PHOTOCHEMISTRY

Photochemistry is the study of reaction of molecules by absorbing radiation from the electromagnetic spectrum in the U.V and visible region and becoming electronically active. The photo physical phenomenon are fluorescence and phosphorescence.

Fluorescence
When a compound absorbs light radiation, its electrons get excited to a higher energy level. The excited electrons comes to the ground state either directly or in steps with the emission of light energy. When this emission of light is instantaneous (1o^-8 sec) , the phenomenon is known as fluorescence.
i.e; When an illuminating system emits light of wavelength different from that of absorbed light, the phenomenon is known as fluorescence and it takes place as soon as the light is absorbed and ceases as soon as the light is stopped.

• Molecules having conjugated double bond or pi-bonds give fluorescence.
• Some electron donating groups such as -OH, -NH2 etc enhances fluorescence.
• Electron withdrawing groups such as -COOH, -NO etc decreases fluorescence.
• Carboxylic groups and aromatic rings decreases fluorescence.
• Certain organic chelating agents increases fluorescence.
• Polarity of the solvent also affects fluorescence.
• As viscosity of the solvent increases, the fluorescence decreases.
• Neutral or alkaline solution of aniline exhibits fluorescence in the visible region, but in acid solution, fluorescence disappears in the visible region ad appears in the U.V region.

Phosphorescence.

When a compound absorbs light energy, the electron goes to a higher energy level from the ground state. The electron then returns to the normal position by the emission of light energy. When this emission of light is observed after some time (10^-3 sec), it is known as phosphorescence.This is usually shown by solid compounds and is also termed as slow fluorescence.

i.e; The molecules with relatively stable excited state may undergo transition to a metastable triplet state and after some time returns to the ground state by the emission of U.V or visible light. This phenomenon is known as phosphorescence.

• Phosphorescence is obtained nicely at room temperature.
• The polarity of the solvent affects phosphorescence.
• Only those molecules which absorbs U.V or visible light shows phosphorescence.
• When increased phosphorescence is required, compounds containing heavy atoms are usually incorporated into the solution.
• Organic compounds containing conjugated ring systems produce phosphorescence intensely.

SPECTROSCOPY

Spectroscopy is the study of the interaction between radiation and matter as a function of wavelength. It is also referred to as interactions with particle radiation. Particle radiation is the radiation of energy by means of fast moving sub-atomic particles known as a particle beam.


The term spectrum refers to the bands into which electromagnetic radiation can be split or resolved. By the study of spectrum, matter and energy is investigated. The study of spectroscopy deals with emission as well as absorption spectra.



Emission spectrum is produced by the emission of radiant energy by an excited atom. When an atom is thermally or electrically excited, electrons in the ground state is promoted to metastable states. When electrons from the metastable state jump to the lower energy state, some energy is released as radiation which is analysed with the help of a spectroscope.



Absorption spectrum is produced by the absorption of radiation of a certain wave length which characterise a particular functional group or a copmound. After absorption the transmitted light is analysed by a spectrometer. Dark pattern of lines corresponding to the wavelengths absorbed is obtained which is the absorption spectrum.


The types of spectroscopy depends on the physical quantity measured, ie; normaly the intensity of energy absorbed or produced.

• Electromagnetic spectroscopy involves interaction of matter with electromagnetic radiation.



• Electron spectroscopy involves interactions with electron beams.



• Mass spectroscopy involves interaction of charged species with magnetic or electric field.



• Acoustic spectroscopy involves the frequency of sound.



• Mechanical spectroscopy involves the frequency of an external mechanical stress.



• Dielectric spectroscopy involves the frequency of an external electric field.

PHOTOELECTRIC EFFECT

The phenomenon of ejection of electrons from the surface of some metals like Cs, K and Rb when light of a certain frequency strikes on it is called photoelectric effect. The emitted electrons are called photoelectrons.
Metals having very low ionization energies exhibit photoelectric effect with visible light. Other metals show this effect with more energetic radiations such as U.V light.

. Photoelectric effect is instantaneous.
· For each metal, there is a characteristic minimum frequency called the threshold frequency below which the photoelectric effect does not occur.
· The kinetic energy of the ejected electron is proportional to the frequency of incident radiation and is independent of its intensity.

If hu is the energy of the striking photon and hu0 is the minimum energy required to eject an electron then the excess energy hu-hu0 is transformed to the photo electron as kinetic energy, 1/2mv2 where m is the mass of electron and v is the velocity. Thus,
hu-hu0 = ½ mv2 = K.E of electron.
This is the Einstein’s photoelectric equation.

ELECTROMAGNETIC RADIATION

Electromagnetic radiation is associated with oscillating electric and magnetic fields which are perpendicular to each other. It is a form of radiant energy which propagates through space in the form of waves. All electromagnetic radiations can travel through vacuum with the velocity of light. Its energy increases with its frequency and decreases with its wave length.

Electromagnetic Spectrum.
The arrangement of different electromagnetic radiations in the increasing order of their wave lengths or in the decreasing order of their frequencies is known as electromagnetic spectrum.

Low energy
Low frequency /Radio / Microwaves/IR waves/Visiblelight/UV waves/X rays/Gamma rays / High energy
High frequency


Visible region consist only a small portion of the total spectrum. Its wave length ranges between 3800 A* (Violet) to 7600 A* (red).

Particle nature of Electromagnetic radiation.
Phenomenon such as diffraction and interference can be explained on the basis of wave nature of electromagnetic radiation. But the phenomenon of photoelectric effect and black body radiation cannot be explained on the basis of wave theory. These properties can be explained on the basis of particle nature of electromagnetic radiation.

ORGANIC CHEMISTRY

Organic Chemistry is the chemistry of hydrocarbons and their derivatives. Since carbon is an essential constituent of all organic compounds, organic chemistry is defined as the chemistry of carbon compounds. But, carbon compounds such as CO, CO2, carbonates, bicarbonates, metal carbides, metal cyanide, etc. are considered as inorganic because of their properties.

The reason for the existence of large number of organic compounds are:-

• Catenation - the self linking property of carbon atoms.


• Isomerism - the existence of different compounds having the same molecular formula.

Hybridisation in carbon.

In the excited state, carbon atom has four unpaired electrons to form four covalent bonds. The four valence orbitals are different, one being an s-orbital and the other three p-orbitals. These four orbitals (2s and three 2p) mix up to produce equivalent orbitals.

The process of mixing up of atomic orbitals of almost equal energies to get an equal number of orbitals of identical shapes and equal energies is known as hybridisation.

Carbon atom can have 3 types of hybridisation : - sp3, sp2 and sp.

sp3 hybridisation.

This is known as tetrahedral hybridisation. The four sp3 hybridised orbitals of carbon atom are directed to the four corners of a regular tetrahedron with an angle of 109*18' between two hybridised orbitals. The orbital has 25% s- character. eg:- methane, ethane.

sp2 hybridisation.

This is known as trigonal hybridisation. The three sp2 hybridised orbitals of carbon atom are directed to the corners of an equilateral triangle with an angle of 120*. The hybrid orbitals has 33.3% s- character. eg:- ethylene.

sp hybridisation.

This is known as diagonal hybridisation. The two sp hybridised orbitals of carbon atom are directed along a line with an angle of 180* to each other. The orbitals has 50% s- character. eg:- acetylene.

Bond length and bond energy are influenced by the type of hybridisation. As the s-character of the hybrid orbitals increases the electronegativity of the atom increases and hence the bond length decreases. Hence the bond energy increases. Therefore, sp hybrid orbitals has the shortest bond length with the highest bond energy.

NUCLEAR CHEMISTRY

Nuclear Chemistry is concerned with the study of atomic nuclei and the natural and artificial changes in it. In nuclear reactions, the elements are transformed into new elements and a large amount of energy is released.

An example of nuclear reactions are given below.
14N7 + 1n0 = 14C 6 + 1p1
This can be simplified as below.
14N7 (n, p) 14C6.

Natural Radioactivity.
This was discovered by the French scientist Henri Becquerel in 1896. All elements with atomic numbers higher than 83 is radioactive.
The spontaneous emission of radiations by atomic nuclei resulting in their disintegration is called natural radioactivity. There are three kinds of radiations:- alpha, beta and gamma.

• Alpha radiations are nuclei of helium atom. Its velocity is nearly 1/10th of light velocity. Of the three radiations it has the least penetrating power. It has the highest ionising power.
• Beta radiations consist of highly energetic electrons. Their speed is almost 90% that of light. Their penetrating power is almost 100 times that of alpha radiations. Their ionising power is nearly 1/100th of that of alpha radiation. They produce fluorescence on ZnS screen.
• Gamma radiations are electromagnetic radiations. Their penetrating power is immensely high. They have no ionising power. They produce weak fluorescence on ZnS screen.

Cause Of Radioactivity

An atom is radioactivity because of its unstable nucleus. In the nucleus there is a ratio between the neutrons and the protons, called the n/p ratio which determines the stability of the nucleus.

If there are too many neutrons, a neutron will be converted to a proton and an electron. The electron will be ejected as beta particle. If there are too many protons, either a helium nucleus is ejected or a positron is ejected.

Artificial Transmutation.

The conversion of one element into another is known as transmutation.

Artificial transmutation is carried out by bombarding an element with projectiles such as protons, neutrons, alpha particles etc. Neutrons are the best projectiles because of its neutral charge. It does not experience any repulsion from the nucleus. Accelerators such as cyclotrons are used to energise the projectiles.

Artificial Radioactivity.

The process of making a stable element radioactive by bombarding it with projectiles is called artificial radioactivity.

The new atoms formed may be stable or unstable.

27Al13 + 4He2 = 30P15 + 1n0

30P15 = 30Si14 + 0e1

Mass Defect and Binding Energy.

The difference between the sum of the masses of the nucleons and the actual mass of the nucleus is called the mass defect. The energy equivalent to mass defect is called binding energy. Higher the value of binding energy per nucleon, the more stable will be the nucleus.