Aqueous Species (1)
● CuSO₄ solid exists in a repeating array, known as a crystal lattice. If you put 2 g of CuSO₄ in 1L of water, what happens?
● As the solid dissolves, the crystal lattice of the ionic compound is broken, thereby freeing up the cations and anions.
● What you get is:
CuSO₄ (s) + H₂O (l) → Cu²⁺ (aq) + SO₄²⁻
Aqueous Species (2)
What is happening?
● As the solid dissolves, water molecules are "solvating" the ions (both cations and anions) and forming a "solvation" shell around them.
● What you have is:
Cu²⁺ ions surrounded by 6 H₂O molecules [Cu(OH₂)₆]²⁺ (aq)
● Also sometimes called dissolution; the process of attraction and association of molecules of a solvent with molecules or ions of a solute. As ions dissolve in a solvent they spread out and become surrounded by solvent molecules.
Electromagnetic Radiation (EMR)
Ultraviolet: < 400 nm (Electronic transitions)
Visible: 400 nm - 700 nm (Electronic transitions)
Infrared: > 700 nm (Vibrational transitions)
Frequency (ν) and Wavelength (λ) (1)
●Where is frequency (ν) highest?
● Where is wavelength (λ) highest?
☞NOTE: The behavior of EM radiation depends on its frequency. Lower frequencies have longer wavelengths. Higher frequencies have shorter wavelengths, and are associated with photons of higher energy.
Frequency (ν) and Wavelength (λ) (2)
c = νλ
● Speed of light is equal to frequency times wavelength.
E = hν
● Planck's relation; energy related to frequency.
E = hc/λ
● Energy is inversely proportional to wavelength.
We see color in the visible region of the spectrum (400-700 nm).
● If the light absorbed by something is visible, this absorption results in the complement of that color being seen by the human eye.
● Absorption of light involves movement of electrons within orbitals.
● Relates the absorption of light to the properties of the material through which the light is traveling.
● A = εcl, where:
A = absorbance
ε = extinction coefficient (dependent on λ)
c = concentration
l = pathlength of cuvette
☞ NOTE: Absorbance is unitless.
● Wavelength of Maximum Absorption
● Why use λmax in spectroscopy?
Using the maximum wavelength gives us the best results. This is because at the peak absorbance, the absorbance strength of light will be at the highest and rate of change in absorbance with wavelength will be the smallest. Measurements made at the peak absorbance will have the smallest error.