Sky Of Chemistry

Fluorescence  When a beam of light is incident on certain substances, they emit visible light or on radiations and they stop emitting light or radiations and they stop emitting light or radiation as soon as the incident light is cut off. This phenomenon is known as fluorescence . The substance that exhibit fluorescence are known as fluorescent substances. When a substance absorbs light energy, electrons move from inner orbits to outer orbits. When these excited electrons return to the ground state, the light is emitted. This emitted light may possess a different frequency than the incident light. X (Normal state) + hv (Photon) ⟶ X* (Excited state) X* ⟶ X** + hv' X** ⟶ X + hv" Where X** represents an energy state between X and X*. The emitted radiation frequencies v' And v" are different from v, the frequency of incident radiation. Some Characteristics of fluorescence: (i) This phenomenon is instantaneous and starts immediately after the absorption of light and stops ...

Luminescence Usually, light is obtained by heating solids or solid particles to high temperatures. By heating a tungsten(W), platinum wire or carbon filament electrically or in a flame, bright light can be obtained. The phenomenon in which thermal energy is converted into the light is known as 'incandescence'. When the emission of visible radiation occurs due to some cause other than temperature, the phenomenon is known as 'Luminescence'. Light is produced at low temperatures, so it is also known as cold light. In both incandescence and luminescence, light is emitted due to the return of electrons from the exciting position to a lesser excitation position or ground state.  Classification of Luminescence 1. Photoluminenscence :  Luminescence caused by light is called photoluminescence. Photoluminescence that stops immediately as soon as the incident light is cut off is called fluorescence. Photoluminescence that continues for an appreciable time even after the incident l...

Factors Affecting Quantum yield [1] Temperature :   The change of quantum yield with temperature is given by the equation d(logΦ)/dT = Q/RT 2 Where Φ is quantum yield and Q is the amount of heat evolved by the formation of a one-mole substance. Photochemical decomposition of ammonia increases by about 15% for every 100ํ ℃ temperature increase. It reaches seven-fold by 500 ℃. [2] Wavelength :   As frequency increases, the power of light energy increases, so quantum yield increases. In other words, the quantum yield decreases as wavelength increases. [3] Light Intensity: As light intensity decreases, quantum yield increases. [4] Inert gases: The addition of inert gases increases the quantum yield. This is because inert gases increase the number of starting chains as they may retard the diffusion of the atoms to the walls. This effect will decrease the rate of the chain-terminating steps. Thus, the rate of reaction increases thereby quantum yield increases.

Class - I: High quantum yield reactions. Reactions in which quantum yield is greater than unity are high quantum yield reactions. The quantum yield of the combination of carbon monoxide and chlorine by the light of a wavelength from 4000 to 4360 Å is 10 3 . CO + Cl 2 ➝ COCl 2 The quantum yield of the combination of hydrogen and chlorine by the light of wavelength less than 4500 Å is 10 4 to 10 6  H 2 + Cl 2 → 2HCl Photochemical decomposition of H 2 O 2 by a wavelength of about 3100 Å has a quantum yield of more than 7. 2H 2 O 2 → 2H 2 O + O 2    Class - II: Low quantum yield reactions. Reactions in which quantum yield is less than unity are low quantum yield reactions. The quantum yield of the combination of hydrogen and bromine to form HBr is about 0.01 H 2 + Br 2 → 2HBr The quantum yield of photolysis of ammonia by a wavelength 2100 Å is nearly 0.2 2NH 3 → N 2 + 3H 2 Class - III: Small integer quantum yield reactions. Reac...

A photochemical reaction involves two processes. One is the  primary process in which light is absorbed by molecules to give excited molecules. Another is a  secondary process in this process exciting molecule initiates a series of thermal reactions. In such reactions, many reactant molecules may undergo chemical change by absorbing one quantum only. In some cases, the activated molecule undergoes deactivation. Thus, less than one molecule may react per quantum. The overall result of the photochemical reaction is expressed in terms of Quantum yield.  The quantum yield or efficiency(Φ) is defined as: " It is the number of molecules which undergo chemical transformation per quantum of absorbed energy. " Mathematically, Φ = No. of molecules reacting in a given time/ No. of quanta absorbed in the same time Φ = No. of moles reacting in a given time/ No. of Einsteins absorbed in the same time Φ = Rate of chemical reaction/ No. of Einsteins absorbed  The quantum yields ar...

Laws of Photochemistry (1) Grotthuss-Draper law (First Law of Photo-chemistry): Only the light which is absorbed by a molecule can be effective in producing photochemical changes in the molecule. "When light y on any substance, only the fraction of incident light which is absorbed by the substance can bring about a chemical change, reflected and transmitted light do not produce any such effect." This law is purely qualitative. It doesn't give any relationship between the amount of light absorbed by a system and the number of molecules reacted. (2) Stark-Einstein's Law (Second Law of Photo-chemistry): "It states that for each photon of light absorbed by a chemical system, only one molecule is activated for a photochemical reaction." Einstein applied quantum theory to photochemical reactions and gave the law. " When an atom or molecule absorbs light of a given frequency, it absorbs one quantum only" The energy absorbed by one mole of the reacting mol...

When light is incident upon a homogeneous medium, a part of the incident light is reflected, a part is absorbed and the rest is transmitted. I 0 = I a + I t + I r                                                                        ...........................(1) Where, I 0 = Incident light             I a = Absorbed light             I t = Transmitted light             I r = Reflected light. Ir is very small, so eq.(1) becomes, I 0 = I a + I t                                                                    ......................(2) Lambert's...

Thermochemical(Dark) Reactions Photochemical Reactions (1) It involves absorption or evolution of heat. (1) It involves absorption of light. (2) It can occur in dark as well as in light. (2) Presence of light is a must. (3) Temperature has a significant effect on the rate of reaction. (3) Temperature has very little effect on the rate of reaction. (4) These reactions are always accompanied by a decrease in free energy. In these reactions,   free energy may increase or decrease. (5) Activation energy is obtained by intermolecular collisions. (5) Activation energy is obtained by a particular type of radiation. (6) Thermal excitation increases in a random manner. i.e. translational, rotational and vibrational energy of all the molecules increases. (6) By selecting a particular type of radiation particular at...

Introduction Photochemistry is the study of chemical effects produced by light radiations ranging from 2000 to 8000 Å wavelength. Type of Chemical Reaction: A chemical reaction is one in which, the identity of molecules is changed due to the breaking and formation of chemical bonds. There are two types of chemical reactions. (i) Dark or Thermal reaction: The reaction which is influenced by temperature, the concentration of reactants, catalyst etc. except light radiations are known as thermal reactions. N 2 + 3H 2 ⇌ 2NH 3 H 2 + I 2     ⇌ 2HI (ii)Photochemical Reactions: The reaction which is influenced by the action of light is known as photochemical reactions.  Some examples are following, 2HBr ⟶ H 2 Follow us on Social media for the latest information about the blog.👇👇