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Chapter 3

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Western University
Chemistry 1027A/B
Felix Lee

Chapter 3: Gases Topic 3.1: Gases 3.1.2 How Do the Three States of Matter Differ? -3 main states of matter: solid, liquid, gas -a substance may exist in only one state, or it may be present in 2-3 states Solids -at some temperature, every substance (except possibly Helium) will form a solid -in the solid state, atoms, molecules or ions are locked into a fixed position relative to others in the substance -forces which hold this “lattice” together can be very strong (ionic solids) or extremely weak (inert gases) -strength of these interactions determines the melting point -strong the interaction, higher the melting point -atoms and molecules in a solid are not moving relative to each other but are always vibrating -vibrational energy increases with temperature -at some point, magnitude of vibrational modes overcome attractive forces -atoms/molecules lose their fixed positions and become a liquid Liquids -attraction between atoms/molecules keeps them touching, although the species are free to move about -substance will take the shape of the container that holds it -increased motion in liquid-atoms/molecules take up more space -solid is denser than its corresponding liquid (except water) Gases -at a high enough temperature forces holding atoms/molecules together is completely overcome (gas) -each atom/molecule has a much more significant translational energy -translational energy: the atom/molecule moves in a specific direction, there is a change in position -gas will expand to fill its container -collisions of gas with walls of the container are the “pressure of the gas” 3.1.3 How Was the Ideal Gas Law Developed? -describes the relationship between moles of gas, pressure, volume and temperature -only true under “ideal” conditions -pressure must not be too high (or molecules will interact-ideal gas law assumes gases do not interact) -temperature must not be too close to the condensation temperature of the gas -under the above conditions, the following relationship exists: Boyle’s Law -for a fixed amount of gas at a constant temperature, the gas volume is inversely proportional to the gas pressure -temperature and moles must be constant -P1V1=P 2 2 for the same gas Charles’ Law Chapter 3: Gases -the volume of a fixed amount of gas at a constant pressure is directly proportional to the Kelvin temperature -pressure and moles stay constant - -Charles’ Law and Boyle’s Law can be combined: Avogadro’s Law -equal volumes of different gases, at the same temperature and pressure, contain equal numbers of molecules -at constant temperature and volume, P n Topic 3.2: The Ideal Gas Law 3.2.2 What is the Molar Volume of a Gas? -for solids and liquids, the mass and volume of 1 mole of a substance depends on the substance being considered -Avogadro’s Law: 1 mole of any ideal gas always occupies the same volume at a given temperature and pressure, regardless of the gas in question -equal volumes of different gases at the same temperature and pressure contain the same number of moles -volume of one mole of gas at STP is 22.4L (molar volume of an ideal gas) 3.2.3 How Do We Relate Density and Molar Mass? -the ideal gas equation, PV=nRT can be used to derive a general relationship between gas density and its molar mass: -at a constant temperature, gas density is directly proportional to its pressure -at a constant pressure and temperature, density is related to the molar mass of a gas 3.2.4 What is Dalton’s Law? -in the case of a mixture with several ideal gases, the total volume of the mixture, the total pressure and the total number of moles behave as a single gas with respect to the ideal gas law: -each of the constituents in a gas mixture is said to exhibit a partial pressure that contributes to the total pressure -Dalton’s Law of Partial Pressure: ‘The total pressure exerted by a mixture of gases is the sum of all the partial pressures of these gases’ Mole Fractions and Partial Pressure -if n is the number of moles of an individual constituent in a gas mixture and n the total number of i T moles in the mixture, the mole fraction (X) is -at constant temperature and volume, the pressure is directly proportional to the moles, this means: -Dalton’s Law says that the sum of the partial pressures, P , P , and P , of a mixture of ideal gases A, B, A B C and C equals the total pressure, P T Chapter 3: Gases -if the pressures are expressed in terms of mole fractions then: PA=XA T PB=X B T PC=XC T XA+ X BX =c -the ideal gas law can be applied to each individual constituent of a gas mixture, as well as the gas mixture as a whole -at a constant temperature and pressure, the volume of a gas is directly proportional to the number of moles Vapour-Liquid Equilibrium and Vapour Pressure -at equilibrium between evaporation and condensation, pressure will remain constant -vapour pressure is dependant only on the temperature of the substance, it does not depend on volume of the container or presence of another gas -at higher temperatures, vapour pressure increases as more energy is provided to the liquid molecules (allowing them to escape into gas phase) -solvents that are more volatile (have lower boiling points) have higher vapour pressures -boiling point: the temperature at which the vapour pressure of a solvent is equal to atmospheric pressure Average Molar Mass of a Mixture ̅̅̅̅̅ -average molar mass, , or a mixture of different gases is the weighted average of the molar masses of the different components: ̅̅̅̅̅ ( ) ( ) ( ) -since ̅̅̅̅̅is based on a weighted average, its value is usually closest to the molar mass of the component in the greater quantity 3.2.5 Gas Reaction Stoichiometry -at a constant pressure and temperature, the volume of a gas is directly proportional to the number of moles -at a constant temperature and volume, the pressure of a gas is directly proportional to the number of moles -in the stoichiometry of gas reactions, the stoichiometric coefficients of gaseous reactants and products (ie number of moles) are directly related to the volume and pressure -when pre
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