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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:



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?



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?



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.



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.



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



CONCEPT: MASS SPECTROMETRY

Mass spectrometry involves the __________________, __________________, and __________________ of gaseous ions according to their mass to charge ratios.



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