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Questions and Answers
The wavelength of visible light is defined as being between 400 to 800 nm.
The wavelength of visible light is defined as being between 400 to 800 nm.
True (A)
Photochemistry studies chemical reactions that occur without the absorption of electromagnetic radiation.
Photochemistry studies chemical reactions that occur without the absorption of electromagnetic radiation.
False (B)
When a molecule absorbs electromagnetic radiation, it is promoted to a lower electronic state.
When a molecule absorbs electromagnetic radiation, it is promoted to a lower electronic state.
False (B)
Planck's constant is equal to 6.63 x 10^-34 Js.
Planck's constant is equal to 6.63 x 10^-34 Js.
The energy of a photon is directly proportional to its frequency and inversely proportional to its wavelength.
The energy of a photon is directly proportional to its frequency and inversely proportional to its wavelength.
An Einstein refers to the energy of one mole of photons, which is 6.02 x 10^23 photons.
An Einstein refers to the energy of one mole of photons, which is 6.02 x 10^23 photons.
A photophysical process involves both energy transfer and product formation.
A photophysical process involves both energy transfer and product formation.
Electromagnetic radiation can be absorbed only by a limited number of organic molecules based on their specific energies.
Electromagnetic radiation can be absorbed only by a limited number of organic molecules based on their specific energies.
What initiates all photochemical and photophysical processes?
What initiates all photochemical and photophysical processes?
According to Beer-Lambert's Law, what does the symbol ε represent?
According to Beer-Lambert's Law, what does the symbol ε represent?
What is the relationship between absorbance (A), concentration (c), and path length (l) as described by the Beer-Lambert law?
What is the relationship between absorbance (A), concentration (c), and path length (l) as described by the Beer-Lambert law?
In the equation log(Iout/Iin) = -εcl, what does Iout represent?
In the equation log(Iout/Iin) = -εcl, what does Iout represent?
How does path length (l) affect absorbance (A) according to the Beer-Lambert law?
How does path length (l) affect absorbance (A) according to the Beer-Lambert law?
What happens to intensity of light as it passes through an absorbing substance according to the Beer-Lambert law?
What happens to intensity of light as it passes through an absorbing substance according to the Beer-Lambert law?
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Study Notes
Photochemistry
- Branch of chemistry involving chemical reactions triggered by absorption of electromagnetic radiation
- Involves absorption of light in visible (400-800 nm) and ultraviolet (200-400 nm) regions
Electronic Excitation
- Molecules get activated due to electronic excitation when absorbing electromagnetic radiation
- Involves promotion of an electron from bonding molecular orbital to antibonding molecular orbital
Quantum Theory
- Both matter and light are quantized
- Specific organic molecules absorb specific energies of light for excitation
Photons
- Energy transfer occurs through photons, which have both wave and particle-like properties
- Energy (E) of a photon is given by Planck's law: E = hv
- Where h is Planck's constant (6.63 x 10^-34Js) and v is the frequency of oscillation
- v is related to wavelength (λ) by: v = c/λ, where c is the velocity of light
- Therefore, E = hv = hc/λ, showing energy is proportional to frequency and inversely proportional to wavelength
Energy Units
- Energy of one mole of photons (6.02 x 10^23 photons) is called an Einstein
- Measured in units of kJ mol^-1
Types of Photochemical Processes
- Photo physical process: energy transfer only
- Photochemical process: product formation
Absorption of Light
- Photochemical and photophysical processes start with the absorption of light or ultraviolet radiation, leading to an electronically excited state.
Beer-Lambert's Law
- The extent of absorption varies greatly between substances, with the probability of absorption indicated by the molar absorption coefficient (ε).
- The intensity of light entering a substance (Iin) is greater than the intensity of the emerging light (Iout).
- There is an exponential relationship between the relative absorption (Iout/ Iin) and the concentration (c) and path length (l) of the absorbing substance: Iout/ Iin = 10^(-ecl).
- Taking logarithms to the base 10 gives: log (Iout/ Iin) = -ecl.
- The left-hand-side quantity is the absorbance A.
- The linear relationship between absorbance, concentration, and path length is known as the Beer-Lambert law: A = ecl.
- Absorbance depends on path length (l) and concentration.
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