Chemistry Determination of melting point.pdf

Determination of Melting Point Ice to water Determination of melting point Physical Properties of Organic Compounds The physical properties of a compound include such things as: Color , Odor , Refractive index, Density, Solubility, Melting point & Boiling point. The exact values of the physical prop...

Determination of Melting Point Ice to water Determination of melting point Physical Properties of Organic Compounds The physical properties of a compound include such things as: Color , Odor , Refractive index, Density, Solubility, Melting point & Boiling point. The exact values of the physical properties of a cpd. depend on its molecular structure. The physical properties of a cpd. depends largely upon which kind of bonds hold it’s atoms together in a molecule Determination of melting point There are two kinds of bonds : 1. Ionic Bonds, Formed by the transfer of electrons. 2. Covalent Bonds, Formed by the sharing of electrons Determination of melting point Phases of Matter : Matter ordinarily exists in four phases: A. B. C. D. Solid Liquid Gas Plasma Plasma It is a high temperature phase not encountered in the typical organic chemistry laboratory Determination of melting point As the environment around the materials is changed, for example by varying the temp. or pressure, the systems undergo a phase transition, that is the change from one state of matter into another Determination of melting point There are four Basic types of Intermolecular Force : 1. Ion - dipole: Ion is attracted to polar molecule via this force as ex. (NaCl in water ) 2. Dipole – dipole: Polar molecules attracted to each other as forces between HCl molecules. Determination of melting point There are four Basic types of Intermolecular Force : Determination of melting point There are four Basic types of Intermolecular Force : 3. Dipole - induce dipole : Polar molecules attracted to nonpolar molecules as for ex. ( Oxygen in water ) by dipole – induce dipole interaction. 4. Induce dipole -induce dipole: : London dispersion forces also called van der Waal’s forces Nonpolar molecules attraction for each other due to electron distortion Determination of melting point There are four Basic types of Intermolecular Force : Hydrogen bonding : Determination of melting point Hydrogen bonding : Hydrogen bonding is an especially strong kind of dipole – dipole attraction in which a hydrogen atom serves as a bridge between 2 electronegative atoms, holding one by a covalent bond & the other by purely electrostatic forces. For H-bonding to be important, both electronegative atoms must come from the group: F, O, N. Determination of melting point Hydrogen bonding : Determination of melting point For Your Knowledge : Determination of melting point Melting Point : The melting point of a solid crystalline compound is the temperature at which the solid and the liquid phase of the cpd. are in equilibrium at a certain pressure usually 1 atmosphere Or, It’s the temp. at which the solid begins to change into liquid under a pressure of 1 atmosphere. Determination of melting point Melting Point Range : Is the range between the temp. at which the sample begins to melt (T1) & the temp. at which the sample is completely melted ( T2). Melting point rang = T2 – T1 Determination of melting point Organic chemist used melting point to : 1- Get an indication of the purity of cpd. Sample Pure Impure Decomposed Melting point Sharpe Not sharpen Not sharpen Melting point range 0.5 –1 °C Wider range Wider range Determination of melting point Organic chemist used melting point to : 2- Help in identification of organic cpd. Compound name Compound formula Compound Melting point Sodium chloride NaCl 801 °C Sodium acetate CH₃COONa 324 °C Glacial acetic acid CH₃COOH 16.6 °C Determination of melting point Mixed Melting Point: Question// How can you know whether the 2 samples are the same or different? Answer: We mix them and measure the m.p. for the resultant mixture, If, m.p. of mixture < m.p. of original & wider m.p. range → different cpd. m.p. of mixture = m.p. of original & same m.p. range → same cpd Determination of melting point Mixed Melting Point: Determination of melting point General Technique for m.p. Determination: Capillary tube method : Determination of melting point General Technique for m.p. Determination: Filling Capillary tube The open end of the tube is pressed gently & repeatedly into the powdered sample several times. The sample can be moved to the sealed end by turning the tube over & tapping it on the bench. Determination of melting point General Technique for m.p. Determination: Filling Capillary tube sample height of 2-3 mm in the capillary tube is recommended. Determination of melting point General Technique for m.p. Determination: Any compound being loaded into a capillary tube must be: A. In Powdered form B. Homogeneous C. Fully Dried Determination of melting point Melting Point Apparatus: A. B. C. D. E. Hot -oil m.p. bath Thiele apparatus Long necked flask. Mel-temp apparatus. Digital m.p. measuring apparatus. Determination of melting point Number of Experiment: 1 Name of Experiment: Determination of the Melting Point of an unknown sample. Aim of Experiment: Identification of an unknown cpd. using it's m.p. Determination of purity of a cpd. using the m.p. as a physical property. Determination of melting point Number of Experiment: 1 Procedure : 1. Preparation of the unknown sample as mentioned before. 2. The capillary tube is attached to a thermometer by a rubber ring in such a way that the closed end is attached to the bottom of the thermometer's bulb. Determination of melting point Number of Experiment: 1 Procedure : 3. Then both of them are placed in an oil – bath , ( the rubber ring should be above the surface of the oil -bath ). 4. Heating is started gradually. Determination of melting point Number of Experiment: 1 Procedure : 5. The range between the temperature at which the powdered solid inside the capillary tube begins to liquefy (T1) & the temperature at which a clear liquid is observed inside the capillary tube (T2) is recorded as the observed m.p. range m.p. rang = T2–T1 m.p. = ( T1–T2) NOTES : For oil bath, glycerin or paraffin can be used, but glycerin is preferred since it's safer than paraffin. The temperature of the oil bath should rise very slowly. Gentle heating with continuous stirring is necessary After completing the experiment, the oil liquid must be allowed to cool before using it again. Dr. Ali Mohammed Question Among these 3 samples of the same cpd., which of them is the purest one? m.p. 115-119°C m.p. 118-120 °C m.p. 121-122°C Dr. Ali Mohammed