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 (ν0).
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 (VS ) 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: 1
2
𝑚𝑢2
= 𝑒𝑉𝑆
(in which m, u, and e are the mass, speed, and charge of an electron, respectively)
It was found that 𝑉𝑆 is proportional to the frequency of the incident light but independent
of the light intensity 𝑉𝑆 ∝ 𝜐 − 𝜐0
𝑉𝑆 = 𝑘 𝜐 − 𝜐0
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
ℎ𝜈0 = 𝑒𝑉0
𝜈0 =
𝑒𝑉0
ℎ

5. When a photon of energy hν strikes an electron, the electron overcomes the work function
eV0 and is liberated with kinetic energy
1
2
𝑚𝑢2, Thus, by the law of conservation of energy,
we have
1
2
𝑚𝑢2
+ 𝑒𝑉0 = hν
𝑒𝑉𝑆 =
1
2
𝑚𝑢2 = hν − 𝑒𝑉0
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 , O3, from oxygen molecules, O2,
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 O2, to form O3.
𝑂2 + ℎ𝜈 → 𝑂 + 𝑂
𝑂2 + 𝑂 + 𝑀 → 𝑂3 + 𝑀+
In the second reaction, a third body, M, such as N2 (g), is needed to carry away excess
energy to prevent immediate dissociation of O3 molecules

7. Example
For radiation of wavelength 242.4 nm, the longest wavelength that will bring about the
photodissociation of O2 , 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
𝜈 =
𝑐
𝜆
=
2.998 𝑥 108 𝑚 𝑠−1
242.4 𝑥 10−9 𝑚
= 1.237 𝑥 1015 𝑆−1
Then, calculate the energy of a single photon.
𝐸 = ℎ𝜈 = 6.626 𝑥 10−34 𝑥
𝐽𝑆
𝑝ℎ𝑜𝑡𝑜𝑛
𝑥 1.237 𝑥 1015 𝑆−1

8. (b) Calculate the energy of a mole of photons
𝐸 = ℎ𝜈
= 8.196 𝑥 10−19
𝑥
𝐽
𝑝ℎ𝑜𝑡𝑜𝑛
𝑥 6.022 𝑥 1023
𝑝ℎ𝑜𝑡𝑜𝑛𝑠
𝑚𝑜𝑙
𝐸 = 4.936 𝑥 105 𝐽/𝑚𝑜𝑙
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.