Important Definitions (PART-1)
This article has all the important definition terms of
NCERT Chemistry Class 12 (Unit 1 to 5) which are commonly asked in Boards as
well as in various competitive examination. It will certainly help you to score full
marks.
Unit 1: The Solid State
Crystal Lattice: It is a
regular arrangement of the constituent particles (atoms, ions or molecules) of
a crystalline solid in three dimensional space.
Unit Cell:
A unit cell may be defined as the smallest three dimensional repeating portion
of a crystal lattice.
Primitive Unit Cell: When constituent particles are present only on the corner
positions of a unit cell, it is called as primitive unit cell.
Face Centred Unit Cell: Such a unit cell contains one constituent
particle present at the centre of each
face, besides the ones that are at its corners.
Body Centred Unit Cell: Such a unit cell contains one
constituent particle (atom, molecule or ion) at its body-centre besides the
ones that are at its corners.
End Centred Unit Cell: n such a unit cell, one constituent
particle is present at the centre of any two opposite faces besides the ones
present at its corners.
Octahedral Voids: The vacant space among six spheres
having octahedral arrangement is called is called octahedral void.
Coordination Number: The number of spheres which are touching a given sphere is
called the coordination number.
Vacancy Defect: When some of the lattice sites are vacant, the crystal is
said to have vacancy defect.
Interstitial defects: When some constituent particles (atoms or molecules) occupy
vacant interstitial positions, the crystal is said to have interstitial defect.
Schottky defect: The defect arises due to missing of equal cations and
anions from the crystal is known as Schottky defect.
Frenkel defect: It arises when an ion is missing from its normal position
and occupies an interstitial site between the lattice points.
F-Centre: The colour centre of the crystal due to trap of free
electrons in anion vacancies is called as f-centre.
Diamagnetism: The substances which are weakly repelled by the magnetic
field are known as diamagnetic substances and this phenomenon is known as
diamagnetism.
Paramagnetism: The substances which have permanent dipoles and are
attracted by the magnetic field are known as paramagnetic substances and this
phenomenon is known as paramagnetism.
Ferromagnetism: The substances which are strongly attracted by the magnetic
field and show permanent magnetism even when the magnetic field is removed are
known as ferromagnetic substances and this phenomenon is known as
ferromagnetism.
For more detailed
notes click here.
Molarity (M): The number of moles of solute dissolved per litre of the
solution. For Mathematical expression Click here.
Molality (m): The number of moles of solute dissolved per 1000g (1Kg) of
the solvent. For Mathematical expression
Raoult’s Law: Click here
Ideal Solution: An ideal solution may be defined as the solution which obeys Raoult’s law exactly over the entire range of concentration.
Non-ideal Solution: An ideal solution may be defined as the solution which do
not obeys Raoult’s law exactly over the entire range of concentration.
Azeotropes: The binary mixtures which have same composition in liquid and vapour phase and
boil at a constant temperature are known as Azeotropes.
Colligative properties: The properties of the solutions which
depends only on the number of solute particles but not on the nature of the
solute particles are called colligative properties.
Osmosis: The phenomenon of the flow of solvent
through a semipermeable membrane from pure solvent to the solution is called
osmosis. For more details Click here.
Osmotic pressure: The excess pressure which must be
applied to a solution to prevent the passage of solvent into it through
semipermeable membrane or to stop the process of osmosis is known as osmotic
pressure.
Reverse osmosis (RO): The process of movement of solvent
through a semipermeable membrane from the solution to the pure solvent by
applying excess pressure on solution side is called reverse osmosis.
For more detailed notes click here.
Unit 3: Electrochemistry
Electrochemical cells or
Galvanic cells: The devices in
which chemical energy of spontaneous redox reaction is converted to electrical
energy are called electrochemical cells or galvanic cells. For more details Click here
Electrode potential: The electrical potential difference at the interface of
metal and its solution is known as electrode potential.
Cell potential or
Electromotive force: The difference
between the electrode potentials of the two electrodes constituting an
electrochemical cell is known as cell potential or electromotive force.
Electrochemical series: the arrangement of elements in order of increasing
electrode potential values is called electrochemical series.
Faraday’s laws of
electrolysis:
Conductance: Conductance is defined as the potential for a substance to
conduct electricity. It is the reciprocal of resistance.
Conductivity: The measure of ease at which electric current can pass
through a material. It is the reciprocal of resistivity.
Molar Conductivity: The conductivity of one molar solution is known as molar
conductivity. Mathematical expression Click here.
Kohlrausch’s law:
For more detailed notes click here.
Unit 4: Chemical Kinetics
Rate of Reaction: The rate of disappearance of reactant or rate of appearance
of product is known as rate of reaction.
Rate law:
Molecularity: The number of reacting species (molecules, atoms or ions)
which collide simultaneously to bring about a chemical reaction is called as
molecularity of a reaction.
Order of Reaction: The order of reaction is defined as the sum of the powers
to which the concentration terms are raised in the rate law equation to express
the observed rate of reaction.
Half-life period: The half-life of a reaction is the time in which the concentration of a
reactant is reduced to one half of its initial concentration.
Activation energy: The excess energy (over and above the
average energy of reactants) which must be supplied to the reactants to undergo
chemical reactions is called activation energy.
Or
The energy
required to form activated complex is called the activation energy.
For more detailed notes click here.
Unit 5: Surface Chemistry
Adsorption: The
accumulation of molecular species at the surface rather than in the bulk of a
solid or liquid is termed adsorption. The
substance on the surface of which adsorption occurs is known as adsorbent and
the substances that get adsorbed on the surface is known as adsorbate.
Adsorption isotherm: The relation between the
amount of substance adsorbed by the adsorbent and the equilibrium gas pressure
(or concentration for solutions) at constant temperature is called an adsorption
isotherm.
Catalyst: A substance which alters
the rate of reaction without itself undergoing any change in mass and chemical
composition at the end of the reaction is called as catalyst. The phenomenon of
increasing the rate of reaction by the use of catalyst is called catalysis.
Enzyme: The biological catalysts
produced by living cells which catalyze the biochemical reactions in living
organism is known as enzyme.
Shape selective catalysis: The catalytic reaction
which depends upon the pore structure of the catalyst and the size of the
reactant and product molecules is called shape selective catalysis. Zeolites
are the good example of shape selective catalyst.
Lyophilic colloids: The colloidal solutions in
which the particles of the dispersed phase have a great affinity for the
dispersion medium are called lyophilic colloids.
Lyophobic colloids: The colloidal solutions in
which the particles of the dispersed phase have no affinity for the dispersion
medium are called lyophobic colloids.
Micelles: The cluster or aggregation
of particles formed by association of colloids in solution are called as
micelles.
Peptization: The process of converting
a freshly prepared precipitate into colloidal form by the addition of a
suitable electrolyte is called peptization. The electrolytes used for the
purpose are called peptizing agents.
Tyndall effect: The phenomenon of scattering of light by colloidal particles as a result of which the path of beam become visible is called tyndall effect.
Electrokinetic potential or
Zeta potential: The
potential difference that exist between the fixed layer and diffused layer of
electrical double layer model of colloidal solution is called electrokinetic
potential or zeta potential.
Electrophoresis: The phenomenon of movement
of colloidal particles under an applied electric field is known
electrophoresis.
Electroosmosis: The phenomenon of
movement of the molecules of the dispersion medium under the influence of
electric field whereas colloidal particles are not allowed to move is known as
electroosmosis.
Coagulation or flocculation: The process of settling
of colloidal particles is called coagulation or flocculation or precipitation
of the sol.
Hardy Schulze rule: According to Hardy Schulze
rule, greater the valency of the active ion or flocculating ion, greater will
be its coagulating power.
Coagulation value or
flocculation value: The minimum concentration of an electrolyte in milimoles
which is required to cause the coagulation or flocculation of one litre of a
sol is known as coagulation value or flocculation value.
Protection of Colloids: The process of protecting
the lyophobic colloidal solutions from precipitation by the electrolytes is
called as protection of colloids.
Gold number: The minimum amount of the
protective colloid in miligrams required to just prevent the coagulation of a
10 ml of a given gold sol when 1 ml of a 10% solutioin of sodium chloride is
added to it.
Emulsion: Emulsions are the
colloidal solutions in which both the dispersed phase and the dispersion medium
are liquids.
Emulsifying Agent: The substances which are
added to stabilize the emulsions are called emulsifying agent.
For more detailed notes click here.
Definitions of rest of the Units are coming soon.
By: Satyam Kumar Nigam
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