A- Determination Of Boiling point B- Distillation

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1 EXP. NO. 2 A- Determination Of Boiling point B- Distillation The boiling point of a liquid is the temperature at which its vapor pressure is equal to the surrounding atmospheric pressure. The normal boiling point of a liquid is the temperature at which its vapor pressure is equal to one atmosphere (760 torr). Factors That Affect the Boiling Point 1- Pressure: when the external pressure is: less than one atmosphere, the boiling point of the liquid is lower than its normal boiling point. equal to one atmosphere, the boiling point of a liquid is called the normal boiling point. greater than one atmosphere, the boiling point of the liquid is greater than its normal boiling point. 2- Types of Molecules: the types of molecules that make up a liquid determine its boiling point. If the intermolecular forces between molecules are: relatively strong, the boiling point will be relatively high. relatively weak, the boiling point will be relatively low.

2 Question: At what temperature does water boil? What determines the boiling point of water? Answer: The boiling point of water depends on the atmospheric pressure, which changes according to elevation. The boiling point of water is 100 C or 212 F at 1 atmosphere of pressure (sea level), but water boils at a lower temperature as you gain altitude (e.g., on a mountain) and boils at a higher temperature if you increase atmospheric pressure (lived below sea level). The boiling point of water also depends on the purity of the water. Water which contains impurities (such as salted water) boils at a higher temperature than pure water. This phenomenon is called boiling point elevation.

3 The boiling point of organic compounds can give important information about their physical properties and structural characteristics. Boiling point helps identify and characterize a compound. A liquid boils when its vapour pressure is equal to the atmospheric pressure. Vapour pressure is determined by the kinetic energy of a molecule. Kinetic energy depends on the temperature, mass and velocity of a molecule. When the temperature increases, the average kinetic energy of particles also increases. When the temperature reaches the boiling point, the average kinetic energy becomes sufficient to overcome the force of attraction between the liquid particles. As the force of attraction decreases, the molecules in the liquid state escape from the surface and turn into gas The boiling point of a liquid varies with the surrounding atmospheric pressure. A liquid at a higher pressure has a higher boiling point than when that liquid is at lower atmospheric pressure. The normal boiling point of a compound is an indicator of the volatility of that compound. The higher the boiling point, the less volatile is the compound. Conversely, the lower the boiling point, the more highly volatile is the compound. At a given temperature, if a compound s normal boiling point is lower, then that compound will generally exist as a gas at atmospheric pressure. If the boiling point of the compound is higher, it then exists as a liquid

4 What are the general trends that affect the boiling point? 1. Strength of intermolecular forces The relative strength of intermolecular forces such as ionic, hydrogen bonding, dipole-dipole interaction and Vander Waals dispersion force affects the boiling point of a compound. The influence of these forces depends on the functional group present. We can explain the effect of these forces on the boiling point of compounds with the help of some examples. Consider butane and its three derivatives such as diethyl ether, n- butanol and sodium n- butoxide. n-butane (C 4 H 10 ) contains no polar functional group. The only attraction between the butane molecules is weak Vander Waals dispersion forces. The result is that butane boils at a temperature at which water freezes, and is much lower than diethyl ether. In the case of diethyl ether, the molecules are held together by dipoledipole interaction which arises due to the polarized C-O bond. Its boiling point is 35 o C. Compare its boiling point with that of n- butanol. The boiling point of n- butanol is 117 o C. The greatly increased boiling point is due to the fact that n- butanol contains hydroxyl group, which is capable of hydrogen bonding. But the boiling point of sodium butoxide is higher than that of butanol because the attractive force in sodium butoxide is very strong ionic bond. The intermolecular forces go in the order Ionic > Hydrogen Bonding > Dipole-Dipole > Van der Waals dispersion force.

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6 2. Length of carbon-carbon chain As the number of carbon atoms increases or the length of carboncarbon chain increases, the boiling point also increases. This is because the force of attraction between the molecules increases as the molecule gets longer and has more electrons. It takes more energy to overcome the force of attraction, and so the boiling point rises.

7 3. Branching decreases the boiling point As the length of carbon chain increases, the surface area of the compound will also increase. Van der Waals dispersion force is proportional to the surface area. So the increase of surface area increases the ability of individual molecules to attract each other. Branching in molecules decreases the surface area thereby decreasing the attractive force between individual molecules. As a result, the boiling point decreases. Consider the boiling point of n-pentane and neo-pentane (2,2-dimethyl propane). These are isomers having the same molecular formula (C 5 H 12 ), but differ in their structures. The boiling point of neopentane is much lower than that of n-pentane.

8 4. Polarity Polarity of the molecule determines the force of attraction between the molecules in the liquid state. In polar compounds, the positive end of one molecule is attracted by the negative end of another molecule. That means polar molecules are attracted by opposite charge effect. The polarity of a molecule is determined by its functional group. The greater the polarity, the higher is the boiling point. Boiling point of some common organic compounds

9 Procedure

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11 B- Distillation Distillation is process involving the conversion of a liquid into vapor that is subsequently condensed back to liquid form. Distillation is a physical separation process, and not a chemical reaction Distillation is a procedure that separates a mixture of liquids with different boiling points. Distillation is a useful technique in chemistry labs, where chemists use it to purify a compound, and also in industry, especially in the petrochemical, refining industry and in the manufacture of ethanol. It is for this last that distillation is most famous--alcoholic beverages are produced through a process of distillation..applications of distillation The application of distillation can roughly be divided in four groups: laboratory scale, industrial distillation, distillation of herbs for perfumery and medicinal (herbal distillate), and food processing. Commercially, distillation has a number of applications. It is used to separate crude oil into more fractions for specific uses such as transport, power generation and heating. Water is distilled to remove impurities, such as salt from seawater. Air is distilled to separate its components notably oxygen, nitrogen, and argon for industrial use.

12 Types of distillation 1. Simple distillation 2. Fractional distillation 3. Steam distillation 4. Vacuum distillation

13 Simple Distillation If water is placed in a sealed container and allowed to evaporate, it will eventually reach an equilibrium such that the water vapor is condensing just as fast as the water is evaporating. The pressure of the vapor at this equilibrium is called the vapor pressure. Vapor pressure is different for different substances and varies with temperature. In a mixture of two liquids with different boiling points, the vapor will have more of the liquid that is more volatile, i.e., evaporates more readily. In simple distillation, the liquid mixture is heated and the vapor rises through a tube and is collected and recondensed. The recondensed liquid will have a higher concentration of the more volatile component than the original mix. If the two liquids in the original mix have widely different boiling points, a one-step evaporation and recondensation process is all that is necessary. This process is called simple distillation. Fractional Distillation Fractional distillation is similar to simple distillation, except the same process is repeated in successive cycles. Each cycle produces a mixture richer in the more volatile compound than the mixture before it. Fractional distillation is necessary when the boiling points of the liquids in the original mix are close enough to each other that simple distillation is not enough to purify either compound.

14 Steam Distillation Steam distillation is used to separate heat-sensitive components. Steam is added to the mixture, causing some of it to vaporize. This vapor is cooled and condensed into two liquid fractions. Sometimes the fractions are collected separately, or they may have different density values, so they separate on their own. An example is steam distillation of flowers to yield essential oil and a water-based distillate. Vacuum Distillation Vacuum distillation is used to separate components that have high boiling points. Lowering the pressure of the apparatus also lowers boiling points. Otherwise, the process is similar to other forms of distillation. Some liquids boil at such high temperatures that simple or fractional distillation using the process described above would be impractical or dangerous. Vacuum distillation, however, offers another alternative. The boiling point of a liquid falls when the pressure is reduced. The boiling point of water, for example, is lower at high altitude than at sea level. By reducing the pressure in the container, the boiling point of the liquids in the mixture can be reduced and the mixture distilled at a lower temperature. This technique is called vacuum distillation.

15 Simple Distillation

16 Fractional Distillation

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18 Distillation Under Reduced Pressure