To use the nomograph given the normal boiling point, simply place a straight edge at on the temperature in the central column of the nomograph b.
Rotating the straight edge about this temperature will afford the expected boiling point for any number of external pressures. Simply read the temperature and the corresponding pressure from where the straight edge intersects the first and third columns.
Using the nomograph in Figure 2 and this temperature for reference, rotating the straight edge about this temperature will afford a continuous range of expected boiling points and the required external pressures necessary to achieve the desired boiling point. Although all of us have brought water to a boil many times, some of us may have not realized that the temperature of pure boiling water does not change as it distills.
This is why vigorous boiling does not cook food any faster than a slow gentle boil. The observation that the boiling point of a pure material does not change during the course of distillation is an important property of a pure material. The boiling point and boiling point range have been used as criteria in confirming both the identity and purity of a substance. Of course, additional criteria must also be satisfied before the identity and purity of the liquid are known with certainty.
You will use both of these properties later in the semester to identity an unknown liquid. Occasionally, mixtures of liquids called azeotropes can be encountered that mimic the boiling behavior of pure liquids. These mixtures when present at specific concentrations usually distill at a constant boiling temperature and can not be separated by distillation.
The azeotropic composition sometimes boils lower the than boiling point of its components and sometimes higher. Mixtures of these substances at compositions other than those given above behave as mixtures.
Returning to our discussion of boiling water, if we were making a syrup by the addition of sugar to boiling water, we would find that the boiling point of the syrup would increase as the syrup begins to thicken and the sugar concentration becomes significant. Unlike pure materials, the boiling point of an impure liquid will change and this change is a reflection of the change in the composition of the liquid. In fact it is this dependence of boiling point on composition that forms the basis of using distillation for purifying liquids.
We will begin our discussion of distillation by introducing Raoult's Law, which treats liquids in a simple and ideal, but extremely useful manner. Figure 3. The apparatus used in a simple distillation. Note the position of the thermometer bulb in the distillation head and the arrangement of the flow of the cooling water. This relationship as defined is capable of describing the boiling point behavior of compound A in a mixture of compounds under a variety of different circumstances.
Although this equation treats mixtures of compounds in an oversimplified fashion and is not applicable to azeotropic compositions, it does give a good representation of the behavior of many mixtures. Let's first consider a binary system 2 components in which only one of the two components is appreciably volatile. Raoult's law states that the observed vapor pressure of water is simply equal to the product of the mole fraction of the water present and the vapor pressure of pure water at the temperature of the mixture.
Once the sugar-water mixture starts to boil, and continues to boil, we know that the observed vapor pressure of the water must equal one atmosphere. Water is the only component that is distilling. Since the mole fraction of water in a mixture of sugar-water must be less than 1, in order for the observed vapor pressure of water to equal one atmosphere, must be greater than one atmosphere. As the distillation of water continues, the mole fraction of the water continues to decrease thereby causing the temperature of the mixture to increase.
Remember, heat is constantly being added. If at some point the composition of the solution becomes saturated with regards to sugar and the sugar begins to crystallize out of solution, the composition of the solution will become constant; removal of any additional water will simply result in the deposit of more sugar crystals. During the course of the distillation, the water vapor which distilled was initially at the temperature of the solution.
Suspending a thermometer above this solution will record the temperature of the escaping vapor. Cooling below this temperature will cause most of the vapor to condense to a liquid. This is why the distillate is frequently chilled in an ice bath during the distillation.
The distillation of a volatile material from non-volatile is one practical use of distillation which works very well.
However, often there may be other components present that although they may differ in relative volatility, are nevertheless volatile themselves. Let's now consider the two component system you will be using in the distillations you will perform in the laboratory, cyclohexane and methylcyclohexane. The vapor pressures of these two materials in pure form are given in Table 1.
As you can see from this table, although cyclohexane is more volatile than methylcyclohexane, the difference in volatility between the two at a given temperature is not very great. This means that both materials will contribute substantially to the total vapor pressure exhibited by the solution if the distillation is carried out at 1 atmosphere. The total pressure, P T , exerted by the solution against the atmosphere according to Dalton's Law of partial pressures, equation 2, is simply the sum of the observed vapor pressures of cyclohexane, , and methylcyclohexane, :.
As before, boiling will occur when the total pressure, P T , equals an atmosphere. However since we have two components contributing to the total pressure, we need to determine the relative contributions of each.
Again we can use Raoult's Law but we need more information about the system before we can do so. In particular we need to know the composition of the solution of cyclohexane and methylcyclohexane.
For ease of calculation, let's assume that our original solution has equal molar amounts of the two components. What we would like to determine is whether it would be possible to separate cyclohexane from methylcyclohexane by distillation.
By separation, we would like to determine if it would be possible to end up with two receiver flasks at the end of the experiment that would contain mainly cyclohexane in one and mainly methylcyclohexane in the other. It is clear that at some point we will need to intervene in this. Table 1. Vapor pressures of cyclohexane and methyl cyclohexane as a function of temperature. Otherwise, if we were to collect the entire contents of the original distilling flask, called the pot, into one receiver flask, we would end up with the same composition as we started.
Initially the mole fractions of both cyclohexane and methylcyclohexane are 0. From Raoult's Law equation 1 , Dalton's Law equation 2 and the information in Table 1, we can estimate that boiling will occur at approximately K when the total pressure of the two components equals one atmosphere or The first thing that we should note is that the initial boiling point is higher than the lowest boiling component and lower than the highest boiling component.
Next, we should inquire about the composition of the vapor. Is the composition of the vapor the same as the initial composition of the pot or is it enriched in the more volatile component? If the composition of the vapor is the same as that of the original mixture, then distillation will not be successful in separating the two components. However, we should ask, "What is the composition of the vapor? First we note that:. If the total vapor can be treated as an ideal gas, then according to Dalton's Law, so can each of the components.
Since the two components are in thermal contact and are distilling together, we can expect them to be at the same temperature. We don't necessarily know the volume of the container, but since it is assumed that the volumes of the molecules are very small in comparison to the total volume the gas occupies, whatever the value of V, it is the same for both components. This means we can establish the following ratio:. If we use the experimental values found in Table 1, we conclude that the composition of the vapor is 1.
This simple treatment allows us to understand the principles behind distillation. However it is important to point out that distillation is far more complex than our simple calculation indicates. For example, we just calculated the composition of the vapor as soon as the solution begins to boil and we have correctly determined that the vapor will be enriched in the more volatile component.
This means that as the distillation proceeds, the pot will be enriched in the less volatile component. Since the composition of the pot will change from the initial mole ratio and become enriched in the less volatile component; the new composition in the pot will introduce changes in the composition of the vapor.
The composition of the vapor will also change from the initial ratio we just calculated to a new ratio to reflect the new composition of the pot.
The consequences of these changes are that the temperature of both the pot and the distillate will slowly increase from the initial value to a value approaching the boiling point and composition of the less volatile component. If we are interested in separating our mixture into components, we are left with the task of deciding how much material to collect in each receiver and how many receivers to use.
Obviously this will depend on the quality of separation we are interested in achieving. Generally, the more receivers we use, the less material we will have in each. It is possible to combine fractions that differ very little in composition but this requires us to analyze each mixture. While it is possible to do this, in general, we really want to end with three receivers, one each enriched in the two components of our mixture and a third that contains a composition close to the initial composition.
It is difficult to describe how much material to collect in each receiver since the volume collected will depend on the differences in the boiling points of the components.
Each fraction collected can be analyzed and those with compositions similar to the initial composition can be combined. The main fractions collected can then be fractionated a second time if necessary.
The experiment we have just discussed is called a simple distillation. It is an experiment that involves a single equilibration between the liquid and vapor.
This distillation is referred to as involving one theoretical plate. As you will see, it is possible to design more efficient distillation columns that provide separations on the basis of many theoretical plates.
Before discussing these columns and the advantages offered by such fractionating columns, it is important to understand the basis of the advantages offered by columns with many theoretical plates.
The following is a simplified discussion of the process just described involving a column with more than one theoretical plate. We have just seen that starting with a composition of , cyclohexane: methylcyclohexane, the composition of the vapor was enriched in the more volatile component.
Suppose we were to collect and condense the vapor and then allow the resulting liquid to reach equilibrium with its vapor. The properties of liquid 2 will differ from the original composition in two ways. First, since the composition of liquid 2 is higher in cyclohexane than the initial one; the temperature at which liquid 2 will boil will be lower than before what is the approximate boiling point of a 1.
In addition, the composition of the vapor, vapor 2, in equilibrium with liquid 2 will again be enriched in the more volatile component. This is exactly what happened in the first equilibration first theoretical plate and this process will be repeated with each new equilibration. If this process is repeated many times, the vapor will approach the composition of the most volatile component, in this case pure cyclohexane, and the liquid in the pot will begin to approach the composition of the less volatile component, methylcyclohexane.
In order for this distillation to be successful, it is important to allow the condensed liquid which is enriched in the less volatile component relative to its vapor, to return to the pot. In a fractional distillation, the best separation is achieved when the system is kept as close to equilibrium as possible. This means that the cyclohexane should be removed from the distillation apparatus very slowly. Thus, if the applied pressure is reduced, the boiling point of the liquid decreases.
This behavior occurs because a lower vapor pressure is necessary for boiling, which can be achieved at a lower temperature. Experimentally the setups are arranged more or less the same, with small differences being how the steam is added to the flask: either indirectly if a steam line is available in the building, or directly by boiling water in the flask. The reduced pressure in the apparatus causes the solvent to boil at a lower temperature than normal.
Lisa Nichols Butte Community College.
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