! www.clutchprep.com
!
ANALYTICAL CHEMISTRY - CLUTCH 1E
CH.17 - FUNDAMENTALS OF SPECTROPHOTOMETRY
CONCEPT: THE NATURE OF LIGHT
Visible light represents a small portion of the continuum of radiant energy known as _______________________________.
γ−Rays : ___________________X−Rays : ___________________Ultraviolet : ___________________Visible : ___________________Infrared : ___________________Microwave : ___________________Radio : ___________________
Atomic & Molecular Transitions
The visible light spectrum ranges from ______________ to ______________ .
Its wave properties of electromagnetic radiation are described by two independent variables:
_________ (ν, Greek mu) is the number of waves you have per second and is expressed in units of ______ or ________.
__________ (λ, Greek lambda) is the distance from one crest of a wave to the other and is expressed in units of _______.
Plane-polarized electromagnetic radiation:
ANALYTICAL CHEMISTRY - CLUTCH 1E
CH.17 - FUNDAMENTALS OF SPECTROPHOTOMETRY
Page 2
CONCEPT: PROPERTIES OF LIGHT
The speed of a wave, is the product of ν and λ. In a vacuum, all forms of electromagnetic radiation travel at2.998×108 ms
c = ν⋅ λ The physicists Max Planck and Albert Einstein theorized that light was made of small “packets” of electromagnetic energy, called ________________ and the energy of a single photon could be calculated by:
ΔE = h ⋅ ν = h ⋅ !ν
• Planck’s constant is represented by the variable of h and is equal to 6.626 x 10-34 J · s.
EXAMPLE 1: Calculate the wavelength (in nm) of the red light emitted by a neon sign with a frequency of 4.16 ×108 MHz.
EXAMPLE 2: What is the energy, in joules, of a mole of photons associated with visible light of wavelength 493 nm?
ANALYTICAL CHEMISTRY - CLUTCH 1E
CH.17 - FUNDAMENTALS OF SPECTROPHOTOMETRY
Page 3
PRACTICE: PROPERTIES OF LIGHT CALCULATIONS 1
EXAMPLE 1: A laser pulse produces 1.242 kJ of energy. It was experimentally determined that the pulse contains 3.50 x 1022 photons. Determine the wavelength of light (in meters) emitted by one photon.
EXAMPLE 2: How much total energy (in µJ/mol) would it take to remove the electrons from a mole of hydrogen atoms?
The ionization energy for a hydrogen atom is 2.178 x 10-18 J.
PRACTICE: A low-pressure mercury-vapor lamp has a characteristic emission line at 253 nm. Knowing that this lamp is putting out 11.8 watts of light energy, how many mercury atoms are emitted per second during operation?
ANALYTICAL CHEMISTRY - CLUTCH 1E
CH.17 - FUNDAMENTALS OF SPECTROPHOTOMETRY
Page 4
CONCEPT: ATOMIC EMISSION Absorption involves the taking in of excess energy by an electron, which promotes it to a higher “excited” energy state. Emission involves releasing that same absorbed energy so that the electron falls to a lower energy ground state.
Distance = Energy
Lyman Series : ∞ to n = 1 : _____________
Balmer Series : ∞ to n = 2 : _____________
Paschen Series : ∞ to n = 3 : _____________
Brackett Series : ∞ to n = 4 : _____________
Pfund Series : ∞ to n = 5 : _____________
Humphrey Series : ∞ to n = 6 : _____________
Absorption Emission Emission Line Spectrums
An emission spectrum represents the different frequencies of electromagnetic radiation emitted by an atom as it transitions from a
higher energy state to a lower energy state.
An absorption spectrum is a spectrum of electromagnetic radiation transmitted through a substance, showing dark lines or bands due to absorption of specific wavelengths.
ANALYTICAL CHEMISTRY - CLUTCH 1E
CH.17 - FUNDAMENTALS OF SPECTROPHOTOMETRY
Page 5
PRACTICE: CALCULATING ABSORBANCE EXAMPLE 1: In UV-Vis spectroscopy, the absorbance represents the quantity of light that a sample absorbs, while the percent transmittance represents the amount of light that passes through a sample. Calculate the percent transmittance of a sample if its absorbance is measured as 0.419.
EXAMPLE 2: If your sample has a percent transmittance of 39.1%, determine the absorbance of the compound. PRACTICE: Identify the colors of a compound that absorbs green light, yellow-green light and blue light.
ANALYTICAL CHEMISTRY - CLUTCH 1E
CH.17 - FUNDAMENTALS OF SPECTROPHOTOMETRY
Page 6
CONCEPT: BEER’S LAW Beer’s Law represents a theoretical model that forms a correlation between a substance’s absorbance, A, and its concentration, c:
A = ε⋅c ⋅ l ε = _______________________ A = log I0I
I0 = _______________________
c = _______________________ I = _______________________l = _______________________
The application of Beer’s Law can be seen with the use of a UV-Vis spectrophotometer with a conjugated compound.
When a conjugated system such as butadiene is irradiated with UV light a pi electron can be promoted to a higher energy level and produce the UV-Vis absorption spectrum below:
Ground State
Energy
Excited State
Energy
ANALYTICAL CHEMISTRY - CLUTCH 1E
CH.17 - FUNDAMENTALS OF SPECTROPHOTOMETRY
Page 7
CONCEPT: MASS SPECTROMETRY
Mass spectrometry involves the __________________, __________________, and __________________ of gaseous ions according to their mass to charge ratios.
ANALYTICAL CHEMISTRY - CLUTCH 1E
CH.17 - FUNDAMENTALS OF SPECTROPHOTOMETRY
Page 8