Question

Briefly expain how numerous vaportizations and condenstations results in a more pure distillate

Answer 1

Distillation is an important commercial process that is used in the purification of a large variety of materials. However, before we begin a discussion of distillation, it would probably be beneficial to define the terms that describe the process and related properties. Many of these are terms that you are familiar with but the exact definitions may not be known to you. Let us begin by describing the process by which a substance is transformed from the condensed phase to the gas phase. For a liquid, this process is called vaporization and for a solid it is called sublimation.Both processes require heat. This is why even on a hot day at the beach, if there is a strong breeze blowing, it may feel cool or cold after you come out of the water. The wind facilitates the evaporation process and you supply some of the heat that is required. All substances regardless of whether they are liquids or solids are characterized by a vapor pressure. The vapor pressure of a pure substance is the pressure exerted by the substance against the external pressure which is usually atmospheric pressure. Vapor pressure is a measure of the tendency of a condensed substance to escape the condensed phase. The larger the vapor pressure, the greater the tendency to escape. When the vapor pressure of a liquid substance reaches the external pressure, the substance is observed to boil. If the external pressure is atmospheric pressure, the temperature at which a pure substance boils is called the normal boiling point. Solid substances are not characterized by a similar phenomena as boiling. They simply vaporize directly into the atmosphere. Many of you may have noticed that even on a day in which the temperature stays below freezing, the volume of snow and ice will appear to decrease, particularly from dark pavements on the streets. This is a consequence of the process of sublimation. Both vaporization and sublimation are processes that can be used to purify compounds. In order to understand how to take advantage of these processes in purifying organic materials, we first need to learn how pure compounds behave when they are vaporized or sublimed.

Let's begin by discussing the vapor pressure of a pure substance and how it varies with temperature. Vapor pressure is an equilibrium property. If we return to that hot windy day at the beach and consider the relative humidity in the air, the cooling effect of the wind would be most effective if the relative humidity was low. If the air contained a great deal of water vapor, its cooling effect would be greatly diminished and if the relative humidity was 100%, there would be no cooling effect. Everyone in St. Louis has experienced how long it takes to dry off on a hot humid day. At equilibrium, the process of vaporization is compensated by an equal amount of condensation. Incidentally, if vaporization is an endothermic process (i.e. heat is absorbed), condensation must be an exothermic process (i.e. heat is liberated). Now consider how vapor pressure varies with temperature. Figure 1 illustrates that vapor pressure is a very sensitive function of temperature. It does not increase linearly but in fact increases exponentially with temperature. A useful "rule of thumb" is that the vapor pressure of a substance roughly doubles for every increase in 10 °C. If we follow the temperature dependence of vapor pressure for a substance like water left out in an open container, we would find that the equilibrium vapor pressure of water would increase until it reached 1 atmosphere or 101325 Pa (101.3 kPa, 760 mm

Hg). At this temperature and pressure, the water would begin to boil and would continue to do so

until all of the water distilled or boiled off. It is not possible to achieve a vapor pressure greater than 1 atmosphere in a container left open to the atmosphere. Of course, if we put a lid on the container, the vapor pressure of water or any other substance for that matter would continue to rise with temperature until the container ruptured. Elevation of the boiling point with increase in external pressure is the principle behind the use of a pressure cooker.