Question

Explain Beer-Lambert's Law. Write down the possible electronic transitions in a molecule.

Answer #1

Beer- Lambert's law gives a relation concerning the absorption of radiant energy by absorbing medium. There is a linear relationship between absorbance and concentration of an absorbing species. This law is used to calculate the results of spectroscopic experiments.

This law is a combination of two laws - 1). Beer's law . 2). Lambert's law.

Let Io be the light incident to material and I be the transmitted light.

This means, absorbance (A) = log( Io/I)

and transmission (T) = 1/A

Lambert's law:

The absorbance of light in homogeneous solution is directly proportional to the length of sample through which light passes.

A = € L

where L is the length of the path travelled by light.

€ is molal extinction coefficient

Beer's law:

It states that the absorbance of light in homogeneous solution is directly proportional to the concentration of solution (C) through which it passes.

A= € C

Combining both laws, A= log (Io/I) = €CL

The electronic transitions in a molecule takes place when electrons in a molecule get excited from a lower energy state to a higher energy state.

In a molecule, the type of electronic transitions depends on the type of electrons in a molecule and the energy that can be supplied at a particular wavelength. Out of the two molecular orbitals produced, one is the bonding orbital (lower energy orbital where two electrons will occupy the bonding orbital in ground state configuration) and the other is the antibonding orbital ( higher energy orbital where one electron in the electronically excited molecule occupies the orbital).

The possible transitions are shown below.

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