Spectroscopy in Inorganic Chemistry

Job's Method

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aA + bB ↔ dD

A + kB ↔ mD dividing all coefficients by "a"

k = b/a and m = d/a

R>k R=K R<K

Job's Method

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A and B are known to react to form D, but the stoichiometry is uncertain. A Job's Method study yields the following data. Plot quantity of product versus moles A to determine the stoichiometry.

R moles

A

moles

B grams product

9 0.2 1.8 2.5

5.7 0.3 1.7 3.75

4 0.4 1.6 5.0

2.3 0.6 1.4 4.38

1.5 0.8 1.2 3.75

1 1.0 1.0 3.12

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X=?

red

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COMPLEX ION COMPOSITION BY JOB‘S METHOD 1. Introduction The nickel(II) ion in aqueous solution forms complexes with 1,2-diaminoethane(ethylenediamine) whose ligand replaces two water molecules in the hydrated complex, i.e. acts as a chelate.

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The relative values of k1, k2 and k3 will determine the relative proportions of the complexes.

If k3 > k2 > k1, then addition of ligand will result in formation of Nien3

2+. If k3 < k2< k1, then all four complexes (or three, depending on the

absolute magnitude of the constant) will co-exist in appreciable quantities at equilibrium. The application of Job‘s Method is limited to situations where one stage predominates. In the method, the total molar concentration of nickel(II) plus ligand is kept constant and the ratio varied from zero to infinity. Initially in a solution containing no nickel(II) ion (ratio = 0), no complex will be present and the absorbance of the solution is due solely to diaminoethane. As the concentration of nickel(II) is increased, absorbance will increase (due to formation of complex) until the molar ratio of nickel(II) to ligand equals the ratio in the complex.

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Under the Job conditions, i.e. total molar concentrations of the two species is constant, this represents the maximum possible concentration of complex. Further increases in nickel(II) concentration (with corresponding decreases in ligand concentration) will causea decrease in absorbance until a solution containing no ligand (ratio = 4) has an absorbance due to nickel(II) ions alone. A plot of absorbance against ratio of molar concentration of nickel(II) ion to ligand will show a maximum where the ratio represents the composition of the complex.

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K1 = [Ni(en)2+] / [Ni2+] x [en] = 3.3x107

K2 = [Ni(en)22+] / [Ni2+] x [en]2 = 1.9x106,

K3 = [Ni(en)32+] / [Ni2+] x [en]3 = 1.8x104

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Mathematically

x = mole A 1-x= mole B

mx=m(1-x)/k → x = 1/(1+k) 1-[1/ ]

1

(1/1+k) 1

(1/1+k)

R = (1+k)/x = = k

Fingerprinting

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Electronic Absorption Spectroscopy

Vibrational and electronic energy levels

in a diatomic molecule

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Electronic Absorption Spectroscopy

continuum

Zero point energy

D0 De

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Electronic Absorption Spectroscopy

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V =

De

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Electronic Absorption Spectroscopy