Lambert's and Beer-Lambert's Law of Radiation

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Questions and Answers

What is the direction of the shift in wavelength when a hypochromic shift occurs?

No shift in wavelength

Towards longer wavelength

Varies with the molecule

Towards shorter wavelength (correct)

Why are non-conjugated alkenes inaccessible to UV spectrophotometer?

They have weak absorption bands

They have absorption below 200 nm (correct)

They have strong absorption bands

They have absorption above 400 nm

What is the effect of conjugation of C=C and carbonyl group on the λmax of both groups?

Shifts λmax to longer wavelength (correct)

Varies with the molecule

Shifts λmax to shorter wavelength

No effect on λmax

What is the significance of λmax in UV-Visible spectrum?

It is the wavelength of maximum absorbance Signup and view all the answers

What is the effect of conjugation of double bonds in a compound on the λmax?

Shifts λmax to longer wavelength Signup and view all the answers

What is the significance of 200-300 nm region in UV-Visible spectrum?

It is the region of low absorbance for non-conjugated carbonyl group compounds Signup and view all the answers

What is the direct proportionality of the decrease in the intensity of radiation with the thickness of the solution?

Both intensity of the incident light and concentration of the solution Signup and view all the answers

What is the unit of the extinction coefficient?

Per centimeter Signup and view all the answers

What is the relationship between the transmission of radiation and the concentration of the solution?

Logarithmically proportional Signup and view all the answers

What is the formula for absorbance?

A = log(It/Io) Signup and view all the answers

What is the effect of increasing the concentration of the solution on the transmission of radiation?

Decreases the transmission of radiation Signup and view all the answers

What is the purpose of a calibration curve in spectrophotometric analysis?

To determine the concentration of the solution Signup and view all the answers

What is the relationship between the transmission of radiation and the thickness of the solution?

Logarithmically proportional Signup and view all the answers

What is the formula for the extinction coefficient?

K = 2.303K' Signup and view all the answers

What is the unit of the molar absorption coefficient (ε) in the given example?

dm3 mol-1 cm-1 Signup and view all the answers

What is the purpose of measuring the absorbance of a mixture at different wavelengths?

To analyze a mixture of unknown concentrations Signup and view all the answers

What is the relationship between the molar absorption coefficient (ε) and the absorbance (A) of a solution?

A = εlc Signup and view all the answers

What is the advantage of using a calibration curve in spectrophotometric analysis?

It provides a direct relationship between absorbance and concentration Signup and view all the answers

What is the term for the promotion of an electron from a lower energy state to a higher energy state in a molecule?

Electronic transition Signup and view all the answers

What is the term for the orbital that is typically the lowest energy occupied orbital in a molecule?

σ-orbital Signup and view all the answers

What is the purpose of using the Beer-Lambert Law in spectrophotometric analysis?

To determine the concentration of a species in a solution Signup and view all the answers

What is the relationship between the absorbance (A) and transmittance (T) of a solution?

A = -log(T) Signup and view all the answers

Study Notes

Lambert's Law

The decrease in intensity of radiation with thickness of the solution is directly proportional to the intensity of the incident light.
The law is expressed as: 𝑑𝐼/𝑑𝑙 = −𝐾′ 𝐼, where 𝐾′ is the absorption coefficient.
The extinction coefficient (K) is related to the absorption coefficient (K') by: 𝐾 = 2.303 𝐾′, and its unit is cm-1.
The absorbance (A) is related to the extinction coefficient (K) and the path length (l) by: A = 𝐾𝑙.

Beer-Lambert's Law

The decrease in intensity of radiation with thickness of the solution is directly proportional to the intensity of the incident light and the concentration of the solution.
The law is an extension of Lambert's law, taking into account the concentration of the solution.

Interpreting UV-Visible Spectra

Non-conjugated alkenes show an intense absorption below 200 nm and are therefore inaccessible to UV spectrophotometers.
Non-conjugated carbonyl group compounds give a weak absorption band in the 200-300 nm region.
Conjugation of double bonds in a compound shifts the λmax to longer wavelengths.
Conjugation of C=C and carbonyl groups shifts the λmax of both groups to longer wavelengths.

Molar Absorption Coefficients

The molar absorption coefficient (ε) is a measure of the ability of a molecule to absorb radiation at a given wavelength.
The absorbance (A) of a mixture can be calculated using the molar absorption coefficients of the individual components.

Electronic Transitions

Electronic transitions involve the promotion of an electron from a electronic ground state to a higher energy state.
Typical electronic transitions occur from the Highest Occupied Molecular Orbital (HOMO) to the Lowest Unoccupied Molecular Orbital (LUMO).
σ-orbitals are typically the lowest energy occupied orbitals, while π-orbitals are of somewhat higher energy.
Anti-bonding orbitals (σ* and π*) are of higher energy than their corresponding bonding orbitals.

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Description

Learn about the laws of radiation, including Lambert's Law and Beer-Lambert's Law, which describe the decrease in intensity of radiation with thickness of the solution.