Exp.4 Determination of the M.P. and B.P. PDF

Experiment 4 Determination of Melting Point of Solids & Boiling Point of Liquids Objectives To learn how to determine the melting point (M.P.) and boiling point (B.P.) for the compounds. To learn the factors affecting the melting point (M.P.) and bo...

Experiment 4 Determination of Melting Point of Solids & Boiling Point of Liquids Objectives To learn how to determine the melting point (M.P.) and boiling point (B.P.) for the compounds. To learn the factors affecting the melting point (M.P.) and boiling point (B.P.) of the compounds. Introduction In identifying a compound, a chemist often measures several physical properties and observes a few chemical properties of the compound. Chemical properties are those properties observed only as a substance is changed chemically into another substance. Physical properties are those properties that can be observed or measured without changing the composition of the substance. They are categorized as either Intensive or Extensive: Intensive properties: Properties that do not depend on the amount of the matter present (Color, Odor, Density, Melting point and boiling point etc…). Extensive properties: Properties that depend on the amount of matter present (Mass, Weight and Volume etc……..).  Three States of Matter Molecules are far apart and fill the GAS available space SOLID LIQUID Solid molecules are close Molecules are close together, packed in a together but more relative regular array, and move to each other very little relative to each other Energy Involved in Phase Changes Melting Point (M.P.)  The melting point of a solid is defined as the temperature at which the liquid and solid phases are in equilibrium.  At this temperature a substance changes from the solid phase to the liquid phase.  There is no change in temperature while this change of phase takes place.  Experimentally, M.P. always reported as a range (M.Pi - M.Pf).  M.Pi = when first drop of liquid appears.  M.Pf = when the solid converts completely into liquid. Experimental Melting Point MPi MPf Melting Point (M.P.)  A solid is said to melt sharply if the melting point range is 0.5 - 1.0 0C.  A pure solid will generally melt sharply (narrow MP range) because the forces of attraction between its particles are the same.  An impure solid melts at a lower temperature and over a wider range. The presence of a foreign particle in a crystal lattice interrupts its uniform structure and the forces of attraction are weakened.  M.P. used to: Identify unknown compounds (Finger Print of compounds). Characterize new compounds. Determine Purity of the samples Melting Point (M.P.)  The melting point of a solid or the freezing point of a liquid is the temperature at which the solid and liquid phases coexist in equilibrium.  The freezing point of a liquid is the same temperature as the melting point of its solid. Factors affecting Melting Point range  Sample Size - Amount of compound that is taken its M.P. should not be more than 2mm in the capillary tube; large sample size may cause broadening of the M.P.)  Heating Rate There should not be change in temp. more than 20C /minute; fast heating may cause incorrect reading of the thermometer..  Purity - Pure compounds exhibit sharp M.P., usually 1-20C whereas impure compounds show a broader 1 mm M.P. range, usually from 3-100C.  Human error - The capillary tube far away from the thermometer bulb.  Boiling Point (B.P.)  The temperature at which a substance changes state from liquid to gas and it occurs when the liquid vapor pressure = external pressure on sample. (equilibrium exists between the liquid and the gas).  The temperature at which a liquid boils when the atmospheric pressure is 760 mmHg (1 atm) is called Normal Boiling Point. Boiling Point (B.P.)  Because the B.P. depends on the pressure, the B.P. changes if pressure changes.  If the atmospheric pressure decreases by 10 mmHg, the B.P. decreases by 0.50C, so it is very necessary to record the pressure when measuring B.P. of the liquid.  If B.P. is decreased by a temperature which is within the uncertainty of the thermometer (0.20C) then we do not count that decrease in temperature.  Units of Pressure 1 atm =760 mmHg=760 torr Barometer Factors affecting Boiling Point  Pressure - Boiling point temperature increases when the pressure increases.  Heating Rate - Fast heating will give inaccurate reading of the thermometer. Make sure to heat slowly about 1 - 2 0C increase every minute.  Purity – Soluble impurities will increase the boiling point temperature. Graph of Temperature against Time  Thermometer Thermometer Test Tube Capillary tube open end down Capillary tube Liquid close end down Solid sample Water bath about 1 mm height Water bath Bunsen Burner Bunsen Burner MELTING POINT APPARATUS BOILING POINT APPARATUS  Bunsen Burner Burner Barrel Air Control Vent  Controls the air that mixes with the gas Gas Inlet connected to main valve Report Sheet 1- Melting Point of Naphthalene (KNOWN): a. Temperature at which the first liquid appears ……………….. b- Temperature at which all crystals melt …………. c- Melting point range …………….. 2- Melting Point of the UNKNOWN: UNKNOWN # = ………… a. Temperature at which the first liquid appears ……………….. b- Temperature at which all crystals melt …………. c- Melting point range …………….. 3- Boiling Point of isopropanol (KNOWN): a. Temperature at the boiling point …………. 4- Boiling Point of the UNKNOWN: UNKNOWN # = ………… a. Temperature at the boiling point …………. 5- Barometric pressure ………………