[Chemistry Class Notes] on Vaporization Pdf for Exam

Vaporization can be defined as the method in which the state of the liquid transforms into the state of the vapor. The kinetic energy of the molecules increases as a result of an increase in temperature. The force of attraction between the molecules decreases as a result of the rise in kinetic energy. As a consequence, in the form of vapors, they escape into the vicinity. This approach requires heat energy.

 

Factors affecting the Rate of Vaporization

• The concentration of an evaporating material in the air 

• Air Rate of Flow – This is partly connected to the above concentration points. If ‘fresh’ air (i.e. air that is not already filled with a substance or other substances) is continuously flowing over the substance, the concentration of the substance in the air is less likely to increase with time, thus facilitating faster evaporation. This is the effect of decreasing the boundary layer at the evaporation surface with the velocity of flow, decreasing the distance of diffusion in the stagnant layer.

• The amount of minerals in the liquid that have been dissolved 

• Intermolecular Forces – The greater the forces in the liquid state that hold the molecules together, the more energy one has to get to escape. This is described by the vaporization enthalpy. 

• Pressure – When there is less effort on the surface to prevent the molecules from launching themselves, evaporation occurs faster. 

• Area Surface – As there are more surface molecules per unit of volume that are theoretically able to escape, a material that has a greater surface area can evaporate quicker. 

• The Substance’s Temperature- The higher the temperature of the material, the greater the surface kinetic energy of the molecules, and the faster the rate of their evaporation.

 

Two Modes of Vaporization Exist: Evaporation and boiling. Evaporation is a surface phenomenon while boiling is a bulk phenomenon.

 

Evaporation

Evaporation is a phase transition from the liquid phase to a vapor (substance state below critical temperature) which occurs at a given pressure at temperatures below the boiling temperature. On the surface, evaporation occurs. Evaporation occurs only when a substance’s partial vapor pressure is less than the equilibrium vapor pressure. For example, vapor pumped out of a solution would eventually leave behind a cryogenic liquid due to continuously decreasing pressures.

 

Applications

• Many printing and coating processes include industrial applications; recovering salts from solutions; and drying a range of materials such as lumber, paper, fabric and chemicals. 

• A typical preparatory step for many laboratory tests, such as spectroscopy and chromatography, is the use of evaporation to dry or concentrate samples. Rotary evaporators and centrifugal evaporators are systems used for this purpose. 

• Even if the ambient temperature is below the boiling point of water, when clothes are hanging on a laundry line, water evaporates. Factors such as low humidity, heat (from the sun), and wind accelerate this. 

 

Boiling

Boiling is  a phase change from the liquid phase to the gas phase, but boiling is the formation of vapor below the liquid surface as bubbles of vapor. Boiling takes place when the substance’s equilibrium vapor pressure is greater than or equal to the ambient pressure. The boiling temperature, or boiling point, is the temperature at which boiling occurs. The boiling point varies with the environment’s pressure.

 

Applications 

• Air Conditioning and Refrigeration – By compressing a gas so that it becomes liquid and then allowing it to boil, most forms of refrigeration and some kind of air conditioning function. This absorbs heat from the atmosphere to cool the refrigerator or freezer or to cool the air entering a house. Propane, ammonia, carbon dioxide or nitrogen are among the usual liquids. 

• To Make Potable Water – As a method of disinfecting water, it is the oldest and most efficient way to bring it to its boiling point at 100 ° C (212 ° F), because it does not affect the taste, it is effective despite pollutants or particles present in it, and is a single step process that removes most microbes responsible for causing intestinal disease.

 

Examples of Vaporization

• Industrially, salt is recovered by the vaporization process from sea-water. 

• The process of vaporization allows wet clothes to dry up. 

• The method is used for the isolation of the components of a mixture in many manufacturing processes.

 

Did You Know?

Sublimation is an endothermic process which occurs in its phase diagram at temperatures and pressures below the triple point of a substance, corresponding to the lowest pressure at which the substance can exist as a liquid. Deposition or sublimation, in which a substance passes directly from a gas to a solid level, is the reverse sublimation process. Sublimation has also been used as a generic term to describe a transition from solid to gas (sublimation) followed by a transition from gas to solid (deposition).

High Heat of Vaporization

The high heat of vaporization is defined as the heat required to convert 1 gram of liquid into the gaseous state. As the temperature rises, the hydrogen bonds present in the liquid water start to break. Water has the highest known heat of vaporization.  Whenever heat is provided to the water, molecules start getting dismissed and they vaporize. The surface molecules get cool and hence More kinetic energy is required to evaporate water and hence water has the highest heat of vaporization. 

Whenever a liquid in a closed container is heated, the liquid which is getting converted into a gaseous state, through evaporation is unable to escape. This process continues until there are as many molecules as the liquid state.  At this point, the vapor is said to be saturated vapor, and the pressure associated with it is saturated vapor pressure. 

Summary

• Vaporization helps to get converted from liquid to a gaseous state.

• There are two types of vaporization: boiling and evaporation.

• Vaporization requires more energy as more effort is required to convert the liquid state into the gaseous state.

• The graph of vaporization decreases with temperature and at a critical point called Critical temperature, it diminishes.

• Condensation is the reverse process of vaporization.

• Nils Wallerius was the first person to make some studies on evaporation and vaporization of liquids using some scientific methods.

Questions that can be asked in the Exam:

1.  Explain the factors affecting vaporization.

2.  What is the high heat of vaporization?

3.  Differentiate between boiling and evaporation.

4.  Differentiate between vaporization and boiling.

5.  What is sublimation? Give an example.

6. What is vaporization and explain the process.

7. At what temperature water does not evaporate?

8. Why does water record the highest heat of vaporization?