This presentation include Interaction of electromagnetic radiation with matter. Photochemical and thermal reaction

1. Dr. Y. S. THAKARE
M.Sc. (CHE) Ph D, NET, SET
Assistant Professor in Chemistry,
Shri Shivaji Science College, Amravati
Email: yogitathakare_2007@rediffmail.com
B Sc- III Year
SEM-V
PAPER-III
PHYSICAL CHEMISTRY
UNIT- V
Interaction of Electromagnetic Radiation with Matter
Photochemical and thermal reaction
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2. Interaction of Radiation with Matter
When light falls on matter, part of it reflected, part of it is
transmitted and some part of it is absorbed. In photochemistry we are
concerned with the part of light which is absorbed.
The light radiations of the visible and ultraviolet regions lying
between 800 nm and 200 nm wave length are chiefly involved in
bringing about photochemical reactions. The relative photons should
be of sufficient energy to raise the atom or molecule from the ground
electronic state to an excited electronic state.
When light radiation of suitable energy absorbed by the matter,
electronic transitions, vibrational changes or rotational changes or
combination of these take place. The atoms and molecules become
excited. They give out energy quickly either by losing energy in the
form of heat or by reemitting the light or they may get dissociated
yielding atoms, molecules or free radicals in the primary processes. In
secondary process the excited atoms, molecules or free radicals
produced in primary stage react further giving rise to different
products.29-August -20 Dr. Yogita Sahebrao Thakare

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1. Primary Photochemical Process: A chemical reaction wherein
photon is one of the reactant

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2. Secondary process: This involves reaction of activated
molecules of primary process with other molecules
resulting into activation or deactivation
Dr. Yogita Sahebrao Thakare

6. Types of Chemical Reactions
A chemical reaction is define as one in which the identity of molecule is lost
due to the rupture and formation of chemical bonds.
Chemical reactions are two types.
(a) Dark or Thermal Reactions
The reaction taking place in absence of light are called dark or thermal
reactions. These are the ordinary chemical reactions which are influenced or
induced by temperature, concentration of reactants, presence of a catalyst etc.
except light radiations.
Examples of dark reactions are:
(i) N2+3H2 2NH3
(ii) H2+I2 2HI
(ii) PCl5 PCl3 + Cl2
The Various characteristics of dark or thermal reactions are:
(i) These reactions need collisions for activation.
(ii) Their rate depends on temperature.
(iii) These reactions take place with decrease of free energy.29-August -20 Dr. Yogita Sahebrao Thakare

7. (b) Photochemical Reaction
Photochemical reaction may be defined as any reaction which is induced
or influenced by the action of light on the system.
Examples of Photochemical reactions are
(i) Dissociation.
(ii) Photo-catalytic reactions.
(iii) Rearrangement
(iv) Combination.
(v) Decomposition
(vi) Polymerization.
(vii) Addition reaction.
29-August -20 Dr. Yogita Sahebrao Thakare

8. Comparison between thermal and photochemical reactions:
Thermal reactions
1. These reactions are initiated by collisions between molecules of reactant.
2. In thermal reactions, free energy of molecules decreases.
3. Thermal reactions are simple in nature.
4. Thermal reactions are generally fast.
5. The rate of thermal reactions depends on the temperature.
Photochemical reactions
1. These reactions are initiated by absorption of light radiations of visible and
ultra violet region.
2. In photochemical reaction free energy of molecules increases.
3. Photochemical reactions are complex in nature.
4. Photochemical reactions are slow.
5. The rate of photochemical reaction independent of temperature.
29-August -20 Dr. Yogita Sahebrao Thakare