Chemistry Week 4

Chemistry Week 4 - Liquids TEXTBOOK READING: BLB-10,...

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Liquids TEXTBOOK READING : BLB-10 , Chapter 11.1-3, pp. 442-454. Practice PROBLEMS: (Ch 11) 7, 9, 14, 19, 22, 23 (1) Some Experimental Observations Here is a listing of a and b parameters for the van der Waals equation of state for real gases: What are some relationships that you see in the parameters a and b ? Gas B.P. (K, 1 atm) MW (g/mol) Polarity a (L 2 atm/mol 2 ) b (L/mol) H 2 20.3 2.02 Nonpolar 0.2444 0.02661 He 4.2 4.00 Nonpolar 0.0341 0.02370 N 2 77.4 28.00 Nonpolar 1.3990 0.03913 O 2 90.2 32.00 Nonpolar 1.36 0.03183 CO 81.7 28.01 Weakly Polar 1.485 0.03985 NO 121.4 30.00 Weakly Polar 1.340 0.02789 CO 2 194.7 sub 44.01 Nonpolar 3.592 0.04267 H 2 O 373.0 18.02 Polar 5.464 0.03049 Cl 2 O 277.0 70.9 Nonpolar 6.49 0.0562 Xe 166.1 131.3 Nonpolar 4.19 0.0266 Parameter “ a ” gives some measure of the strength of intermolecular/interatomic forces in these gases. Note that a increases as MW increases, as polarity increases, and as size increases. Polarity refers to the degree that the center of positive charge and the center of negative charge are separated in the molecule. In an atom, the center of positive charge is the nucleus; the center of negative charge (the electrons) is also at the nucleus, so atoms are intrinsically nonpolar. Also, boiling point (B.P.) tends to increase with MW (water is unusual!). Look at the trends in boiling points (temperature, at 1.00 atm, at which the vapor and liquid are in equilibrium with each other) for various substances containing hydrogen:
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Group 123456 T (°C) -300 -200 -100 0 100 He Ne Ar Kr Xe CH 4 NH 3 OH 2 FH The noble gases and the Group 4 compounds with hydrogen (CH 4 -SnH 4 ) show steady increase in boiling point with AW or MW. H 2 O, HF, and NH 3 show anomalously high boiling points due to additional interactions – hydrogen bonding . • Demonstration : Oils on water. Mineral oil and some vegetable oils (nonpolar hydrocarbons; hydrophobic, stronger attractions within the oil than between the oil and water) forms beads on the surface of water (smallest area in contact between the two substances); olive oil (organic molecules with polar portions; hydrophilic, stronger attractions between oil and water than within the oil) spreads out. A symmetrical razor blade (double-edged blade) can float on water, although it is clearly much denser than the liquid. Adding some dish soap causes the razor blade to sink (the soap weakens the interactions between water molecules as the detergent comes between them. Cohesion is the interaction between molecules in the same liquid. Adhesion is the interaction between molecules in different substances. Surface tension represents the energy needed per area to stretch a surface: high surface tension means strong intermolecular forces. With these concepts, water in a glass tube creeps up the walls to produce a U-shaped meniscus—strong adhesion between the polar water molecules and the glass. Mercury shows an inverse meniscus since the intra-atomic cohesion is stronger than the interatomic adhesion forces. This is called capillary action .
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Chemistry Week 4 - Liquids TEXTBOOK READING: BLB-10,...

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