Photochemistry: Overview | Chemistry Optional Notes for UPSC PDF Download

Definition

Photochemistry is the study of the chemical reaction initiated by absorption of energy in the form of light.

Difference between Photochemical Reaction & Thermal Reaction

Grotthus Draper Law

“Only those radiations which are absorbed by a reacting substance or system are responsible for producing chemical change.”
According to this law, all light radiations are not bringing the chemical reaction. Some are increase the kinetic energy of molecule while some are re-emitted. (i.e. fluorescence).

Stark-Einstein Law or Einstein law of photochemical equivalence

“Each molecule of absorbing substance absorb one photon (or quantum) of the radiation in primary process.”
Explanation: A molecule acquire energy by absorbing photon as,
A + hυ ---------> A*
Thus energy of photon is,
E = hυ
Where υ = frequency of absorbing photon.
h = plank’s constant = 6.624 × 10^–34 J.s
The energy of 1 mol photon (i.e. Einstein) is given by,
E = N.h.υ
But, υ = C/λ

N = Avogagro’s no. = 6.023 × 10²³ mol^–.
C = velocity of light. = 3 × 10⁸ m/s.


λ should be in cm

Lamberts law

When a monochromatic radiation is passed through a homogeneous absorbing medium, the rate of decrease in the intensity of radiation with thickness of absorbing medium is directly proportional to the intensity of the incident radiation.

I₀ = initial intensity before passing absorbing medium.
I = intensity after passing absorbing medium.
x = thickness of absorbing medium.
a = extinction coefficient.
𝑰/𝑰_𝒐 = transmission or transmittance (T)

Lamberts-Beer’s law (or Beer’s law)

When a monochromatic radiation is passed through a solution of absorbing medium, the rate of decrease in the intensity of radiation with thickness of absorbing medium is directly proportional to the intensity of the incident radiation and concentration of the solution.

I₀ = initial intensity before passing absorbing medium.
I = intensity after passing absorbing medium.
x = thickness of absorbing medium.
ε = molecular extinction coefficient

Quantum yield or Quantum efficiency (Φ)

It is defined as the ratio of number of molecules reacting in given time to the number of quanta absorbed in the same time.

Reasons for unite Quantum yield

According to Einstein law of photochemical equivalence , in primary process each molecule absorbs one quanta. Hence Quantum yield is unity.

Reasons for high Quantum yield: Φ > 1

• The product of primary process may collide with 2nd molecule & transfer energy. 2nd to 3rd and so on. Thus chain reaction starts and no. of reacting molecule will be high.
• Due formation of intermediate product which act as a catalyst. 
• Exothermic reaction may activate more molecules. 
• Eg. Reaction between Hydrogen and Halogen.

Reasons for low Quantum yield: Φ < 1

• In some reaction deactivation of activated molecule take place in primary process before transfer to product. 
• The product of primary process may react back to form reactants. 
• Eg. Decomposition of HI and HBr, Polymerization of anthracene.

Factors affecting quantum yield

• All primary photochemical process is endothermic. Hence, quantum yield increases with temperature. 
• We know energy absorbed by molecule is inversely proportional to wavelength. Hence, quantum yield will be higher at the lower wavelength and vice versa. 
• As speed of photochemical reaction is proportional to intensity of light. Hence, quantum yield increases with intensity and vice versa. 
• The addition of inert gas in photochemical reaction the quantum yield.

• Photosynthesis of HBr i.e. Photochemical combination of Hydrogen and Bromine to form HBr. The reaction may be represented as
  H2 + Br2 → 2 HBr 
• The quantum efficiency of this reaction is very low i.e. about 0.01 at ordinary temperatures. This is explained by proposing the following mechanism for the above reaction:
  (a) Primary process; Br2 + hv → 2 Br
  (b) Secondary Process:
  (i) Br + H2 endo→ HBr + H
  (ii) H + Br2 → HBr + Br
  (iii) H + HBr → H2 + Br
  (iv) Br + Br → Br2

Photosynthesis of HCl from H2 and Cl2

• This is an example of a reaction whose quantum yeild is very high i.e to This reaction may be represented as
  H2 + Cl2 → 2HCl
  The high quantum yield of this reaction is explained by the chain mechanism (proposed by Nernst in 1918).
  The different steps involved are as follows: 
  (a) Primary process: Cl2 + hv → 2Cl
  Chain initiating step i.e. a chlorine molecule absorbs one quantum of light and dissociates to give CI atoms.
  (b) Secondary process: 
  (i) CI + H2 → HCl + H | Chain propagating steps
  (ii) H + Cl2 → HCl+ CI I
  (iii) CI + CI → Cl2 } Chain terminating step
• It is interesting to note that the mechanism of the above reaction is similar to that of the photosynthesis of HBr, yet the quantum yield of this reaction is very high whereas that of the photosnthesis of HBr is very low. 
• The difference is explained on the basis of the reaction
  (i) of the secondary processes.
• In the present reaction, the reaction
  (i) of the secondary processes which immediately follows the primary process is exothermic and therefore, takes place very easily and consequently the chain reaction is set up very easily. 
• In the photosynthesis of HBr, the reaction
  (i) of the secondary process is endothermic and thus has very little tendency to take place.

Photosensitization Reaction

• Photosensitization is a reaction to light that is mediated by a light-absorbing molecule, which is not the ultimate target. There are many substances that do not normally react when exposed to light. lf, however, another substance is added to it, photochemical reaction readily proceeds. 
• The substance thus added does not undergo any chemical change. It merely absorbs light energy and then passes it on to the reactant substance. The added substance is called a photosensitizer since it sensitizes the reaction. The process is termed as photo-sensitization. The photosensitizer acts as a carrier of energy from the excited molecule to the reactant molecule. Photosensitization can involve reactions within living cells or tissues, or they can occur in pure chemical systems. 
• If only hydrogen gas is irradiated by the ultraviolet light of λ = 253.70 nm the molecules do not dissociate to the atoms. 
• But if the same radiation acts on hydrogen in presence of Hg-vapour, the hydrogen molecules undergo dissociation to the hydrogen atoms. 
• Hg + hv → Hg*
• Hg* + H₂ → Hg + 2H