Chapter 5 Gases Gases o Gases are composed of particles that are flying around very fast o Gas molecules are constantly in motion o As they move and strike a surface they push on that surface Push Force o The pressure of a gas depends on several factors Number of gas particles in a given volume Volume of the container Average speed of the gas particles How to measure Air Pressure o Atmospheric pressure is measured with a barometer o Column of mercury supported by air pressure o Force of the air on the surface of the mercury counter balances the force of gravity on the column of mercury Pressure Units o 1 atm 760 mmHg o 1 torr 760mmHg 1 atm 1 torr o The SI unit is Pascal Pa 1 Pa 1 N m2 1 atm 101 325 Pa o 1 atm 29 92 inHg o 1 atm 14 7 psi o The pressure of a gas trapped in a container can be measured with an instrument called a manometer Manometer Boyle s Law In this sample the gas has a larger pressure than the atmosphere o Robert Boyle 1627 1691 o Pressure of a gas is inversely proportional to its volume Constant T and amount of gas Graph P vs V is curve Graph P vs 1 V is straight line o As P increase V decreases by the same factor o P x V constant o P1V1 P2V2 o When you decrease the volume of the container with the same number of molecules in the container more molecules will hit the wall at the same rate Charles s Law o Jacques Charles 1746 1823 o Volume is directly proportional to temperature Constant P and amount of gas Graph V vs T is straight line o As T increases V also increases o Kelvin T Celsius T 273 o V constant x T If T measured in Kelvin Charles s Law A Molecular View o The pressure of gas inside and outside the balloon are the same o At high temperatures the gas molecules are moving faster so they hit the sides of the balloon harder causing the volume to become larger Avogadro s Law o Amedeo Avogadro 1776 1856 o Volume is directly proportional to the number of gas molecules V constant x n Constant P and T More gas molecules larger volume o Count number of gas molecules by moles o Equal volumes of gases contain equal numbers of molecules The gas doesn t matter Ideal Gas Law o By combining the gas laws we can write a general equation o R is called the gas constant o The value of R depends on the units of P and V We will lose 08206 atm L mol K and convert P to atm and V to L o The other gas laws are found in the ideal gas law if two variables are kept o Allows us to find one of the variables if we know the constant other three Standard Conditions 0 C Molar Volume o STP Standard Temperature and Pressure o Standard Pressure 1 atm o Standard Temperature 273 15 K o Solving the ideal gas equation for the volume of 1 mol of gas at STP gives 22 4 L 6 022 x 1023 molecules of gas Notices the gas is immaterial o We call the volume of 1 mole of gas at STP the molar Volume It is important to recognize that one mole measures of different gases have different masses even though they have the same volume Density at Standard Conditions o Density is the ratio of mass to volume o Density of a gas is general given in g L o The mass of 1 mole molar mass o The volume of 1 mole at STP 22 4 L o Density Molar Mass g 22 4L Molar Mass of a Gas o One of the methods Chemists use to determine the molar mass of an unknown substance is to heat a weighed sample until it becomes a gas measure the temperature pressure and volume and use the ideal gas law o Molar Mass Mass in grams moles Mixture of Gases o When gases are mixed together their molecules behave independent of each other All the gases in the mixture must have the same volume All completely fill the container each gas s volume the volume of the container All gases in the mixture are at the same temperature Therefore they have the same average kinetic energy o Therefore in certain applications the mixture can be thought of as one gas 78 by Volume Composition of Dry Air o Nitrogen N2 o Oxygen O2 o Argon Ar o 9 by Volume o Carbon Dioxide CO2 21 by Volume 04 by Volume Partial Pressure o The pressure of a single gas in a mixture of gases is called its partial pressure o The sum of the partial pressures of all the gases in the mixture equals the total pressure if gases behave independently Do not react to each other o Dalton s Law of Partial Pressures o PTotal PA PB PC Mole Fraction o The fraction of the total pressure that a single gas contributes is equal to the fraction of the total number of moles that a single gas contributes o The ratio of the moles of a single component to the total number of moles in the mixture is called the mole fraction c Gases Volume 100 o The partial pressure of a gas is equal to the mole fraction of that gas times the total pressure Collecting Gas by Water Displacement Reactions Involving Gases o The principle of reaction stoichiometry from Chapter 4 can be combined with the gas laws for reactions involving gases o In reactions of gases the amount of a gas is often given as a volume Instead of moles As we ve seen you must state pressure and temperature o The ideal gas law allows us to convert from the volume of the gas to moles then we can use the coefficients in the equation as a mole ratio o When gases are at STP use 1 mol 22 4 L o P V T of Gas A Mole A Mole B P V T of Gas B Kinetic Molecular Theory o The particles of the gas either atoms or molecules are constantly moving o The attraction between particles is negligible o When the moving gas particles hit another gas particle or the container they do not stick and continue moving in another direction hence collision is elastic o There is a lot of empty space between the gas particles o The average kinetic energy of the gas particles is directly proportional to the Kelvin temperature 1 Because the gas particles are constantly moving they strike the sides of the container with a force 2 Because the gas keeps moving around and spreading out until they fill the container they take the shape and the volume of the container they are in 3 Because there is a lot of unoccupied space in the structure of a gas the gas molecules can be squeezed closer …