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Unformatted text preview: Mel$ng Points and Mixed Mel$ng Points Goals: 1. Learn how to measure mel$ng points 2. Learn how to use mel$ng points to establish the purity of a compound Changes in Temperature of a Compound at Constant Pressure Temperature b.p. m.p., f.p. Solid A (S) B C Liquid S L (L) D E L G Gas (G) Energy Added B = mel$ng point/freezing point. Temperature Constant from B to C. D = boiling point/condensa$on point. Temperature Constant from D to E. Psolid = Pliquid (P = Vapor Pressure) Pliquid = Papplied A Pressure Versus Temperature Phase Diagram Line ET = Solid‐Liquid Equilibrium E Psolid = Pliquid D P3 e.g. T3 = Mel$ng Point at P3 Point D = Cri$cal Point (“Supercri$cal Fluid” is Formed) Pressure P2 PT P1 C Solid Liquid T Gas T1 T3 TT T2 Point T = Triple Point (Solid, Liquid, and Gas are all in equilibrium) (Sublima$on/Deposi$on Curve) Temperature Curve CT = Vapor Pressure of a Solid at a Given T e.g T1 = Sublima$on Point at P1 Curve TD = Vapor Pressure of a Liquid at a Given T (Boiling Point/Condensa$on Curve) e.g. T2 = Boiling Point at P2 A Pressure Versus Temperature Phase Diagram Characteris$c Of Most Organic Molecules E P2 D Pressure P1 C Solid T Liquid Increasing pressure on most organic molecules increases the intermolecular forces. Solids generally have greater intermolecular forces than liquids due to closer packing of molecules. P1 T1 Gas T1 Temperature At (P1, T1), the liquid is the preferred phase. Holding T1 constant, but increasing the pressure to P2, the solid becomes preferred. To go back to a liquid, T would have to be increased. A Pressure Versus Temperature Phase Diagram E P2 D Mel$ng points and freezing points are assumed to be taken at atmospheric pressure unless otherwise stated. Liquid T Pressure P1 C Solid As you ice skate, the ice itself is at a subzero temperature (P1, T1). Under the ice skate blade, however, the pressure is much higher (P2, T1). Gas T1 Temperature This increased pressure along with a slight increase in temperature due to fric$on, leads to a thin layer of water, which is what allows you to skate. Note that the mel$ng point/freezing point is rela$vely independent of pressure. In the case of H2O, the Line ET has a nega$ve slope (as on the diagram above). Physical Constants and Observable Characteris$cs Can Help Iden$fy an Unknown Solid by “Narrowing the Field ” • Visual Cues: Color of the solid; Powder or crystal; If crystalline, what shape are the crystals. • Smell: Not common for solids, but if you can smell it then this really narrows the ﬁeld. • Taste: Please don’t do this inten$onally! • Solubility in acid/base; Solubility in polar, non‐polar solvents. • Mel$ng Point Mel$ng points recorded using a typical mel$ng point apparatus are never a point, but rather a range. Mel$ng point range is measured from Tstart to Tﬁnish Tstart : ﬁrst visible drop of liquid (“swea$ng”) Tﬁnish : disappearance of last bits of solid Eﬀect of Impuri$es on Mel$ng Points (Freezing Points) Case 1: Impurity is insoluble in liquid melt. For example, sand in water. Result: NO EFFECT Case 2: Impurity is soluble in liquid melt. For example, salt in water. Result: MELTING/FREEZING POINT DEPRESSION. Mel$ng/Freezing Point Depression E Pressure Solid Liquid T D Assume compound A is the solvent. Since XA is always less than 1 in a solu$on with a soluble solute, PA is lowered. C TT new Gas Temperature PA lowered. Raoult’s Law States: PA = XAPA°. The par$al pressure of compound A is the product of its mole frac$on (XA) with its “standard” vapor pressure (PA°) at a given temperature. Mixed Mel$ng‐Point/Composi$on Diagrams The mel$ng/freezing depression for a mixture of compound A (m.p. TA) and compound B (m.p. TB) varies according to the mole frac$ons of A and B. Liquid Solu$on of A & B TA TF Liquid Solu$on TB Liquid Solu$on + Solid B TS T E +Solid A Eutec$c Solid A & B 1.0 0.8 0.6 0.4 0.2 0.0 0.0 0.2 0.4 0.6 0.8 1.0 Mole Frac$on Example: The mel$ng range for an 80:20 mixture of A:B should be TE to TF. The actual range is TS to TF. Conclusions 1. A broad mel$ng range may signify: A. The presence of a soluble impurity. For crystals may be “wet.” B. Mixture of Enan$omers C. Decomposi$on of the solid upon hea$ng example, the 2. A sharp mel$ng range may signify A. A pure compound (perform mixed mel$ng point determina$on with an authen$c sample if possible) B. You are at the eutec$c point. ...
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- Spring '08
- Organic chemistry