CHM171L / A11 Physical Chemistry Laboratory 2
Quarter S.Y. 2010-2011
, Palomaria, Ralph Matthew
Regulacio, Anna Rafaela
Professor, CHM171L/A11, School of Chemical Engineering, Chemistry and Biotechnology,
Mapua Institute of Technology;
Student, CHM171L /A11 , School of Chemical Engineering,
Chemistry and Biotechnology, Mapua Institute of Technology
This experiment is on the topic of photochemistry. Photochemistry is the study of the
interactions between light and
. It describes
proceed with the absorption of light. To visualize the effect of light on chemical reactions is
the goal of the experimentation. Photochemical reactions require a light source that emits
wavelengths corresponding to an electronic transition in the reactant. It can be noticed
here that the area exposed to the sunlight turned blue while the covered portions became
Photochemistry, a sub-discipline of chemistry, is the study of the interactions between light
and atoms or molecules.
Photochemistry describes chemical reactions that proceeds with
the absorption of light. Everyday examples include the degradation of plastics and the
formation of vitamin D in sunlight.
Light is just another term for electromagnetic radiation, a source of energy. The first law of
photochemistry, known as the Grotthuss–Draper law (for chemists Theodor Grotthuss and
John W. Draper), states that light must be absorbed by a chemical substance in order for a
photochemical reaction to take place.
The second law of photochemistry, the Stark-Einstein law, states that for each photon of
light absorbed by a chemical system, only one molecule is activated for a photochemical
reaction. This law, also known as the photoequivalence law, was derived by Albert Einstein
at the time when the quantum (photon) theory of light was being developed.
Many chemical reactions occur only when a molecule is provided the necessary "activation
energy". A simple example can be the combustion of gasoline (a hydrocarbon) into carbon
dioxide and water. In this reaction, the activation energy is provided in the form of heat or a
spark. In case of photochemical reactions light provides the activation energy.
Simplistically, light is one mechanism for providing the activation energy required for many
reactions. If laser light is employed, it is possible to selectively excite a molecule so as to
produced a desired electronic and vibrational state. Equally, the emission from a particular
Group 8 │ September 16, 2010