Photochemical reactions, 8(2)

Photoelectric Effect and Photochemical Reactions. Photons of Light and Chemical Reactions. photodissociation of O2. It was found that Stopping voltage is proportional to the frequency of the incident light but independent of the light intensity

1. Photoelectric Effect and Photochemical Reactions Dr. K. Shahzad Baig Memorial University of Newfoundland (MUN) Canada Petrucci , et al. 2011. General Chemistry: Principles and Modern Applications. Pearson Canada Inc., Toronto, Ontario. Tro , N.J. 2010. Principles of Chemistry. : a molecular approach. Pearson Education, Inc.

2. photoelectric effect A phenomena in which when light strikes the surface of certain metals, electrons are ejected. The salient feature of photoelectric effect are; electron emission only occurs when the frequency of the incident light exceeds a particular threshold value ( ν ). When this condition is met , the number of electrons emitted depends on the intensity of the incident light, but the kinetic energies of the emitted electrons depend on the frequency of the light.

3. Stopping voltage (V S ) of photoelectrons as a function of frequency of incident radiation.

4. At the stopping voltage, the kinetic energy of the photoelectrons has been converted o potential energy:   (in which m , u , and e are the mass, speed, and charge of an electron, respectively) It was found that is proportional to the frequency of the incident light but independent of the light intensity       The constant k is independent of the metal used, but varies from one metal to another. The work function is a quantity of work and, hence, of energy  

5. When a photon of energy h ν strikes an electron, the electron overcomes the work function e V and is liberated with kinetic energy , Thus , by the law of conservation of energy, we have       The additional fact that the number of photoelectrons increases with the intensity of light indicates that we should associate light intensity with the number of photons arriving at a point per unit time.

6. Photons of Light and Chemical Reactions Chemical reactions that are induced by light are called photochemical reactions . The reactions by which ozone molecules, are produced , O 3 , from oxygen molecules, O 2 , are represented below For the first reaction is UV radiation of wavelength < 242.4 nm is required. O atoms from the first reaction then combine with O 2 , to form O 3 .     In the second reaction, a third body, M, such as N 2 (g), is needed to carry away excess energy to prevent immediate dissociation of O 3 molecules

7. Example For radiation of wavelength 242.4 nm, the longest wavelength that will bring about the photodissociation of O 2 , what is the energy of (a) one photon, and (b) a mole of photons of this light? Solution ( a) First, calculate the frequency of the radiation   Then, calculate the energy of a single photon.  

8. ( b) Calculate the energy of a mole of photons     when the energy of a single photon is expressed, the energy is rather small and perhaps difficult to interpret . However, the amount of energy carried by a mole of photons is something we can easily relate to. As shown above, light with a wavelength of 242.4 nm has an energy content of 493.6 kJ/ mol , which is similar in magnitude to the internal energy and enthalpy changes of chemical reactions.