1 Types f Intermlecular Frces: Strength Interactin Picture Descriptin est Lndn Dispersin Frces (induced dipleinduced diple) + + + + + + Attractin between temprary induced diples in nn-plar mlecules. Diple-Diple + + Attractin between tw diples (plar parts f mlecules). Example: plar liquids, such as water + Charge attractin between a Hydrgen Bnding O H hydrgen bnded t a strngly O electrnegative atm (N, O, r F), and an atm with a lne pair. In-Diple + + Strngest In-In + Attractin between in and ppsitely charged end f a diple (plar mlecule). Example: NaCl disslving in water Attractin between tw ppsitely charged ins. Example: inic salts, such as NaCl Jhn Cngletn
2 Slubility and Intermlecular Frces In the disslving prcess the slute is usually the smaller quantity and it is the substance that is being disslved whereas the slvent is the larger quantity and the substance that des the disslving. The disslving prcess can be brken dwn int the fllwing steps. Slute-Slute Interactins The attractive frces (intermlecular) that hld the slute mlecules tgether must be vercme, breaking the mlecules apart. This step requires energy and the ptential energy increases here. Slvent-Slvent Interactins The attractive frces (intermlecular) that hld the slvent mlecules tgether must be vercme, breaking the mlecules apart thus making rm fr the slute mlecules. This step requires energy and the ptential energy increases here als. Slute-Slvent Interactins During this prcess, the mlecules f the slute interact with the mlecules f the slvent. The slvent particles surrund the slute particles and frm attractive frces between them. This step decreases the ptential energy. In rder t disslve, the slvent mlecules and the slute mlecules must break apart frm each ther and this prcess raises the energy. The new interactins between the slvent and the slute must be strng enugh t lwer the energy back dwn. Jhn Cngletn
3 We can lk at IM frces t cmpare the interactins befre disslving t the interactins after disslving. If the net frces are strnger (r similar with regard t IM frces) after disslving, then the slute will disslve in the slvent. Let s examine the fllwing scenaris. Slute IM Frces Slvent IM Frces Slvent-Slute IM Frces Lndn Dispersin Lndn Dispersin Lndn Dispersin Slubility Sluble Strng (Plar r H-bnding) Strng (Plar r inic) Strng (diple-diple r H-bnding) Sluble Strng (Plar r H-bnding) Lndn Dispersin Insluble General Slubility Rules f Thumb Hydrcarbns Alkyl Halides Esters, Ketnes, Aldehydes Cyan, Ethers, Amides Alchls, water Amines, carbxylic acids Nn-Plar (Parts) Nn-Plar (Parts) Intermediate Plarity (Parts) Highest Plar (Parts) Jhn Cngletn
4 Melting Pint Thery MP Basics In rganic chemistry, we can use the melting pint f a cmpund t elucidate the identity f a slid unknwn cmpund and/ r t determine the purity f an islated prduct. Hwever, we need t cmpare ur experimentally determined MP with that f the true cmpunds MP. Melting "Pint" The true melting pint is the temperature at which the slid and liquid phases f a pure substance are in equilibrium (at 1 atm f pressure). The capillary melting "pint" (Melting Range) is the temperature range ver which a sample f a cmpund in a capillary tube (~1-2 mm) liquefies. It starts when the sample begins t liquefy and ends when it has cmpletely liquefied. IN GENERAL: Pure Cmpunds have melting ranges less than abut 2 C. Ranges greater than abut 2 C, indicate the presence f impurities. Depressed ranges are als indicatins f impurity. Yur technique can als give yu larger and depressed ranges Jhn Cngletn
5 In this experiment, yu will: 1. Take the MP f knwn (t gain expertise) 2. Take MP f unknwn (t narrw yur chices t tw cmpunds) ne fast, t get a rugh idea f melting range ne slw, fr an accurate measurement Apprximate Melting pint Available fr mp mixture Substance name 102 C Oxalic acid dehydrate 102 C -tluic acid 115 C acetanilide 115 C flurene 122 C beta-naphthl 122 C benzic acid 133 C trans-cinnamic acid 133 C Urea 141 C dimethyl terephthalate 141 C salicylamide 150 C 2-brmbenzic acid 150 C benzilic acid 164 C p-phenylphenl 164 C Triphenylmethanl 3. Take mixed MPs (t cnfirm yur unknwn) Althugh a cmpund's melting pint is an intrinsic prperty f that cmpund, many cmpunds cincidentally share mp's. QUESTION: Hw can we be sure we have made a prper identificatin f a cmpund by melting pint? ANSWER: Run a mixed melting pint: Jhn Cngletn
6 Melting Pint Cmpsitin Curve Jhn Cngletn