19. Light absorption in solutions.

Question 1

What does the Beer-Lambert law state? Give its formula and define each upcoming term.

Answer 1

Beer-Lambert law states that light absorption in dilute solutions depends on the concentration of the solute.

Formula: lg J₀/J = A = ε(λ) * c * x

Question 2

What happens on the atomic scale when light is absorbed or emitted? Refer to the model below and name it.

Answer 2

The model is called Bohr’s model.

Absorption & emission:

1. incoming photon is absorbed by the atom
2. e^- is excited
   
   1. either: remains excited = absorption, therfore no further photon emission
   2. or: e^- returns to ground state, a photon is emitted = emission

Question 3

The molar extinction coefficient ε(λ) depends on.. ? What is its unit?

Answer 3

• type of molecules that are dissolved
• wavelength

Unit: L/(mol*cm)

Question 4

What is an absorption spectrum? Answer the questions below refering to the spectrum:

1. What does the height of the curves say about ε(λ)?
2. What does the height of the curves say about thickness/conc.?
3. What does the height of the curves say about the behavior of e^- in the solution?
4. How can the color of the substance be obtained from the absorption spectrum?

Answer 4

An absorption spectrum is the graphical display of the wavelength dependence of absorption.

1. Materials absorb at different λ ranges and the extinction coefficients ε(λ) in the absorption bands can differ significantly (different height) if conc./thickness remain constant.
2. If conc./thickness are increased the height increases proportionally, because the absorbance is dependent on the conc./thickness (Beer’s law)
3. Locations of the maxima in the absorption spectrum are related to the E_excitation of e^- characteristic for that molecule.
4. The visible color of the solution is the complementary color of the color in which the light is absorbed (the λ).

Question 5

How is the absorption spectrum measured? Why do we need it in practice? Describe the 3 steps of the measurement.

Answer 5

The equipment used to measure the absorption spectra is called “absorption spectrometer”.

→ unknown c_ion can be determined in complex solutions (complex = ions + ligands)

Stepwise measurement:

1. Unknown c_ion at different λ’s measured, λ that corresponds to the maximum of the complex solution determined (complex = ions + ligands)
2. λ of maximum absorption is set, and OD of different complex solutions (increasing c_ligand) measured
3. Data plotted (y-axis = OD, x-axis = c_ligand)

→ point where the curve breaks = saturation point, c_ligand equals to unknown c_ion

Question 6

What is the difference between spectrometry and spectroscopy?

Answer 6

Spectrometry or spectrophotometry is the quantitative analysis of the spectrum

Spectroscopy is the qualitative analysis of the spectrum

Question 7

How can the transition energy of a photon be calculated for an absorption spectrum based on its wavelength?

Answer 7

According to Planck’s law:

ΔE = E_photon = Eexcitedstate - Egroundstate = h * f = h * c/λ