

4Ln + 302 ----------------->2Ln203

When heated in halogens, the lanthanides bum producing LnX3, which will also be made by heating the oxides with the appropriate ammonium halide:

2Ln + 3 X2------------> 2 LnX3

Ln2O3 + 6 NH4X-----------> 2LnX3 + 6NH3+ 3H2O

Cerium with fluorine forms CeF4:

Ce + 2F2-------------> CeF4

The metals react exothermically with hydrogen though heating to 600- 700 K is often needed to initiate the reactions. Their hydrides are non stoichiometric compounds having ideal formula MH2 and MH3. The hydrides are remarkably   stable to heat up to 1200 K.  The hydrides react with water liberating hydrogen gas.

MH3 + 3H2O -------------------> M (OH) 3+ 3H2

On heating, the lanthanides react with boron giving borides of the type MB6 and MB4, with carbon giving carbides M2C3 and MC2 and with nitrogen giving nitrides MN. A wide variety of their oxosalts, like carbonates, nitrates, sulphates, phosphates, oxalate, etc., are known.

All the actinides are unstable with respect to radioactive disintegration, though the half-lives of the most abundant isotopes of uranium and thorium are so long that for many purposes their radioactivity can be neglected. Like lanthanides, actinides are also electropositive and reactive metals. They react with water, hydrogen, oxygen Halogens and acids. Their hydrides are non-stoichiometric having ideal formulae MH2 and MH3. The metals also react with most non-metals especially if heated.

Posted Date: 7/12/2013 6:37:34 AM | Location : United States







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