Practical organic chemistry 2 Determination of boiling point Prepared by: Pshtiwan Ghareeb Ali Bsc. in Pharmacy
Definition The particles in a liquid are arranged less regularly and are freer to move about than in a crystal. Boiling involves the breaking away from the liquid of individual molecules or pairs of oppositely charged ions (see Figs.). This occurs when a temperature is reached at which the thermal energy of the particles is great enough to overcome the cohesive forces that hold them in the liquid.
The boiling point of a pure liquid is defined as the temperature at which the vapor pressure of the liquid exactly equals the pressure exerted on it by the atmosphere. At an external pressure of 1.0 atm (760 torr), the boiling point is reached when the vapor pressure equals 760 torr. However, at other pressures the boiling point of the liquid will be different. See the table. When the boiling point of a substance is determined, both the atmospheric pressure and the experimental boiling point need to be recorded.
B.p. is a physical constant used for: 1. Identification of unknown liquid compound 2. Purification
Factors affecting boiling point Pressure Molecular weight Structure of the molecule & intermolecular interactions Branching Polarity Van derwaals interactions H-bonding Impurities
Pressure
Molecular weght
Branching Polarity
Van derwaals interactions Hydrogen-bonding
Impurities The b.p. of pure H2O is 100 C, but that b.p. can be elevated by adding a solute such as a salt? The number of solvent molecules at the surface of the solution is less than for pure solvent. The surface molecules can be considered diluted by the less volatile particles of the solute. The rate of exchange between solvent in the solution and the air above it is lower (vapor pressure of the solvent is reduced) A lower pressure means that a higher temperature is necessary to boil the water in the solution, hence boiling point elevation For Melting point? Effect of impurities?
Effects of adding salt to water? detail When salt is added to water, then the intermolecular forces between water molecules gets altered due to dissociation of NaCl into sodium and chloride ions. The hydrogen bonding( a weak bond) between water molecules gets affected and the ion-dipole interaction between ions (sodium and chloride ions) and dipole (water) starts occuring. Ion-dipole interaction is comparatively strong bond. Hence more heat is required by water molecules to come out as gaseous phase and vaporise.
Effects of impurities A simple explanation of this is that the impurities dilute the concentration of water (the number of water molecules per unit volume decreases), and the number of molecules that can vaporize at any give temperature decreases. The result is that a higher temperature is required to achieve the same vapor pressure. Concentrated sugar-water solutions that are used for making candies and caramel boil at temperatures exceeding 150 C.
Effects of impurities
How determined 1-A small-scale simple distillation apparatus (sample size ~ 10ml) The boiling point of 5-10 ml or more of a pure liquid compound can be determined by a simple distillation. 2- Semi-micro scale method 3- Microscale method
1.5 ml Microscale Procedure
Melting point apparatus assembly Microscale Method
Procedure for Semi-micro scale method Place a thermometer 1-1.5 cm above approximately few milliliters of the liquid in a long test tube. Slowly heat the liquid to boiling so that the thermometer bulb is immersed in the vapor. Allow the temperature to remain at a constant value for 30 sec or more. This value is the boiling point of the liquid. While you are heating the liquid, it is helpful to record the temperature at 1-minute intervals
Semi-micro scale method to determine the boiling point of a liquid