PHYSPHAR-LAB (dragged).pdf

CYCLE SEM Physical Pharmacy LAB PHARM3A | TUTOR 1 1 MODULE 2: MELTING POINT DETERMINATION PRE LABORATORY DISCU...

CYCLE SEM Physical Pharmacy LAB PHARM3A | TUTOR 1 1 MODULE 2: MELTING POINT DETERMINATION PRE LABORATORY DISCUSSION Usually it is conducted for solid materials under normal conditions Physical Property a characteristic of matter that is not Factors That Affects Melting Point associated with a change in its chemical a. Molecular composition composition. b. force of attraction Familiar examples of physical properties c. presence of impurities can all affect the include density, color, hardness, melting and melting point of substances. boiling points, and electrical conductivity. Composition of Molecules Physical Change When molecules are tightly packed together, a change in the state or properties of matter substance has a higher melting point than a without any accompanying change in its substance with molecules that do not pack chemical composition (the identities of the well. substances contained in the matter), such as For example, symmetrical neopentane dissolution and dilution. molecules have a higher melting point than isopentane, in which molecules do not pack well. o Molecular size also affects the melting point. When other factors are equal, smaller molecules melt at lower temperatures than larger molecules. o For example, the melting point of ethanol is -114.1 degrees Celsius/- Melting Point 173.4 degrees Fahrenheit, while the temperature at which the solid and liquid forms melting point of the larger ethyl of a pure substance can exist in equilibrium. cellulose molecule is 151 degrees o As heat is applied to a solid, its Celsius/303.8 degrees Fahrenheit. temperature will increase until the Macromolecules have giant structures made melting point is reached. up of many nonmetal atoms joined to adjacent o Most mixtures and amorphous solids atoms by covalent bonds. Substances with melt over a range of temperatures. giant covalent structures, such as diamond, graphite and silica, have extremely high Melting Point Range melting points because several strong The temperature at which a compound begins covalent bonds must be broken before they to melt and the temperature at which the can melt. compound is totally melted. Forces of Attraction The melting point range is a valuable and easily determined physical constant for A strong attraction between molecules results characterizing an organic compound. in a higher melting point. In general, ionic compounds have high melting Importance of Melting Point points because the electrostatic forces For its storage & transport - A higher melting connecting the ions – the ion-ion interaction – point indicates greater intermolecular forces are strong. and therefore less vapour pressure. Melting In organic compounds, the presence of point test is not required for every chemical. polarity, especially hydrogen bonding, usually leads to a higher melting point. AD | PHARMACY 2A | 2021 1  MODULE 2: MELTING POINT DETERMINATION The melting points of polar substances are higher than the melting points of nonpolar substances with similar sizes. For example, the melting point of iodine monochloride, which is polar, is 27 degrees Celsius/80.6 degrees Fahrenheit, while the melting point of bromine, a nonpolar substance, is -7.2 degrees Celsius/19.04 degrees Fahrenheit. Presence of Impurities Impure solids melt at lower temperatures and may also melt over a wider temperature range, known as melting point depression. The melting point range for pure solids is narrow, usually only 1 to 2 degrees Celsius, known as a sharp melting point. Impurities cause structural defects that make the intermolecular interactions between the molecules easier to overcome. A sharp melting point is often evidence that a sample is fairly pure, and a wide melting range is evidence that it is not pure. For example, a pure organic crystal has uniform molecules, perfectly packed together. However, the crystals are impure when they occur in a mixture of two different organic molecules because they don't fit together well. It takes more heat to melt the pure structure. POST LAB DISCUSSION SAMPLE STANDARD MELTING POINT Aspirin USP 135 Degrees Phenacetin 134 Degrees – 136 Degrees Acetaminophen 168 Degrees – 172 Degrees AD | PHARMACY 2A | 2021 2