Photochemical Equivalence Law Explanation on Vedantu
The photochemical equivalence law is a basic concept relating to light-induced chemical reactions, which states that for every unit of radiation absorbed, a molecule of the substance reacts. A quantum is defined as a unit of electromagnetic radiation with an energy equal to the product of a constant (or Planck's constant - h) and the frequency of the radiation, which is symbolized by the Greek letter nu (ν).
Discovery and Expressing the Photochemical Equivalence Law
In Chemistry, quantitative measures of the substances can be expressed in terms of gram moles, one single gram mole consisting of 6.022140857 × 1023 (which is an Avogadro's number) molecules. As a result, the photochemical equivalence law can be restated as follows: 6.022140857 × 1023 quanta of light are absorbed for every mole of the material that reacts.
The photochemical equivalence law will be applied to the part of a light-induced reaction, which is known to be the primary process; it means the initial chemical change, which results directly from the light absorption. In most photochemical reactions, usually, the primary process can be followed by so-called secondary processes, which are normal interactions between the reactants that do not require the absorption of light.
Such reactions resultantly do not appear to obey the one quantum - one single molecule reactant relationship. This law is further restricted to the conventional photochemical processes using the light sources with moderate intensities; high-intensity light sources like the ones used in laser experiments and flash photolysis are referred to cause the so-called bi-photonic processes; it means the absorption by a molecule of a substance of the light two photons.
Also, the photochemical equivalence law was at times referred to as the Stark–Einstein law after the German-born Physicists Albert Einstein and Johannes Stark, who independently formulated the law between the years 1908 and 1913.
Photochemical Reactions
A photochemical reaction is defined as a chemical reaction, which results when the reacting substance has been exposed to radiation. The activation of the reacting molecule can be achieved by the absorption of photons of the appropriate energy. Often, a radiation photon, which is known as quantum, is given as the primary unit of radiation of energy hv, where v is the frequency of radiation.
These particular reactions are affected by the absorption of radiation of visible and ultraviolet regions of wavelength. As per Einstein's law of Photochemical equivalence, "Each quantum of the radiation absorbed activates one molecule in the major step of a photochemical process."
The reason behind rate photochemical reactions are independent of the concentration of product and reactant is given by this law. This law also states that every molecule takes one quantum of radiation for its activation, including the subsequent chemical reaction. So, the rate of Photochemical reaction increases with increasing intensity of light and can be independent of the concentration of the product and reactant.
Einstein Law of Photochemical Equivalence
Einstein's Law of Photochemical Equivalence is a fundamental law of photochemistry, which establishes that every photon absorbed causes a single elementary reaction. The reaction can consist of the chemical transformation of the substance's molecules or in their physical excitation and the emission of the energy that is absorbed (or in the thermal energy transformation). The number N of the reacted molecules can be related to the energy E, which is absorbed by the system by the equation as:
E = Nhv = Nh \[\frac{c}{λ}\]
where 'v' is given as the frequency of radiation, 'c' is given as the velocity of light, 'λ' is given as the wavelength of the light, and 'h' is given as Planck's constant. The criterion for the applicability of Einstein's law of photochemical equivalence is typically the quantity " (the photochemical reaction's quantum yield), which is equal to the ratio of the number of reacted molecules of a given material to many light quanta absorbed. As per the law, 'γ' should be equal to unity.
From the Einstein law of photochemical equivalence, the deviations that are frequently observed in nature are explained by secondary processes usually. This law was discovered by Albert Einstein in 1912. Einstein's photochemical equivalence law is applicable to that segment of light-induced reaction which is known as the primary process; that is, the preliminary chemical amends that lead to the outcome directly from the incorporation of light. In most photochemical reactions, the above-mentioned primary process is more often than not followed by the allegedly known secondary processes that are standard interactions amid reactants that do not need to absorb light.
As a result, any of such reactions appear to not be in abeyance of the commonly known one quantum–one molecule reactant relationship. The law additionally has limitations involving the conventional photochemical process which involves the usage of light sources using reasonable intensities; high-intensity light sources on the other hand such as those that are being used in flash photolysis along with laser experiments are said to be causing the so-called biphotonic processes; i.e., the process of absorption happening throughout a molecule of a substance for two photons of light.
Photoisomerization of Azobenzene
Photoisomerization is a molecular behavior in chemistry, where photoexcitation causes the structural transition between the isomers. Both the reversible and irreversible photoisomerization reactions do exist. However, usually, the term "photoisomerization" indicates a reversible process. For rewritable DVDs, CDs, and 3D optical data storage, photoisomerizable molecules are already used in pigments. In addition, photoisomerizable molecules have recently sparked interest in molecular devices such as molecular motors, switches, and electronics.
The below figure shows the photoisomerization of azobenzene:
(Image will be uploaded soon)
Photoisomerization behavior may be roughly categorized into many classes. Two major classes of it are trans-cis (or 'E-'Z) conversion and the open-closed ring transition. Examples of the former include azobenzene and stilbene. This type of compound contains a double bond, and inversion or rotation around the double bond affords isomerization between the two states. Examples of the latter are given as diarylethene and fulgide. This compound type undergoes bond cleavage and bond creation upon irradiation with specific wavelengths of light. Still, the other class is given as the Di-pi-methane rearrangement.
Discuss the Structure of Azobenzene in Brief
The structure of the Azobenzene molecule comprises two phenyl rings that are linked to one another through an N=N double bond. This kind of bond is also known as the parent or the main compound as known in the azobenzene class of compounds.
Manipulation of Azobenzene Photoisomerization by way of Strong Light– also known as Molecule Coupling
The process of formation of hybrid light–molecule states that are known as polaritons provides a fresh plan and strategy to control the photochemistry inside the molecules. To exploit its potential to the complete limits, one has to put together a toolbox of polaritonic phenomenologies that add on the ones of standard photochemistry.
By using a method of a state-of-the-art computational photochemistry strategy accompanied by a strong molecule coupling system, here we also make known a variety of mechanisms not relating to polaritonic chemistry: coherent population oscillations amid polaritons, the process of quenching by capturing the dead-end polaritonic states, and the adjustment of the photochemical response pathway alongside quantum yields.
We will currently have our focus on azobenzene photoisomerization, which holds in itself the necessary characteristics of complex photochemical reactions like we can say the occurrence of conical intersections and reaction coordinates involving multiple internal modes. A chemically comprehensive simulation offers a structure to rationalize how the process of strong molecule coupling has its effects on the photochemistry of real molecules.
Why is the Process of Photoisomerization Considered Important?
The process of Photoisomerization helps in modifying the polarizability inside a molecule that shows the end results as changes in the linear optical properties (i.e., indices of refraction alongside absorption can be seen easily), and also to alterations occurring in the nonlinear optical properties in the second and third-order given that the isomer stops any such characteristics.
The Main Purpose of Irradiating the Azobenzene
The trans-azobenzene with no trouble isomerizes to the cis isomer by the process of irradiation of the trans isomer that has a wavelength in the range of 320–350 nm. The reaction then produced is reversible and apart from that the trans isomer can be brought back only when the cis isomer gets irradiated with light having wavelengths of 400–450 nm, or if it is heated up.
FAQs on Photochemical Equivalence Law
1. What is the Stark-Einstein Law of Photochemical Equivalence?
The Stark-Einstein Law, also known as the Law of Photochemical Equivalence or the second law of photochemistry, states that for every one quantum of light (photon) absorbed by a chemical system, only one molecule is activated for a photochemical reaction. This establishes a direct one-to-one correspondence between the number of absorbed photons and the number of activated molecules in the primary step of the reaction.
2. What are the two fundamental laws that govern photochemical reactions?
The two basic laws that govern the field of photochemistry are:
The Grotthuss-Draper Law (First Law): This law states that only the light which is absorbed by a substance can bring about a photochemical change. Light that is transmitted or reflected does not cause any reaction.
The Stark-Einstein Law (Second Law): This law provides a quantitative relationship, stating that each molecule participating in the primary step of a photochemical reaction absorbs exactly one photon of the radiation causing the reaction.
3. How is the energy absorbed by one mole of molecules calculated in a photochemical reaction?
The energy (E) of a single photon is given by the Planck-Einstein relation, E = hν = hc/λ, where 'h' is Planck's constant, 'ν' is the frequency, 'c' is the speed of light, and 'λ' is the wavelength of the radiation. According to the law of photochemical equivalence, one molecule absorbs one photon. Therefore, the energy absorbed by one mole of molecules is the energy of one mole of photons, which is called one Einstein. It is calculated as E_mole = N_A * hν, where N_A is Avogadro's number.
4. Why is the law of photochemical equivalence strictly applicable only to the primary photochemical process?
The law of photochemical equivalence applies strictly to the primary process, which is the initial step where a molecule absorbs a photon and becomes electronically excited. It does not necessarily apply to the overall reaction because the excited molecule can undergo various secondary processes. These secondary steps, such as decomposition, rearrangement, or reaction with other molecules, are essentially thermal reactions and do not follow the one-photon, one-molecule rule. These subsequent steps are what cause the overall quantum yield to deviate from unity.
5. What does the 'quantum yield' (Φ) of a reaction signify, and why is it not always equal to one?
The quantum yield (or quantum efficiency), symbolised by Φ, is a measure of the efficiency of a photochemical reaction. It is defined as the ratio of the number of molecules reacted to the number of photons absorbed.
Φ = (Number of molecules reacted) / (Number of photons absorbed)
The quantum yield is not always equal to one because of the secondary processes that occur after the primary absorption step.
Φ > 1: If the secondary processes involve a chain reaction, a single activated molecule can cause many other molecules to react, leading to a high quantum yield.
Φ < 1: If the excited molecules lose their energy through non-reactive pathways like fluorescence, phosphorescence, or deactivation by collision before they can react, the quantum yield will be low.
6. What are the key differences between a photochemical reaction and a thermochemical (or dark) reaction?
Photochemical and thermochemical reactions differ fundamentally in how they are initiated and proceed:
Source of Activation Energy: Photochemical reactions are activated by absorbing light (photons), whereas thermochemical reactions are activated by heat energy from molecular collisions.
Temperature Dependence: The rate of thermochemical reactions is highly dependent on temperature. In contrast, the rate of the primary step in photochemical reactions is largely independent of temperature.
Gibbs Free Energy (ΔG): Thermochemical reactions are spontaneous only if ΔG is negative. Photochemical reactions can occur even if ΔG is positive because the energy from light can drive a non-spontaneous process.
7. Provide an example of a reaction that illustrates the principles of photochemical equivalence.
A classic example is the decomposition of hydrogen iodide (HI). The overall reaction is 2HI + light (hν) → H₂ + I₂. The process involves:
Primary Process: An HI molecule absorbs one photon to get activated or dissociate: HI + hν → H + I. This step strictly follows the law of photochemical equivalence.
Secondary Processes: The resulting atoms react further with other HI molecules: H + HI → H₂ + I, and I + I → I₂.
Because of the secondary reactions, one initial photon absorption leads to the decomposition of two HI molecules, resulting in an overall quantum yield (Φ) of approximately 2 for this reaction.
