Chemistry Week 3

Chemistry Week 3 - States of Matter Gases TEXTBOOK READING:...

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States of Matter Gases TEXTBOOK READING : BLB-10 , Chapter 10, pp. 398-441. Practice PROBLEMS: (Ch 10) 7, 38, 40, 48, 52, 60, 77, 85, 92, 94, 103 All matter exists in one of three states of aggregation, i.e., gaseous , liquid , or solid , under most conditions of temperature and pressure. We are most familiar with normal conditions (298 K = 25 ° C and 1 atm pressure). At this temperature and pressure, only a few substances are gases (for example): Elements Compounds H 2 , N 2 , O 2 , O 3 , F 2 , Cl 2 HF, HCl, HBr, HI, CO, NO He, Ne, Ar, Kr, Xe, Rn CO 2 , NO 2 , N 2 O, SO 2 , H 2 S, NH 3 , HCN CH 4 , C 2 H 6 , C 3 H 8 , C 2 H 4 , C 2 H 2 Under normal conditions, gases are generally “small” molecular species with nonmetallic elements (low molecular weights). The atoms within a gas molecule are held together by chemical bonds (energies on the order of 10 2 kJ/mol); gas molecules are not held together : All gases have the following characteristics: Gases assume the volume and shape of the container; Gases are the most compressible of the states of matter; Gases mix evenly and completely; Gases have much lower densities (larger volumes) than solids and liquids. (1) State Variables The macroscopic behavior of gases is closely linked to their microscopic characteristics. For example, the volume occupied by a gas far exceeds the total volume of all gas molecules, and this is coupled with the notion that the intermolecular forces between molecules in a gas are very weak. Here are the four important macroscopic variables and a statement regarding their microscopic interpretation: MACROSCOPIC VARIABLE MICROSCOPIC INTERPRETATION No. of Moles (mol): Number of gas molecules; Volume (L, cm 3 ): Distances between gas molecules far exceeds distances within m o l e c u l e s ; Temperature (K, °C): Average kinetic energy of the gas molecules (speeds). Pressure (atm, mm Hg, Pascals): Strength and frequency of collisions between gas molecules and walls of the container (measured as momentum transfer, momentum = mass × speed);
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The Kelvin temperature scale arose from gas phase properties, and the fact that T (K) must be greater than or equal to 0 K parallels the interpretation as a measure of kinetic energy. The conversion between Kelvin and Celsius: K = °C + 273.15 Pressure is a measure of force exerted per unit area of the surface in contact with the gas (NOTE: energy = force × distance; pressure = (force × distance/area × distance) = energy/volume). Therefore, pressure can be measured by comparing the “weight” of a gas against the known weight of a fluid (mercury: high density and low thermal expansion coefficient). Units of pressure include mm Hg, atm, Pascals 1 atm = 760 mm Hg (torr) = 1.01325 × 10 5 Pa Standard temperature and pressure (STP) is 273.15 K and 1.0 atm. Variables can be divided into those that depend on the amount of substance (extensive ) and those that do not (intensive ). Given the definitions and microscopic interpretations of the four macroscopic variables for gases, temperature and pressure are intensive; volume and moles are extensive.
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This note was uploaded on 03/27/2008 for the course CHEM 201 taught by Professor Miller during the Fall '07 term at Iowa State.

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Chemistry Week 3 - States of Matter Gases TEXTBOOK READING:...

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