The Inner Transition f-Block Elements

 The Inner Transition f-Block Elements

   ·      28 elements from atomic number 58(Ce) to 71(Lu) —14 elements and from atomic number   90(Th)  to 103(Lr)  —14 elements . these have been arranged in two horizontal rows below the periodic table. These elements are collectively called f- block elements

     Since the last electron enters one of the f-orbitals

    ·      These are also known as inner transition series

    ·      f-block consists of two series of elements known as Lanthanides or Lanthanons and Actinides or Actinons.

     ·      The lanthanide series follows lanthanum (At. No. 57), because lanthanum closely resembles the lanthanoids

·            Position in periodic table -the 14 members of lanthanide series have been placed along with lanthanum in the third group and sixth period and similarly 14 members of the actinide series have been placed with actinium in the third group and seventh period

    ·        The justification for assigning one place to these elements has been given on the basis of their similar properties. The properties are so similar that the fifteen elements from La to Lu can be considered as equivalent to one element. The same explanation can be given in the case of actinides.

    ·      In case, these elements are assigned different positions in order of their increasing atomic numbers, the symmetry of the whole arrangement would be disrupted. Due to this reason, the two series of elements, i.e., lanthanides and actinides are placed at the bottom of the periodic table

    ·      4f- series( lanthanide series) - There are fourteen elements from cerium (At. No. 58) to lutetium (At. No. 71) in this series. 4f-orbitals are gradually filled up. In past, elements were called rare earths.

    ·      5f-series (Actinides series): There are fourteen elements from thorium (At. No. 90) to lawrencium (At. No. 103) in this series. 5f-orbitals are gradually filled up.

   The elements from atomic number of 93 onwards are called transuranic or trans-uranium elements

 

GENERAL CHARACTERISTICS OF LANTHANIDES

 Electronic configuration –

     ·      General Electronic configuration -       (n-2)f^{1 -14}(n-1)d^0-1ns²     or   4f^1-14 5d^0-1 6s² 

     ·      The energies of 5d- and 4f-orbitals are nearly similar and thus their fillings show certain irregularities.

    

·        All tri-positive ions (the most stable oxidation state of all lanthanoids) have 4f^1-14 electronic configuration  

Atomic size and ionic size

     ·      There is decrease in atomic and ionic radii from lanthanum to lutetium due to lanthanoid contraction. However, the shielding of one 4f electron by another is less than a d-electron by another with the increase in nuclear charge along the series.

     ·      The decrease in atomic radii is quite not regular as it is regular in M³⁺

    Lanthanoid contraction –

     is a regular decrease in atomic radii by filling of 4f before 5d orbital. Since 4f orbital causes poor shielding effect(screening) than 5d.

    Consequences of lanthanoid contraction –

    ·      Similarity in atomic size of 2^nd and 3^rd  transition series.

    ·      Variation in basic strength of hydroxide

       Basic strength decreases from La(OH)₃ to Lu(OH)₃ due to lanthanoid contraction ,size of M³⁺ decreases its result is the increase in covalent nature

    Oxidation states

    ·      common oxidation à +3

    ·      In   La, Gd, Lu are especially stable because M³⁺, ions have stable electronic configuration as 4f⁰, 4f⁷ ,4f¹⁴ respectively

    ·      Ce(4f⁰ )and Tb(4f⁷ )exhibit à +4  , also show+3

    ·      Lanthanoids have a tendency to attain the oxidation state +3 due to most stable.

    ·      Ce⁴⁺ is good oxidising agent  while Sm⁺² is good reducing agent

    Ce⁴⁺  +  Fe²⁺------------> Ce³⁺  +  Fe³⁺

    2 Sm²⁺  +  H₂O -------> 2Sm³⁺  +  2OH^-  + H₂

    ^·              Eu²⁺ is strong reducing reagent while Tb⁺⁴ is an oxidant

    ·      The E° Value for Ce4+/Ce3+ is +1.74 V which suggests that it can oxidise water. However, the reaction rate is very slow and hence Ce(IV) is a good analytical reagent.       

    Colour

     The colour is due to f-f transitions since they have partly filled f-orbitals

        Mischmetal consists of a lanthanoid metal (nearly 95%) and iron (nearly 5%) and traces of S,C, Ca, and Al . these are used in Mg-based alloy to produce bullets, shell and light flint.

Gas laws - Boyle's law

 

The gaseous state

·      General characteristics are

a.     Shape and volume- gases neither has definite shape nor definite volume

b.     Highly compressible

c.      Gases exert pressure

d.     Have much lower density than the solid and liquids

e.     Diffusion- gases intermix completely in all proportions without any mechanical aid

f.       Liquefaction – can be liquified by cooling and by applying pressure.

This simplicity of gases is due to weak or  negligible force of interaction between gaseous particles

The gas laws

·      From the study of the behaviour of gases, certain generalisation were made. i.e the behaviour of gases are governed by some general laws, these generalisation are called gas laws.

·      These laws are related to measurable properties(pressure, volume, temperature and mass) of gases

These laws are as follows

a.     Boyle’s law(pressure-volume relationship)

b.     Charles’law (Temperature-volume relationship)

c.      Gay lussac’s law(pressure-Temperature relationship)

d.     Avogadro law(volume-Amount relationship)

a.     Boyle’s law (pressure-volume relationship)

·      In 1662 Robert Boyle proposed a relationship between Pressure and temperature. It is called Boyle’s law

·      The Pressure of a fixed amount of a gas are inversely related to its volume at constant temperature and.

                          P α 1/V   ---------(1)

           PV = K₁    ----------(2)      

     

 Let us consider  initial pressure and volume as P_1, V₁    After applying external pressure on gas, the final pressure and volume are as      P_2, V₂

   At constant amount of substance and Temperature

Then, according to Boyle’s law-

 P₁V₁ =K₁     ---------(1)

 P₂V₂ = K₁ ---------(2)

i.e.

    P₁V₁ = P₂V₂ =K₁     --------(3)

Graphical representation of Boyle’s law-

    ·      If graph is plotted between P and V we get curve called rectangular hyperbola)

   The curve clearly shows that when volume is increased, pressure decreases and vice versa.

   Similar curves are obtained at other temperature. Higher curve corresponds to higher temperature.

   Each curve corresponds to a different constant temperature, is known as isotherm (constant temperature curve) 

    ·      If the graph is plotted between P and 1/V, a straight-line graph is obtained passing through origin.

    ·      At high pressure, gasses deviate from Boyle’s law and so at high pressure, a straight-line graph cannot be obtained.

`            relationship between pressure and density

·      If density(d) = m/V    or V = m/d     Here m=mass of gas and V=volume of gas

Then we can give relationship between pressure and density by putting the value of volume (V) in Boyle’s equation as-

                              P m/d = K₁                                     (Boyle’s law  PV= K₁ )

                              So, P α d

Significance of Boyle’s law-

     ·      Gases are compressible      

     ·      Low pressure at high altitude, cause altitude sickness(anoxia) symptoms-uneasiness, sluggish feelings, headache

       Jet and aeroplanes fly at very high altitude with emergency oxygen supply in case of pressure falls   

 

Intermolecular forces vs Thermal Energy

 

Thermal energy

·      It is the energy of a body arising by motion of its particles/atoms/molecules

·       Is proportional to the temperature of substance.

·      It is the measure of average kinetic energy of molecules of matter

·      Is responsible for movement of particles

·      This movement of particle is called thermal motion

Intermolecular forces vs Thermal Energy

    ·      Intermolecular forces tend to keep the molecules together but thermal energy of molecules tends to keep them apart.

     ·      Three states of matter are the result of the balance between intermolecular forces and the thermal energy of the molecules.

 

·      Gas ----------> Liquid ----------->Solid

Predominance of intermolecular interactions

·      Gas <--------- Liquid <----------Solid

Predominance of thermal energy

p-block d-and f-block elements Questions

 

Catalytic properties of transition metals

 

Catalytic properties of transition metals

    ·      Good catalysts due to

   a.  the presence of free valencies and  variable oxidation states

   b.  these provide surface for reaction 

  

some Examples are as

    ·      Iron (III) catalyses the reaction between iodide and persuphate ions-

2I^- +   S₂O₈^2- -------> I₂ + 2SO₄^2-

Explanation of catalytic action of Fe³⁺ in above reaction

Step-1 

2Fe³⁺ + 2I^-    -------> 2Fe²⁺ + 2I₂

Step-2

 2 Fe²⁺ +   S₂O₈^2- -------> 2Fe³⁺   + 2SO₄^2-

in this catalytic action of Fe, Fe shows variable oxidation state i.e. Fe³⁺ changes into Fe²⁺ Again into Fe³⁺

    ·      Pt-used as a catalyst in the manufacture of H₂S0₄.       

    ·      Fe-used as a catalyst in the manufacture of NH₃ by Haber process. A small amount of molybdenum is added as a promoter.

    ·      Ni.-used as a catalyst in the hydrogenation of oils.

    ·      V₂0₅-used as a catalyst for the oxidation of S0₂ into S0₃ for the manufacture of H₂S0₄ in the contact process.

    ·       Mn02-used as a catalyst in the decomposition of KCI0₃ for preparation of oxygen.

Formation of Interstitial Compounds and Alloy

 

·      

 Formation of Interstitial Compounds-

     ·      interstitial compounds are those which are formed when small atoms like H, C, N, B etc are trapped inside the crystal lattice of metals

      ·       The general characteristic physical and chemical properties of these compounds are:

        a).  High melting points which are higher than those of pure metals.

        b) Retain metallic conductivity i.e. of pure metals.

        c). Very hard and some borides have hardness as that of diamond.

        d). Chemically inert.

Alloy Formation –

     ·     transition elements form alloy because of similar in atomic size and other characteristics of transition metal

     ·      Alloys are hard and having high melting point.

             e.g., Brass- (Cu + Zn) 

                     Bronze -(Cu + Sn) etc.

       Hg when mix with other metals form semisolid amalgam except Fe, Co, Ni, Li.