Hack tech fact

 Packing of particles in crystal: In a crystal the constituent particles are closely packed. If maximum available space is occupied by the constituent particles, a crystal is said to be closely packed and have maximum density and stability. The close packing, however depends on the shape and size of the constituent particles in close packing. Assuming the constituent particles to be hard spheres of equal size.

 Bragg's model of X ray diffraction : According to Bragg's, a crystal which is made up of series of equally spaced atomic planes, can be treated as a transmission grating and a reflection grating as well. When X-rays are incident on a crystal face, some of the rays will pass undeflected and some other rays will penetrate into the crystal and strick the atoms in successive planes. From each of these plans the X-rays are reflected (i.e., X-rays strick with electrons in the atoms of the structural units and undergo a change in direction), in all directions. For the formation of an intense diffraction pattern in any direction the condition is that the angle of incidence should be equal the angle of reflection of the beam for the direction studied.               Bragg's equation can be applied either to the reflection or the diffraction experiment. In the case of reflection from the crystal surface, the angle ∅ stands for the angle between the crystal surface and the duration of

Nature of diffraction patterns : The big spot in the centre of the pattern corresponds to the unscattered beam. The other spots represent the scattered beam through different characteristics angles. The spreading of X-rays from each lines of atoms in the crystal gives rise to region of various intensities. Diffraction patterns depends on the symmetry of the crystals, and helps in determining the crystal structure.

Structure of simple ionic crystals:   In ionic crystals lattice positions are occupied by positive and nagatives ions in equivalent amounts. In such crystals positive ions are surrounded by negative ions and vice-versa. Since the coulombic forces holding the oppositely charged ions together in an ionic crystal are non directional, the arrangement of ions in the crystal is largely controlled by the sizes and charges of the ions concerned. Normally, each ion is surrounded by the largest number of oppositely charged ions. The number of oppositely charged ions surrounding an ion is the co ordination number of the ion and is related to the relative sizes of the positive and nagative ions. We will discuss below the different types of ionic compounds, depending upon the relative number of positive and nagative ions present in them.

 Imperfections or Defects in crystalline solids : In an ionic crystal the particles are well orderly arranged in a regular pattern. An ideal crystal is that which has same until cell containing the same lattice points through out the whole crystal. But, such ideal crystal only exists at zero Kelvin temperature i.e., entropy of it's particle is zero. It means that there is no movement of the constituent particles at 0K. But, none of the crystals are basically ideal and it suffers certain defect at temperature above 0K. This defect may arise due to some irregularities in the arrangement of constituent particles in the crystal lattice and are called imperfections or crystal defects. These are of two types. Point defects:                 These defect are due the irregularities in the arrangement of atoms around a point or an atom in a crystalline solid. These are also called atomic defects. Line defects:                   These defects are due to the irregularities in the arrangement o

Rutile structure: The radius radio is in the range 0.73 to 0.41. in rutile (TiO²) each Ti⁴+ ion is octahedrally surrounded by six O²- ions whereas each other O²- ion is surrounded by only three Ti⁴+ ions arranged at the tree corners of a plane triangle. The Co-ordination number of Ti⁴+ and O²+ ions are 6 and 3, respectively. It is not exactly a close packed structure. Ti⁴+ ions in rutile may however, be considered as forming a sufficiently distorted body centred cubic lattice.           Example  compounds  having radius ratio below 0.41 are SiO² and BeF² but these are only a few. The Co-ordination number of Si⁴+ and Be²+ is four and that of O²- is two. However, these are appreciably co-valent.

 Fluorite structure :  In CaF², the radius radio is 0.732 which gives rise to a body - centred cubic (bcc) structure. In this case each Ca²+ ion is surrounded by eight F- ions, so the co-ordination number of Ca²+ is 8. Since in an ionic compound containing different numbers of cation and anions, the cation and anion have different co-ordination numbers, it follows that the co ordination number of F- ion is four (since the number of F- ions is double the number of Ca²+ions). Thus, CaF² has 8 : 4 arrangements. In fluorite, Ca²+ ions are too small to touch each other hence, the structure is not strictly close packed structure. 

 Results of Schottky defects:  Because of missing of ions, the ionic compounds show following changes in the properties of ionic crystals.  1- The density of the crystal decreases. 2- The stability and lattice energy of crystal decreases. 3- The electrical conductivity of the crystal increases. It is because when electricity is applied, the ions move to the vacant place and this process continues in the whole crystal lattice resulting in the increase of electrical conductivity of crystals. Alkali metal halides such NaCl,KCl,KBr, CsCl etc. Normally show this defect.

Results of Frenkel defect:    1- It increase the electrical conductivity of the crystal because of the presence of vacancy in crystal lattice.  2- It decrease the stability or lattice energy of the crystal. 3- This defect does not change the density of crystal because the number of ions per unit volume remain same in the crystal. 4- It increase the dielectric constant of the crystalline solid as the similarly changed ions come closer. This defects are generally found in (i) AgCl,AgBr,Agl etc. It is because Ag+ ion being small in size occupy the interstitial sites leaving its own position in crystal lattice (ii) Further this defect is also noticed in ZnS crystal because of small size of Zn²+ ion which can fit in its own interstitial sites. (iii) Alkali metal halides don't exhibit this defect because of large size of alkali metal ions.

Position of nobel gases in the periodic table: The inert gases were not discovered at the time when Mendeleef gave his periodic table. He also could not imagine the presence of such elements devoid of chemical reactivity and thus left no place for these elements in his periodic table. As these elements are chemically inert, they should be placed in between the highly electronegativity halogens (VIIA) and highly electropositive alkali metals (IA), I.e., in the zero group. Further, it has been found that all these elements have fully filled stable electronic configuration, i.e., they have no tendency either to lose, gain or share electrons with the atoms of other elements. In other words, their valency is zero. Therefore, they have assigned zero group in the periodic table. Zero group is also numbered as group 18  in the modern periodic table. Basing upon their electronic configuration, it has been found that expect helium all other inert gases have eight electrons in their valence shell

 Schottky defect  1- It is found in case of ionic crystals. 2- The electrical conductivity of the crystal increases. 3- The stability of the crystal decrease. 4- Equal number of cation and anions are missing from their normal lattice sites. 5- It decrease the density of the crystal. 6- Dielectric constant value remain same. 7- It is observed in case of ionic compounds having high co-ordination number. 8- these are noticed in case of ionic compounds where cation and anions are almost of equal sizes. 9- These are observed in case of alkali metal halides.

 Application of Henry's law: (I) In the soft drinks, e.g., soda water, beer, in order to increase the concentration of CO² the bottles are normally sealed under high pressure of CO² High pressure of CO² inside the bottle increases the solubility of CO² as per Henry's law. When the bottle is opened, the pressure on the surface of the liquid decreases and as a result, CO² comes out of the bottle along with liquids of drinks. (II) Deep sea divers used compressed air for oxygen. According to Henry's law, the nitrogen of air dissolves in the blood of sea divers due to high pressure when the diver is in the deep sea. When the diver comes to the surface, the pressure decreases and the nitrogen dissolved in the blood escape out thereby  causing a painful sensation called cassion sickness or bent. In order to avoid this sort of difficulties, He and O² mixture is taken in place of air which is less soluble in blood. (III) Henry's law explains the function of lungs. When air enter

Frenkel defect : (I) It is also found in case of ionic crystals.  (II) The electrical conductivity of crystal increases. (III) The stability of the crystal decreases.  (IV) Ions leave their normal position and occupy some where else in the interstitial sites in crystal lattice.  (V) The density of crystal remain same. (VI) Dielectric constant value increases. (VII) It is observed in case of compounds having low co-ordination number. (VIII) These are noticed in case of ionic compounds where cation are smaller in size than anion. (IX) These are observed in case of silver halides.

Solubility of liquid in liquid: The solubility of two different liquids depends upon the nature of force of attraction among them. For example, (I) Ethyl alcohol is soluble in water due to intermolecular H-bonding among them. (ii) Benzene and carbon tetrachloride are soluble because of van der Waals force of attraction among them.  

 Azeotropes (constant boiling liquid mixtures) : The solution which show positive deviation from Raoult's law, at one of the intermediate compositions have total high vapour pressure or low boiling point. On the otherhand, the solutions showing negative deviations from Raoult's law, for one instermediate compositions, shows lowest vapour pressure highest boiling point. For liquid pairs of such intermediate compositions, the compositions of both liquid and vapour phases is the sane. As a result, both the components present in the liquid mixture will boil at the same temperature without undergoing any change in composition.         This type of liquid mixture which boils at the same temperature without undergoing any change in composition is known as constant boiling mixture or azeotropic mixture or azeotroes.  Azeotropic liquid mixtures present in binary solution are of two type:  (I) Minimum boiling azeotropes:  These are formed in case binary liquid mixtures showing positive d

Maximum boiling azeotropes : These are formed in case of binary liquid mixtures showing negative deviations from Raoult's law. Such azeotropes have boiling point higher than either of its pure components for example, a mixture containing 68% nitric acid (boiling point 3.73 K) form a constant boiling azeotropes having boiling point. 393.5K.   

Minimum boiling azeotropes: These are formed in case binary liquid mixtures showing positive deviation form Raoult's law. For example, a liquid mixture 95.57 mass percent of ethyl alcohol and 4.43 mass percent of water represents a constant boiling azeotropes. This mixture boils at 351.1K which is less than the boiling point of both ethyl alcohol (351.3 K) and water (373 k). This is known as minimum boiling azeotrope because the partial vapour pressure of both the components are maximum. At it's boiling point, both water and ethyl alcohol distill over as if they are the constituents of pure liquid.

Membrane bombardment theory: According to this theory, Osmosis arises due to unequal bombardment of solvent molecules  on both sides of a semipermeable membrane. Since, the surface covered by solvent molecules is more in solvent side than in solution side, there are lesser bombardment per unit area of the surface on the solution side than on the solvent side. Hence, due to the unequal bombardment, the solvent molecules on the solvent side will diffuse more faster through the membrane than on the solution.

The Adsorption theory: According to this theory a membrane is permeable to the molecules which get absorbed on it. For example, water molecules (solvent) are absorbed on the surface of a membrane due to polar interactions and maintain a connection between the water molecules of both solvent side and solution side. As a result, the solvent molecules pass easily through the membrane while solute molecules can not.

Solution theory :  According to this theory, the substance which dissolves a semipermeable membrane can pass through it. For example, phenol works as a semipermeable membrane between pure water and calcium nitrate. It allows water to pass through it because it is dissolved in water other hand, on the Ca(NO³)² molecules donot pass because it does not dissolved in phenol 

Vapour pressure theory: According to this theory pure solvent molecules can pass through the semipermeable membrane when it's pressure is higher than that of its solution.

Sieve theory:    In this theory semipermeable membrane is considered as a sieve consisting of large number of fine pores. These pores allow the molecules of solvent which are similar in size through them as compared to the size of solute molecules. But this theory fails explain the cases where solute molecules are smaller in size than solvent molecules.

Potential electrolytes (or ionogens) : These are incapable of conducting electricity in molten or liquid state. On dissolving, they form ions as a result of their interaction with solvent molecules. All acids, SnCl²,AlCl³, are examples of this type. These are also referred to as pseudo - electrolytes.

True electrolytes (or ionophores) :   These are ionic in pure liquid state and their melt conducts electricity, e.g., NaCl, NaNo³, K²SO⁴ etc. When dissolved, the solvent tear off the ions from it's lattice, solvates them and makes them mobile.  

Electrolytes - Electrolytes are classified as strong electrolytes and weak electrolytes on the basis of their relative conductance. Strong Electrolytes: These substances are high conducting in dissolved or molten state and possess ionic and polar or hydrogen bonded molecular crystalline structure. Halides, hydroxides, acetates of Group I A, and II A, element, nitrates, sulphates and chlorates of monovalent and divalent cations. HCl, HBr, Hl, H²SO⁴, HClO⁴, HNO³, . Are example of this type.

Oxidation potential  (Eox)  (1)- It is the tendency of an electrode to lose electrons. It is indicated as E m/mn+. (2)- It is the opposite value of reduction potential. Ex. Eox Of Cu is -0.34V  (3)- Oxidation potential of an electrode                        1                ___________                conc.of ions.

 Electrode potential (Emn+/m) : Cell potential (Ecell): Electrode potential- It is generally the reduction potential of the electrode when it is in contact with the solution containing ions of the electrode material  Cell potential- It is the difference of the reduction potentials of the two electrodes placed in contact with their respective electrolytic solutions in the two half cells i.e. E cell=  Ecathode -   Eanode

Ionic conductance: The conductance of an electrolytic solution depends upon the conductances of the ions forming the electrolyte. We know that the molar     (or. equivalent) conductance of an electrolyte solution increase with dilution and at infinite dilution it approaches a maximum limiting value. At infinite dilution, an electrolyte dissociates completely into ions and each ion contributes it's definite share to the total molar (or, equivalent) conductance of the electrolyte. In other words l, the molar (or, equivalent) conductance of an electrolyte solution is an additive property, I.e., the sum of the conductances of the ions (called the molar (or, equivalent) ionic conductances) in the solution.

Reduction potential (Ered)   1- It is a tendency of an electrode to gain electron. It is indicated as E mn+/M . 2- It is the opposite value of oxidation potential. Ex. Ered of Cu is + 0.34V. 3- Reduction potential of an electrode conc. Of ions.

 Electromotive force: 1- It is the difference between electrode potentials of two electrodes. 2- It is measured by a potentiometer. 3- EMF is the maximum voltage obtained from cell. 4- It is responsible for the flow of steady current.

Potential deference: 1- It is the actual difference in potential of the two electrodes when they are connected externally and current is flowing through the circuit . 2- It is measured by a voltameter. 3- It is less than EMF (It is because EMF) is the limiting potential difference at zero current with no consumption of energy. 4- It is not responsible for the flow of steady current.    

Chemical kinetics: The branch of physical chemistry which deals with the study of the rates of chemical reaction and with the description of the mechanism by which they proceed is called chemical kinetics.     The study of chemical kinetics involves (i) the study of the rate of  reaction and rate laws, (ii) the factors, such as temperature, pressure, concentration and catalyst that influence the of a reaction, and (iii) the mechanism in which a reaction takes place.