Chemqueries

Tuesday, 2 July 2019

Types of Adsorption

Different types of Adsorption

Adsorption may be  classified into flowing types i.e.
(a) On the basis of concentration
(b)On the basis of nature of force existing between adsorbate and adsorbent molecule

(a)On the basis of concentration 

On the basis of concentration of adsorbate, molecule adsorption can be of following two types
(1)Positive Adsorption
(2)Negative Adsorption

(1)Positive Adsorption:

  • If the concentration of adsorbate is more on the surface as compared to its concentration in the bulk then it is called positive adsorption.
  • For example, when a concentrated solution of KCL is shaken with blood charcoal, it shows positive adsorption.

(2)Negative Adsorption: 

  • If the concentration of adsorbate is less than its concentration in the bulk then it is called negative adsorption.
  • For example, when a dilute solution of KCL is shaken with blood charcoal, it shows positive adsorption.

(b)On the basis of nature of force existing between adsorbate and adsorbent molecule:

On the basis of nature of force existing between adsorbate and adsorbent molecule adsorption again of following two types i.e.
(1)Physical Adsorption or Vanderwaals Adsorption
(2)Chemical Adsorption or Activated adsorption

(1)Physical Adsorption or Vanderwaals Adsorption:

If the physical or Vander Waals' force of attraction hold the adsorbate molecule to the surface of the adsorbent, it is termed as physical adsorption(physisorption). Since physical forces involve Vander walls' forces it is,
  • Reversible
  • Involves physical forces
  • Appreciable only at low temperature below the boiling point of adsorbate
  • Not very specific
  • Cause multilayer adsorption
  • Generally has geat of adsorption less than 10 KCal/mol.
  • Due to the formation of multilayers, physical adsorption decreases after sometimes.

(2)Chemical Adsorption or Activated Adsorption:

If the chemical forces hold the adsorbate molecule to the surface of the adsorbent, it is termed as chemical adsorption(chemisorption). Since activated chemisorbent involves a high degree of specificity like chemical forces, the chemisorption is 
  • Irreversible
  • Involves transfer of electrons between gas and solid
  • Appreciable at high temperature 
  • Maybe rapid as well as slow
  • May involve activation energy in the adsorption process
  • Highly specific
  • Leads almost to monolayer
  • Generally has the heat of adsorption greater than about 20 KCal/mol
          

Friday, 10 May 2019

Adsorption

What is Adsorption?
  • The phenomenon of adsorption was introduced by Scheele during the discovery of uptake of gases by charcoal.
  • The term adsorption was given by Kayser and this concept of adsorption was developed by Kayser and Raymonds.
  • When a solid surface is exposed to a gas or a liquid, molecules from the gas or liquid accumulates at the surface. The phenomenon of concentration of a gas or a liquid at a solid surface rather than in its bulk is called Adsorption.
  • Adsorption is caused by a type of Vanderwall's force which exists between the molecule known as London Dispersion Force
  • The substance that concentrates at the surface is called adsorbate and the solid at the surface of which the concentration occurs is called adsorbent.
Example of Adsorption:
  • Finely divided charcoal if stirred into a dilute solution of methylene blue, the dye molecule are adsorbed by the charcoal particles and the depth of color of the solution decreases appreciably.
  • If gas like SO2 , Cl2 , NH3S is treated with powdered charcoal in a closed vessel, the gas molecule adsorbed on the charcoal surface and the gas pressure is reduced.
  • The common adsorbents are charcoal(vegetable and animal),  silica gel(prepared by heating a mixture of sodium silicate and 10% HCl at 50-degree centigrade), a metal such as Ni, Cu, Ag, Au, Pt, and colloids.

Tuesday, 9 April 2019

Artist's Colour Wheel

The observed color of a compound from the color of absorbed light can be determined by using the Artist's color wheel diagram. Here complementary colors are shown on opposite sides of the color wheel.
  • If a compound absorbs light of one color(say orange), then it reflects(transmit) light of blue color. The transmitted or reflected light of blue color attacks on the retina of our eyes and the compound is seen to be blue colored. The color of the transmitted light is called the complementary color of absorbed light.
  • At the heart of color theory, complementary color are the opposite hues on the color wheel.
  • In their most basic form, they are one primary color and that is created by mixing the other two primary color. For example, the complementary color of yellow is purple which is a mix of blue and red.
  • With that knowledge, it is rather easy to remember the first set of complementary color i.e.         yellow and purple, blue and orange, red and green as shown in the above diagram.
  • If we add the tertiary color, those are made up of one primary and one secondary color, we will find that that color are also complementary i.e.  yellow-orange  and  blue-purple(indigo)                                                                                       orange-red  and  blue-green(aqua)                                                                                                 red-purple(pink) and  green-yellow



Tuesday, 8 January 2019

Limitations of Crystal Field Theory

Crystal Field Theory was given by Hans Bethe and Van Vleck. This theory is based on the assumption that the interaction between metal ion and ligands is purely electrostatic in nature. When the ligands approach the central metal atom or ion, the five degenerate  d-orbital of the central atom become different i.e.they split into the different energy level under the influence of the electrostatic field of ligands.

Limitations of Crystal Field Theory:

Crystal field theory explains successfully the structure of complex, magnetic properties, color, and electronic spectra, thermodynamic and kinetic aspects of the complexes. However, this theory has some serious limitations.
  • In CFT only d-electrons of the metal ion are considered, the other orbitals such as s, are not taken into consideration.
  • This theory has not considered the covalent character in transition metal complexes. It treats the metal-ligand as purely ionic.
  • CFT cannot explain the relative strength of ligands as given in spectrochemical series, i.e. it cannot explain why H2 is a stronger ligand as compared to OH- ion.
  • CFT has also not considered the Pi bonding in complexes either it is metal to ligand or ligand to metal.
  • This theory has no significance to the orbits of the ligands. Therefore it cannot explain any properties related to ligand orbitals and their interaction with metal orbitals.
  • It don't explain the effect of π bond on Δ0.
  • The compounds like in which metal is in zero oxidation states and the ligand is neutral have no electrostatic attraction between the metal and the ligands.

Evidence of metal-ligand covalent bonding in complexes

1.Cr(CO)6 is a volatile compound and Ni(CO)4 is a liquid. This indicates that there is a covalent bonding between metal and ligand instead of the ionic. If there would be an ionic bond, then Cr(CO)should be non-volatile and  Ni(CO)4 is solid.

2. Electron Spin Resonance(ESR):

  • The ESR spectrum of [IrCl6]2- suggests that the single unpaired electron is only 70% localized on the metal atom, and 30% is localized on the chloride ion. This indicates that there is sharing of electron and hence some covalency between metal and ligands.

3. The Nuclear Magnetic Resonance(NMR):

  • The fluoride NMR has detected the delocalization of electron in the fluoro complexes, of the paramagnetic metal ion. It is possible only when an unpaired electron spends more than negligible time on  19F nucleus.

4. The Nephelauxetic Effect:

  • The electronic repulsion in d-orbitals of transition metal cations gives rise to a number of energy levels depending upon the arrangement of electrons in d-orbitals.
  • The energy difference between two energy states can be expressed in terms of interelectronic repulsion parameters, called Racah parameters B and C. The difference in energy between two levels having same spin multiplicity can be expressed in terms of  only B, and the difference in energy between  two  energy levels having different spin multiplicities can be expressed in terms of B and C.
  • It is observed experimentally(i.e. from electronic spectra of complexes)that the magnitude of B and C decreases when the complex is formed. The reduced value of B and C indicates that electron density is reduced on metal cation i.e. electron cloud is delocalized over both the metal cation and the ligands. This suggests that there is some covalency between metal cation and ligands.
  • The more the value of B and C is reduced, the greater the delocalization of electron cloud and greater the covalency. The delocalization of the electron cloud over the metal cation and the ligand is called the nephelauxetic effect.

5. Nuclear Quadrupole Resonance(NQR);

  • The NQR spectrum of some of the complexes containing halide ions as ligands like [PtCl4]2- ,  [PdCl4]2-suggests that metal-ligand bond is partly ionic and partly covalent.

Monday, 7 January 2019

Application of CFSE

1.Enthalpy of Hydration:

  • When one mole of an ionic crystal is dissolved in water, water molecules gather from the ion and this process is called hydration. In this process, some amount of energy is released which is called Hydration energy.
  • Hydration energy of a metal cation increases with the increase in effective nuclear charge and decrease in ionic radii because these two factors bring the water molecules closer to the metal cation resulting in the increased electrostatic attraction between the metal cation and the water molecule.
  • For dipositive transition metal cation of 3d-series, the effective nuclear charge increases and ionic radii decrease across a period. So hydration energy should increase regularly from Ca2+ to Zn2+
  • So, Hydration energy  α  charge of the cation  ̸   size of the cation
  • For example, Hydration energy of Co2+ <  Co3+ because here as the size of both the ions are same, the ion having higher charge has greater hydration energy.

2.lattice Energy:

  • When one mole of an ionic crystal is formed from its constituent gaseous ions, some amount of energy is released which is called lattice energy.
  • Or energy required to break one mole of ionic crystal into its surrounding gaseous ion is called lattice energy.
  • According to Born Lande's equation lattice energy of an ionic crystal increases with the increase in the product of Z+and Z-and decrease in the interionic distance(r0).
  • The lattice energy for the halides of dipositive metal ions of the 3d-series transition element should increase from  Ca2+ to Zn2+ion and a straight line should be observed.

3.Ionic radii of Divalent Metal ions of 3d-series transition element:

  • The ionic radii of dipositive and tripositive metal cations of  3d-series transition metals in the low spin or high-spin octahedral field might be expected to decrease regularly from Ca2+to Zn2+ . 
  • The reason is that there is an increase of force of attraction between metal cations and ligands due to the increase in effective nuclear charge and the poor shielding effect of d-electrons due to which ligands and metal cation approach each other more closely.

Wednesday, 2 January 2019

Pairing Energy

Pairing Energy:

         The energy required to force the two unpaired electrons in one orbital is called pairing energy. When more than one electrons are paired, P becomes the mean pairing energy. It may be obtained from the analysis of electronic spectra.
If  ∆0 > P, it favors the low spin complex
If  ∆0 < P, it favors the low spin complex
If  ∆0 = P, high spin and low spin complex equally exist
In general, for 4d and 5d series transition metal complexes, the magnitude of  ∆is greater than that of P. 
  • In weak field octahedral complex of 3d-series transition metal with oxidation number less than equal to +3, the value of is  ∆small and there will be no pairing of electrons. Therefore in weak field complexes of d4, d5, d6, and dconfiguration, there is no pairing of electrons. These complexes have the maximum number of unpaired electrons are called high spin or spin free complexes. The term high spin or spin free is used because these complexes have the same number of spin as in d-orbital of free metal cations.
  • In strong field octahedral complex of 3d-series transition metal with oxidation number, in general, greater than equal to +2, the value of ∆0 is large. In strong field complexes of d4, d5, d6, and dconfigurations, the pairing of d-electrons will take place in according to Hund's rule. These complexes have the maximum number of paired electrons are called low spin or spin paired complexes. The term low spin or spin paired is used because these complexes have more number of paired electrons(or spin)than that of the free metal cation.
  •  It is to be notated that week field octahedral complexes are always not the high spin complexes. The metal cation of 3d-transition series with the oxidation number of greater than equal to +4 and 4d and 5d series transition metal cations always form low spin complexes with weak ligands.
  • For example, [NiF6]2- ion (oxidation state of Ni is +4) is low spin and diamagnetic, though F-is a weak ligand. [Rh(H2O)6]3+is low spin and diamagnetic, though is a weak ligand.
  • An exception is observed for 3d-series transition metals in which Co3+ form low spin complexes with H2O and O2- though H2O and  O2-are weak ligands.