CRYSTALLIZATION BY: TAHSEEN ISMAIL. Crystallization Definition: “Crystallization is the (natural or artificial) process of formation of solid crystals.

CRYSTALLIZATION CRYSTALLIZATION

BY:BY:

TAHSEEN ISMAILTAHSEEN ISMAIL

CrystallizationCrystallization

Definition:Definition:

““CrystallizationCrystallization is the (natural or is the (natural or artificial) process of formation of solid crystals artificial) process of formation of solid crystals precipitating from a solution, melt or more rarely precipitating from a solution, melt or more rarely deposited directly from a gas.” deposited directly from a gas.”

““Crystallization is also a chemical solid-Crystallization is also a chemical solid-liquid separation technique, in which mass liquid separation technique, in which mass transfer of a solute from the liquid solution to a transfer of a solute from the liquid solution to a pure solid crystalline phase occurs.”pure solid crystalline phase occurs.”

Crystal:Crystal: “ “A A crystalcrystal or or crystalline solidcrystalline solid is a solid is a solid

material, whose constituent atoms, molecules, or material, whose constituent atoms, molecules, or ions are arranged in an orderly repeating pattern ions are arranged in an orderly repeating pattern extending in all three spatial dimensions.” extending in all three spatial dimensions.”

““The scientific study of crystals and crystal The scientific study of crystals and crystal formation is formation is crystallography.”crystallography.” “ “The process of crystal formation via The process of crystal formation via mechanisms of crystal growth is called mechanisms of crystal growth is called crystallization crystallization or or solidification.”solidification.”

TYPES OF CRYSTALS:TYPES OF CRYSTALS:

a) a) BasedBased on physical & chemical on physical & chemical properties. properties.

1) Covalent crystals1) Covalent crystals

2) Metallic crystals2) Metallic crystals

3) Ionic crystals3) Ionic crystals

4) Molecular crystals4) Molecular crystals

Covalent Crystals: Covalent Crystals: ““This is a crystal This is a crystal

which has real chemical covalent between which has real chemical covalent between all of the atoms in the crystal.” A single all of the atoms in the crystal.” A single crystal of a covalent crystals is really just crystal of a covalent crystals is really just one big molecule.one big molecule.

Covalent crystals can have extremely Covalent crystals can have extremely high melting points. high melting points.

ExampleExample: Diamond or Zinc sulphide. : Diamond or Zinc sulphide.

Metallic Crystals:Metallic Crystals:

““Individual metal atoms Individual metal atoms occur on lattice sites while the outer occur on lattice sites while the outer electrons from these atoms are able to electrons from these atoms are able to flow freely around the lattice.” flow freely around the lattice.”

Metallic crystals normally have high Metallic crystals normally have high melting points and densities. melting points and densities.

Ionic Crystals:Ionic Crystals:

““A crystal in which the A crystal in which the individual atoms don't have covalent individual atoms don't have covalent bonds between them, but are held bonds between them, but are held together by electrostatic forces.” together by electrostatic forces.”

Ionic crystals are hard and have Ionic crystals are hard and have relatively high melting points. relatively high melting points.

Example: Sodium chloride (NaCl). Example: Sodium chloride (NaCl).

Molecular Crystals:Molecular Crystals: ““A crystal in which there A crystal in which there

are recognizable molecules in the are recognizable molecules in the structure and the crystal is held together structure and the crystal is held together by non-covalent interactions like van der by non-covalent interactions like van der Waals forces or hydrogen bonding”. Waals forces or hydrogen bonding”.

They are soft and have lower melting They are soft and have lower melting pointspoints

Example: SugarExample: Sugar

b) Based on crystal system.b) Based on crystal system. 1) Triclinic1) Triclinic 2) Monoclinic2) Monoclinic 3) Orthorhombic3) Orthorhombic 4) Tetragonal4) Tetragonal 5) Trigonal5) Trigonal 6) Hexagonal6) Hexagonal 7) Cubic7) Cubic

Crystalline phases:Crystalline phases:

1) 1) PolymorphismPolymorphism is the ability of a solid to is the ability of a solid to exist in more than one crystal form. For exist in more than one crystal form. For example, water ice is ordinarily found in the example, water ice is ordinarily found in the hexagonal form Ice, but can also exist as the hexagonal form Ice, but can also exist as the cubic Ice, the rhombohedral ice. cubic Ice, the rhombohedral ice.

2) 2) AmorphousAmorphous phases are also possible phases are also possible with the same molecule, such as amorphous ice. with the same molecule, such as amorphous ice. In this case, the phenomenon is known as In this case, the phenomenon is known as polyamorphism. polyamorphism.

Mechanism:Mechanism:

It involves 2 steps.It involves 2 steps.

1) Nucleation (creation of crystalline 1) Nucleation (creation of crystalline nuclei)nuclei)

2) Crystal growth (nuclei grow in to 2) Crystal growth (nuclei grow in to crystals)crystals)

NUCLEATION:NUCLEATION:

It is the step where the solute It is the step where the solute molecule dispersed in solvent, start to molecule dispersed in solvent, start to gather into clusters, elevating solute conc. gather into clusters, elevating solute conc. in small region. in small region.

It may arise spontaneously or by It may arise spontaneously or by introduction of minute crystal of dissolved introduction of minute crystal of dissolved substance in the solution. substance in the solution.

Seeding:Seeding:““Addition of same solute in that Addition of same solute in that

solution creates crystal nuclei this process solution creates crystal nuclei this process is known as seeding.”is known as seeding.”

Theory of Miers:Theory of Miers:It postulates that a definite It postulates that a definite

relationship exists between the conc. & relationship exists between the conc. & temp. at which crystals will spontaneously temp. at which crystals will spontaneously form in an initially un seeded soln.form in an initially un seeded soln.

This form of relationship is a super solubility This form of relationship is a super solubility curve roughly parallel to and above the normal curve roughly parallel to and above the normal solubility curve. solubility curve. Such a curve may be imagined as solubility for Such a curve may be imagined as solubility for exceedingly fine particles hence have solubility exceedingly fine particles hence have solubility greater than large crystals. greater than large crystals. Solubility curve (diagram).Solubility curve (diagram).Metastable zone:Metastable zone:

““There will no appreciable There will no appreciable spontaneous nucleation. But above solubility spontaneous nucleation. But above solubility curve there is copious spontaneous nucleation.” curve there is copious spontaneous nucleation.”

‘‘E’ spontaneous crystalline solution its E’ spontaneous crystalline solution its concentration falls to that given by normal concentration falls to that given by normal solubility curve at prevailing temp. i.e, ‘G’.solubility curve at prevailing temp. i.e, ‘G’.

Solution represented at point ‘F’ in meta Solution represented at point ‘F’ in meta stable zone will be unchanged unless stable zone will be unchanged unless seeds are added and when nucleation seeds are added and when nucleation occurs the conc. will again drop to ‘G’.occurs the conc. will again drop to ‘G’.

CRYSTAL GROWTH:CRYSTAL GROWTH:

Crystal growth is the Crystal growth is the subsequent growth of nuclei that succeed in subsequent growth of nuclei that succeed in achieving the critical cluster size.achieving the critical cluster size.

nucleation and growth occurs nucleation and growth occurs simultaneous while super saturation exist.simultaneous while super saturation exist.

Super saturation:Super saturation:

““It is the driving force of crystallization It is the driving force of crystallization hence rate of nucleation and growth is hence rate of nucleation and growth is driven by the existing super saturation in driven by the existing super saturation in the solution.”the solution.”

Size of crystals:Size of crystals:Depends on conditions of crystallization Depends on conditions of crystallization

1) very small size:1) very small size:By rapid cooling with frequent stirring By rapid cooling with frequent stirring

of soln., almost saturation at their bp usually of soln., almost saturation at their bp usually avoided.avoided.2) Median size:2) Median size:

Soln. saturated about 60 -80 c.Soln. saturated about 60 -80 c.3) Large size:3) Large size:

BY evaporation the solvent may be BY evaporation the solvent may be facilitated by seeding. facilitated by seeding.

Mother liquor:Mother liquor:Liquid remaining after crop of crystals.Liquid remaining after crop of crystals.

Again used till whole crop of crystal is Again used till whole crop of crystal is obtained.obtained.

Applications of crystallization:Applications of crystallization:

1) 1) Crystal production:Crystal production:

Macroscopic crystal, tiny size crystal.Macroscopic crystal, tiny size crystal.

2) 2) Purification:Purification:

Use to obtain very pure Use to obtain very pure substances and to verify their purity.substances and to verify their purity.

thatz allthatz all