Photochemistry process involves the initiation of a chemical reaction through the absorption of a photon by an atom or molecule.
h = Planck constant = 6.626 × 10–34 J-s
c = speed of light in vacuum = 3 × 108 ms–1
v = frequency of light
and λ = wavelength of light
So, S1, S2, T1 & T2 are electronic levels.
S → singlet
T → triplet
The Beer-Lambert law states that for a given material sample path length and concentration of the sample are directly proportional to the absorbance of the light.
The Beer-Lambert law is expressed as:
A = εLc
But Beer-Lambert law is a combination of two different laws: Beer’s law and Lambert law.
I0 = intensit y of incident light
I = intensity of transmitted light
and Ia = Intensity of the light absorbed = I0 – I
Absorbance of solution, A =
A = …(1)
where A = absorbance
C = concentration of solution
∈ = molar extinction coefficient or molar absorption coefficient (unit = concentration length–1)
l = path length
The absorbance of a solution is additive whereas the transmittance is multiplicative.
Q.1. A monochromatic light is an incident on the solution of 0.05 molar concentration of an absorbing substance. The intensity of the radiation is reduced to one-fourth of the initial value after passing through 10 cm length of the solution. Calculate the molar extinction coefficient of the substance.
Sol. From bear Lambert law
2 × 0.3010 = 0.5 ∈
∈ = 1.204 dm3 mol–1 cm–1
Q.2. A substance when dissolved in water at 10–3 M concentration absorbs 10 percent of incident radiation in a path of 1 cm length. What should be the concentration of the solution in order to absorb go percent of the same solution?
Sol. ∵ 10% absorbed then 90% transmitted
then T = = 90%
∵ 90% absorbed then 10% transmitted
C = 0.0218 mol dm–3.
Q.3. In carbon-dating application of radioisotopes, 14C emits
(1) β-particle (2) α-particle (3) γ--particle (4) positron
Correct answer is (1)
Q.4. With the increase in temperature, Gibb’s free energy for the adsorption of a gas on a solid surface.
(1) Becomes more positive from a positive value
(2) Becomes more negative from a positive
(3) Becomes more positive from a negative value
(4) Becomes more negative from a negative value
Sol. From Langmuir isotherm, the fractional coverage is
∵ P ∝ T
Then higher the pressure or temperature lower the value of fractional coverage q.
R(g) + M(surface) RM (surface)
if q decreases then the formation of RM decreases. i.e. rate of formation of RM decreases.
This indicates that Gibbs from the energy of adsorption becomes positive.
So, increase the temperature, the Gibbs free energy of adsorption of a gas on a solid surface become more positive from a negative value.
The correct answer is (3).
Q.5. One of the assumptions made in the conventional activated complex theory is:
(1) Equilibrium is maintained between the reactants and the activated
(2) Equilibrium is maintained between the reactants and the product
(3) Equilibrium is maintained between the products and the activated complex
(4) Equilibrium is maintained between the reactants, the activated complex and the products
Sol. According to Eyring, the equilibrium is maintained between reactants and the activated complex
(AB)# is the activated complex.
The correct answer is (1)
Q.6. For a reaction, the rate constant k at 27°C was found to be k = 5.4 × 1011 e–50 The activation energy of the reaction is
(1) 50 J mol–1 (2) 415 J mol–1 (3) 15000 J mol–1 (4) 125000 J mol–1
Sol. From Arrhenius equation
k = Ae-Ea / RT &(1)
k = 5.4 × 1011 e–50 …(2)
From equation (1) & (2) we get
The correct answer is (4)