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Chapter 3 Melting points and Boiling points

Chapter 3 Melting points and Boiling points. Melting Points. The melting point (mp) of a pure solid organic compound is a characteristic physical property. Mp’s are dictated by the degree of van-der- Waals, hydrogen bonding, and dipole-dipole interactions between molecules.

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Chapter 3 Melting points and Boiling points

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  1. Chapter 3Melting points and Boiling points

  2. Melting Points • The melting point (mp) of a pure solid organic compound is a characteristic physical property. • Mp’s are dictated by the degree of van-der- Waals, hydrogen bonding, and dipole-dipole interactions between molecules. • Mp determination helps to characterize a compound and indicate purity (mp range). • Determined using a Mel-Temp apparatus and a capillary tube to hold the sample.

  3. Melting Points • Do calibration of the thermometer using naphthalene (80-82°C). • Take the melting point of pure urea and cinnamic acid. Take the melting point of pure urea - cinnamic acid mixture. There are three mixtures (1:3, 1:1, 3:1), one group only need to do one. • Determine the melting points of unknowns; all the unknowns are listed on page 54.

  4. Melting Points(comments) • Tightly pack 2-3 mm of sample into a capillary by tapping a capillary. • If the mp is known, the apparatus can be heated rapidly to within 20°C before slowing the rate of heating to 1°C/minute. • Melting process begins when the first liquid drops are observed and ends when the last trace of solid melts.

  5. Boiling Points • The boiling point of a pure liquid compound is also a characteristic physical property. • Bp’s are dictated by the degree of van-der- Waals, hydrogen bonding, and dipole-dipole interactions between molecules. • Bp determination does not help to indicate purity, as bp is affected less by impurities.

  6. Boiling Points • Bp is defined as the temperature at which the vapor pressure of the liquid exactly equals the pressure exerted on it. Therefore, correction for current atmospheric pressure is required. (P mm Hg / 760 mm Hg) x measured bp = corrected bp

  7. Boiling Points(comments) • Use the apparatus shown in Figure 3.13 on p. 58. • Thermometer bulb must not touch anything. • Boiling chips are necessary to provide the proper surface for vapor bubble formation. • True boiling is indicated by droplets forming on and dripping from the bulb while observing a constant temperature. Postlab questions: 5, 6, 7, 13 (pp. 59-60)

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