crystallization

Question 1

crystallization

Answer 1

is a chemical solid-liquid separation technique in which mass transfer of a solute from the liquid solution to a pure solid crystallaine phase occurs
-crystallization is a process of formation of large crystals in pure state from their solution

Question 2

general process

Answer 2

-take a china dish and add water
-heat it
-add impure salt
-continue stirring and adding more salt until no more salt dissolve
-filtr it
-allow to cool it
-aftersome time, crystal forms

Question 3

crystal

Answer 3

-can be defined as a solid particles, which is formed by the solidification process in which structural units are arranged by a fixed geometric patern or lattice
-a discrete solid particle bounded by definite faces intersecting at definite agents, and showing certain symmetry characteritics is called crystals. The building block of the crystal is called unit cell. it is the basic unit of crystal and possesing all the properties of its pattern (solid)

Question 4

crystal lattice

Answer 4

-crystal lattice is defined as an orderly arrangement of particles in three dimensional space. The three dimensional arrangement of particle in a crystal is also known as space lattice
-the smallesr geometric portion, which repeat to build up the whole crystal, is called a unit cell
-a crystal is bounded by place surface called faces
-the angle between the two perpendiciular faces to the intersecting faces is termed as acial angle

Question 5

aixal length

Answer 5

can be defined as the distance between the centre of two atms

Question 6

crystal system

Answer 6

-all crystals are constructed from repeating unit called unit cells. All unit cells in a specific crystal are the same size and contain the same number of molcules or ions arranged in the same way
-there are seven primitive unit cells

Question 7

cubic

Answer 7

a=b=c
a=B=y=90 degree
number of lattice=3

Question 8

tetragonal

Answer 8

a=b=/c
a=b=c=90
2

Question 9

orthorhombic

Answer 9

a=/b=/c
90
4

Question 10

monodinic

Answer 10

a=/b=/c
a=b=90=/y
2

Question 11

triclinic

Answer 11

a=/b=/c
a=/b=/y90
1

Question 12

trigonal

Answer 12

a=b=c
a=B=y <120, =/90
1

Question 13

hexagonal

Answer 13

a=b=/c
a=b=90, and y120
1

Question 13

process of crystallization

Answer 13

-in chemical engineering crystalization ooccurs in a cystallizer. crystallization is therefor an aspect of percipitation, obtained through a variation of the solubility conditions of the solute in the solvent, as compared to percipitaion due to chemical reaction
-there are three major events in the process:
formation of solution
nucleaion
crystal growth

Question 13

super-saturation of solution

Answer 13

-durin process of C the first step….. i.e the concentration of the solution in the solution must be greater than its solubility
-there are various methods for this purpose depending upon how the solubility of solute varies with temp, for exampe, sodium chloride solution is super saturated only by evaporation of water, while in case of KNO3 either evaporation or cooling is used

Question 14

nuleation

Answer 14

• a step where the solute moleules dispersed in the solvent to from clusters together
  -these clusters become stable under the current oeprating conditions
  -aggregation of molecules builds larger and larger molecules, becomes a nucleas at some point
  -these stable clusters constitute the nucleai
  -howevr, when the clusters are not stable they redissolve. therefore, the clusters need to reach a critical size in order to become stable nuclei
  -it is a stage of nucleation that the atomes arrange in a defined and periodic manner that defines the crystal growth
  -nucleation is the initiation of a phase change in a small region, such as the formation of a solid crystal

Question 15

primary nucleation

Answer 15

-initial formation of a cystal where there are no other crystals present or where if there are crystals present is the system, they do not give any infleunce on the process
a- homogenous nuceation: it is not influenced in way any by solid. these solids inclue the walls of the crystallizer vessel and particles of any foregin cunstance
b- hetergenous n: this ocuurs when solid particles of foreign substaces cause an increase in the rate of nucleation that would otherwise not be seen withou

Question 16

secondary nucleation

Answer 16

formation of nuclei attributable to the influence of the existing microscopic crytals in the magma. it is attribualbe to fluid shear, the other due to collisions between already existing crystals with either a solid surface of the crystalizer or with other crystals themselves

Question 17

crystal growth

Answer 17

-once the first smalle crystal , the nucleas forms it acts as convergence point for molcecules of solute touching or adjacent to the crystal so that it increases its own dimension in a succesive layers
-in the crystallization process nuclei formation should be under control, since the number of nuclei will control the size of the crystal
-large crystals may be obtained as a sresult of slow cooling of solution
-nucleation may be inhibited by the presence of impurites especially if of high molecular weight in the solution

Question 18

collection of cystals

Answer 18

-after srystallization the solution is filtered under redued pressure
-when whole of the mother liquor is drained out, the crystalls are washed with pure cold solvent to remove adhering impurities and then are dried in an over or in vacuum desiccators
-crystal size also depend upon colvent used, for example drisionfluvin(antifungal)
-when crystallize out from benzene, chloroform and acetone, three different size of crystals are formed

Question 19

methods of preventing crystal formation

Answer 19

-use of narrow size range
-grincing the solid in the presence of dispersing fluid
-use of surface active agents which are adsorbed on the surface of cystals