Sublimation by Waqas Siddiqe

Best Slides and Content about the Process of Sublimation Author Waqas Siddiqe

1. Sublimation PRESENTER : WAQAS SIDDIQE ROLL NO : 059

2. Sublimation: • Sublimation  is the transition of a substance directly from the  solid  to the  gas  state ,without passing through the liquid state. • Sublimation is an Endothermic process that occurs at temperatures and pressures below a substance's  triple point   in its  phase diagram which corresponds to the lowest pressure at which the substance can exist as a liquid.

3. Now, the question is that what is triple point??? • The term "triple point" was coined in 1873 by  James Thomson , brother of  lord kelvin. • the particular temperature and pressure at which the solid, liquid, and gaseous phases of a given substance are all at equilibrium with one another. •  It is that temperature and pressure at which the  sublimation curve,  fusion  curve and the  vaporisation curve meet. 

4. Principle of sublimation: Solid sublimes only when the pressure of its vapours is below that of the tripple point for that substance. This principle can passes directly from solid to vapour state without passing through the liquid phase. Hence, for the solid to sublime, the pressure of the vapours must be lower than the triple point. The soliD sublimes directly to gas.

5. Sublime: When a solid changes directly to gas (or vice versa) it is known as sublimation and the solid is said to sublime . Examples : familiar substances that sublime readily includes: • dry ice. •camphor • iodine

6. How sublimation occurs??? • Sublimation is caused by the absorption of heat which provides enough energy for some molecules to overcome the  attractive forces   of their neighbors and escape into the vapor phase. Since the process requires additional energy, it is an  endothermic  change. • The  enthalpy of sublimation  (also called heat of sublimation) can be calculated by adding the  enthalpy of fusion  and the  enthalpy of vaporization.

7. Sublimation Results in change in physical state……. The term  sublimation   refers to a  physical change  of  state  and is not used to describe the transformation of a solid to a gas in a chemical reaction. For example, the dissociation on heating of solid  ammonium chloride  into hydrogen chloride and ammonia is  not  sublimation but a chemical reaction. Similarly the combustion of candles , containing  paraffin wax , to  carbon dioxide and  water vapor  is  not  sublimation but a chemical reaction with oxygen .

8. Examples: Naphthalene: Naphthalene is a solid that sublimes at  standard atmospheric temperature  with the sublimation point at around 80°C or 176°F. At low temperature, its vapour pressure is high enough, 1 mmHg at 53°C, to make the solid form of naphthalene evaporate into gas. 

9. Dry ice: Solid  carbon dioxide  ( dry ice ) sublimes everywhere along the line below the triple point (e.g., at the temperature of −78.5 °C (194.65 K, −109.30 °F) at  atmospheric pressure , whereas its melting into liquid CO 2  can occur only along the line at pressures and temperatures above the triple point (i.e., 5.2 atm, −56.4 °C).

10. Desublimation: Desublimation  is a  phase transition  in which  gas  turns into  solid  without passing through the liquid state. • It is the reverse of  sublimation . Examples: • Formation of snow in clouds
• Soot on walls of chimneys

11. How Sublimation is done in laboratory??? • The impure substance is placed on china dish which is then heated gently. • The dish is covered with perforated filter paper over which is placed an inverted funnel. • The other surface of funnel is kept wet by covering it with wet filter paper. • The vapors rising from the solid pass through filter paper and deposit as pure solid.

12. Purpose of sublimation: Sublimation  is a technique used by chemists to purify compounds. A solid is typically placed in a  sublimation  apparatus and heated under vacuum . Under this reduced pressure, the solid volatilizes and condenses as a purified compound on a cooled surface (cold finger), leaving a non-volatile residue of impurities behind .

13. Practical Applications: Practical Applications of Sublimation are: • Sublimation  and erosion cause ablation, a process that wears down glaciers. • Sublimation  of iodine may be used to reveal latent fingerprints on paper. • Sublimation  is used to purify compounds. ... • Because dry ice  sublimates  so readily, the compound is used to produce fog effects.

14. Pharmaceutical applications: • This process is also employed to purify volatile solids contaminated with non-volatile impurities. • Helpful in separating volatile substances from non- volatile solids. • This process is used in the purification of many pharmaceutical substances such as such as iodine, camphor, naphthalene, benzoic acid, mercuric chloride, ammonium chloride are purified by this process. • Preparation of highly porous gastroretentive tablet .

15. Pharmaceutical applications • Sublimation is preferred over crystallization for < 100mg purification as there is little loss of material. • Determine mass, % yield, and mp for unknown characterization