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K-12 Chemistry Project : Crystal Field Stabilization Energy ( CFSE) of Coordination complexes

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As a K-12 teacher the project gets the students to understand the preparation of iodoform along with the mechanism.

color wheel with wavelength in nm

color wheel with wavelength in nm

The purpose of this study is to determine the Crystal Field Stabilization Energy (CFSE) and use this calculated energy of the crystal field to calculate the wavelength of emission. Many metal-containing compounds, especially those of transition metals, form coordination complexes.

Transition elements form coordination complexes as they have small cationic sizes, empty d-orbitals and a high polarizing power.

A coordination complex is a compound where the center is a metal atom surrounded by an array of atoms/molecules or ions.

Transition metal complexes display color as they absorb and emit light in the visible region of the electromagnetic spectrum.

These complexes show electronic transitions by the absorption of light. It is due to the bright colors that they exhibit that they are used as pigments. Transition elements have incompletely filled d-orbitals where the d-electrons undergo d-d transitions. It is the d-d transitions that allow these complexes to exhibit bright color. Charge transfer bands is another reason why transition metal complexes exhibit color. In a d–d transition, an electron in a d- orbital on the metal is excited by a photon to another d orbital of higher energy. A charge transfer band is observed when electron density is promoted from an electron dense ligand to a metal-orbital – It is a Metal to Ligand Charge Transfer or MCLT. The converse can also occur when there is an excitation of an electron in a ligand-based orbital from an empty metal-based orbital (Ligand to Metal Charge Transfer (LMCT). This is observed in the visible region of the electromagnetic spectrum. Electrons are able to absorb certain frequencies of electromagnetic radiation to get promoted to higher energy orbitals. These frequencies have a certain energy which corresponds to the energy difference between the different orbitals. Now most substances are only able to absorb frequencies of radiation which are outside the visible light spectrum. This means that it reflects all other types of radiation, including the full spectrum of visible light. So our eyes see a mixture of all the colors; red, green, blue, violet, etc

  1. Prepare a 0.5M 50 ml solution of the salt
  2. Rinse the burette with ammonium thiocyanate solution
  3. Fill the burette with ammonium thiocyanate solution up to the 25 ml mark
  4. Now add the ammonium thiocyanate solution from the burette till there is a change in the color of the salt

In our project we prepared coordination complexes of Nickel, cobalt and copper salts along with measuring the temperature changes accompanying the formation of the complexes.

EXPERIMENT ONE

Formation of Nickel Thiocyanate complex

Salt Used: Nickel Sulphate hexa hydrate

Molar Mass: 262.86g

Formula: NiSO4.6H2O


Mass of salt taken: 6.5715g


Temperature of water: 303 K


OBSERVATIONS


Temperature of the salt dissolved: 303 K


Temperature of the complex: 304 K


Colour change: Light green to dark green


CALCULATIONS


Enthalpy change = mc(T2-T1)

= 56.5715x4.18x1

= 236.46 J

Enthalpy change for 1 mole = 236.46x262.86x2/6.5715

= 18917.5 J


Wavelength = hc/E = 6.626x10-34x3x108x6.02x1023/18917.5

= 632 nm


Wavelength 632 nm corresponds to the color red which is the complementary color green

EXPERIMENT TWO

Formation of Cobalt thiocyanate complex

Salt Used: Cobalt chloride hexa hydrate

Molar Mass: 237.93g

Formula: CoCl2.6H2O


Mass of salt taken: 5.948g

OBSERVATIONS

Temperature of water: 303.5K

Temperature of the salt dissolved: 303 K


Temperature of the complex: 302 K


Color change: Maroon to Pink


CALCULATIONS


Enthalpy change = mc(T2-T1)

= 55.948x4.18x1

= 233.8 J

Enthalpy change for 1mole = 233.8x237.93x2/5.943

= 18709.79 J


Wavelength = hc/E = 6.626x10-34x3x108x6.02x1023/18709.9

= 639 nm

Wavelength 639 nm corresponds to pink

EXPERIMENT THREE

Formation of Copper thiocyanate

Salt Used: Cobalt sulphate penta hydrate

Molar Mass: 249.68 g

Formula: CuSO4.5H2O


Mass of salt taken: 6.239 g

OBSERVATIONS

Temperature of water: 303 K

Temperature of the salt dissolved: 302 K

Temperature of the complex: 303 K


Color change: Blue to Green


CALCULATIONS


Enthalpy change = mc(T2-T1)

= 56.239x4.18x1

= 235.07J

Enthalpy change for ,1 mole = 235.07x249.68x2/6.239

= 18815.36 J


Wavelength = hc/E = 6.626x10-34x3x108 x6.02x1023/18815.36

= 636 nm

Wavelength 639 nm corresponds to red which is the complementary color of green

EXPERIMENT FOUR

Formation of Ferrous thiocyanate

Salt Used: Ferrous sulphate hepta hydrate

Molar Mass: 278.01 g

Formula: FeSO4.7H2O


Mass of salt taken: 6.95 g

OBSERVATIONS

Temperature of water: 304 K

Temperature of the salt dissolved: 302 K

Temperature of the complex: 303 K


Color change: Yellow to Blood Red


CALCULATIONS


Enthalpy change = mc(T2-T1)

= 56.95x4.18x1

= 238.05 J

Enthalpy change for 1mole = 238.05x278.01x2/6.95

= 19044.76 J


Wavelength = hc/E = 6.626x10-34x3x108 x6.02x1023/19044.76

= 628 nm

Wavelength 628 nm corresponds to red

The experimental work for this project was conducted by Aneesh Sukumar and Vibhav Sinha of the R.N. Podar Sr. Secondary High School (CBSE) under the guidance of Ms Anjali B Gharpure

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