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The Kinetic Molecular Theory

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  Course Outline General Chemistry--Unit 2 04/25/09  The Kinetic Molecular TheoryDescription of Mixtures �Before You Begin: To master this material you will need to understand kinetic molecular theory andknow the types of intermolecular attractions.  Types of Mixtures A mixture is a sample of matter in which two or more different substances arephysically combined such that each retains its own identity. The properties of amixture are an average of the properties of the components of that mixture. Forexample, if you add sugar and water together you get a sugar water mixture. Themixture will be a colorless liquid with density, viscosity, boiling point and vaporpressure almost the same as that of pure water. It will taste sweet like sugar.When the water evaporates, sugar crystals will form. Neither the sugar nor thewater will have been fundamentally changed during the process.In contrast, when two or more substances are chemically combined, a newsubstance is formed. This new substance has properties that may be quitedifferent from the properties of the substances that were used to produce it. If yeast is added to the sugar water mixture, the single celled organisms will digestthe sugar and use it to produce carbon dioxide, ethanol, and energy. Thesubstances that result are very different from those of the sugar that the yeastingested.A homogeneous mixture , also called a  solution,  is a mixture in which thesubstances are uniformly blended on the smallest scale. A homogeneous mixture ‘looks like’ one substance, because it is so uniform. Air is an example of ahomogeneous mixture. Air is a mixture of gases, mostly nitrogen and oxygen withsmall amount of carbon dioxide, water vapor and other gases. The  solvent is themajor component of a solution (the part that does the dissolving). The solute  isthe minor component(s) of the solution (the part that gets dissolved). A heterogeneous mixture is one in which the substances are not uniformly mixed.  These mixtures often look “lumpy” because we can see the different constituents.Concrete is an example of a heterogeneous mixture made up of rock, sand, andcement. A colloidial suspension  or colloid  is a mixture in which the substancesare uniformly mixed on a large scale, but these mixtures have particles which aretoo large and will eventually separate. Fog and milk are examples of colloids. Likeother colloids, fog and milk have a cloudy appearance, because colloids exhibit the Tyndall Effect . The individual particles that make up a colloid are too small toosee, but they are large enough to scatter the light that passes through them. Wecan see a car’s headlights from the side in the fog because a beam of light isscattered due to the Tyndall Effect. Examples of Mixtures of the Various States of Matter Before you begin this material, you need to understand the kinetic moleculartheory description of the states of matter and gas law calculations from unit 2. Gas as a Solute Although people tend to think of solutions as solids dissolved in liquids, mixtures of gases are solutions. They are homogeneous because gases diffuse throughouttheir container keeping the mixture ‘mixed.’ Air is a mixture of gases that variesin composition with weather and altitude, but, in any given area, it is a uniformmixture composed of nitrogen and oxygen with traces of other gases.The Composition of Air GasPercent byVolume in DryAir Nitrogen78.084Oxygen20.964Argon0.934CarbonDioxide0.035Neon0.0018Helium0.00052  Methane,CH 4 0.00017Krypton0.00011Hydrogen0.00005 This data is from Earth Fact Sheet, by NASA.The amount of water in the atmosphere varies (hot air can hold more watervapour than cold air) but totals roughly 1 percent. Gaseous Mixtures and Dalton’s Law of Partial Pressure Dalton’s law of partial pressures states that for a mixture of ideal gases the totalpressure of the mixture will be equal to the sum of each gas’s partial pressure, thepressure it would exert under the same conditions of volume and temperature if that gas were alone in the container. This law implies that any gas in a mixture of gases behaves independently of theothers. Kinetic molecular theory explains that gas particles are not attracted to orrepelled by one another. When they collide with each other or the walls of theircontainer, they bounce elastically. Because the total number of collisions dependson the number of collisions of all of the different types of particles, the totalpressure equals the sum of each gas’s individual pressure. If the gases are notideal, when they collide they interact (perhaps by reacting chemically or bysticking together due to intermolecular force). In that case, the pressure would belower than that predicted by Dalton’s law. Concept Check:  Three gases are in a container, A, B, and C. Each has the samepartial pressure. The container is cooled below the boiling point of the gas B,which then condenses into a liquid. What effect does this have on the totalpressure and the partial pressures of each gas? Answer:  The partial pressure of gas B drops radically. A liquid phase does notcontribute to the pressure of a gas except for its vapor pressure (the pressure dueto the small amount of liquid that evaporates in a closed container). The partialpressures of A and C do not change because the behavior of an ideal gas isindependent of the presence (or absence) of other ideal gases. The total pressuredrops by an amount equal to the difference between the srcinal pressure of gas Band its vapor pressure. Aerosols The components of a mixture do not have to be the same phase. Fog is anexample of a liquid (water) suspended in a gas (air). Smoke is an example of amixture in which the solutes are solids, liquids, and gases while the solvent is agas (air). When organic material burns it combines with oxygen to form non-metal oxides, especially carbon dioxide and water vapor. These are gases.Inadequate supplies of oxygen lead to the breakdown of the organic material intoa complex mixture of other compounds. According to the 1964 Surgeon General’s  Report on Smoking and Health “The particulate matter of MS [mainstreamcigarette] smoke contains at least 3,500 individual compounds.” The fine particlesand droplets form a suspension which scatters light by the Tyndall effect givingsmoke a hazy appearance.Mixtures composed of particles of solids/liquids suspended in a gas are called aerosols . Climate change theorists estimate that about 10% of the atmosphere’saerosols are due to human activities (such as burning fossil fuels) as opposed tonatural events (such as volcanic eruptions). Some computer models project thatthe atmosphere may cool due to reduction is transmitted solar radiation byatmospheric aerosols.  Liquid as a Solute Gases in a Liquid and Henry’s Law Gases can mix with liquids to form heterogeneous mixtures like whipped cream.They can also dissolve in a liquid to form a solution. The solubility of a gas in aliquid depends on the nature of the substances involved and on the pressure of thegas. Henry’s law states that the concentration of the dissolved gas is directlyproportional to its pressure: where C is the concentration of the gas, P is the pressure of the gas, and k is aproportionality constant that is unique for each gas/liquid combination and varieswith temperature. Kinetic molecular theory can help explain Henry’s law. If a gasis in contact with the surface of a liquid, the gas particles collide with the liquidparticles. Some of these collisions are elastic, but, in some cases, the gas particleswith lower than average kinetic energy will not be able to overcome theintermolecular attraction of the liquid. Some gas particles will stick to and mixwith the liquid. The number of collisions increases as the pressure increases, so anincrease in pressure increases the concentration of dissolved gas. The constant, k,is different for each combination of liquid and gas because the strength of theintermolecular force depends on factors such as polarity and the size of themolecule. The value of k also varies with temperature. As temperature increases,the kinetic energy of the gas particles increases, and fewer of the collisions resultin solution.  Concept Check: Carbonated beverages are bottled so that greater than oneatmosphere of carbon dioxide is in the container. Describe what happens on amolecular level when a can of soda pop is opened. Answer:  The unopened can has a relatively high pressure of carbon dioxide gascompared to the amount of carbon dioxide partial pressure in the air. Gaseouscarbon dioxide molecules strike the liquid, and the ones with low kinetic energycannot escape the intermolecular attractions. These become part of the liquidphase. At the same time, some of the dissolved carbon dioxide molecules haveenough kinetic energy to escape the attractions of their neighbors and vaporize.These processes are in equilibrium (happening at the same time and at the same

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Jul 23, 2017

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Jul 23, 2017
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