Properties Of Gases Chapter 5

Nature Of gases  11 elements are gases under normal conditions. The six noble gases are monoatomic. Other gases are diatomic or polyatomic molecules. Organic gases include methane(CH 4 ),propane(C 3 H 8 ) e.t.c.  Inorganic gases include ammonia(NH 3 ), and sulfur dioxide(SO 2 ).

Pressure  A sample of gas can be specified by the following properties:  number of moles of molecules present, volume of the sample, the temperature and the pressure. If we change any of these properties by adding more molecules, compressing the sample, or changing the temperature then we change the state of the gas.  Pressure (P) = force/area =F/A

 Force=mass x gravity/A =mg/A  The SI unit of pressure is Pascal (P a )  1P a =1kg/m.s 2 = 1N/m²  1 bar = 10 5 P a = 100kP a  1atm = x 10 5 P a = kP a  1atm =760 Torr  1atm= 14.7 lb/inch² Units Of Pressure

The Gas Laws  Boyles Law: If a gas is compressed at a constant temperature, the pressure of the gas increased in a certain way.  Pressure ∞ 1/volume or P = constant/V  This law states that the pressure of a fixed amount of gas at constant temperature is inversely proportional to the volume.

Boyle’s Law

Class Practice  What pressure would a sample of argon exert when compressed from 500ml to 300ml at constant temperature, given that the initial pressure is 750 Torr ?

Charles’s Law  For a fixed amount of gas under constant pressure, the volume varies linearly with the temperature.  Volume ∞ absolute temperature  or V=constant x T  Pressure ∞ absolute temperature  or P= constant x T  The volume of a fixed amount of gas at constant pressure is directly proportional to the absolute temperature.

 The pressure of a fixed amount of gas at constant volume is proportional to the absolute temperature.  Charles's Law - under conditions of constant pressure and volume, there is a direct relationship between the volume and absolute temperature for an ideal gas.V1 / T1 = V2 / T2  Charles's Law may also be called Gay-Lussac's Law. Charles did the original work, which was verified by Gay-Lussac.

Class Practice  In a study of an internal combustion engine, nitrogen gas in a 125 ml cylinder at 18°C is heated to 322°C. What is the final volume of the gas?

Avogadro’s Principle  At constant temperature and pressure, the volume of a gas is proportional to the amount of gas. This can be done by comparing the molar volumes of gases.  Molar volume= volume occupied/number of moles  V m = V/n  Volume occupied ∞ number of moles  V= constant x n

Avogadro’s principle  The volume occupied by a sample of gas at constant pressure and temperature is proportional to the number of moles of molecules present.

Class Practice  A 5.00L tank of carbon dioxide gas contains mol of CO 2(g). A chemist needs mol CO 2 (g) at the same temperature and pressure as the tank.What volume container will the chemist need for the sample?

The Ideal Gas Law  Charles law and Avogadro’s principle concluded that volume of a gas is directly proportional to temperature. V∞T and the number of moles V∞ n.  Boyle’s law has told us that volume is inversely proportional to the pressure V ∞ 1/P  Combining the three equation:  V∞ n x (T/P) or PV ∞ nT or PV= nRT  This is the ideal gas law equation.  R=PV/ nT R= L.atm/K.mol

 The ideal gas law, a limiting law contains all the relations describing the response of ideal gases to changes in pressure, volume, temperature, and moles of molecules.

Molar Volume  The ideal gas law can be used to predict the molar volume of gas under any conditions of temperature and pressure.  V m =V/n=nRT/P/n=RT/P  At standard temperature and pressure (STP) which means)0°C(273.15K) and 1.00atm the molar volume is  V m =( L.atm/K.mol) x (273.5K)/1.00atm=22.41L/mol

Class Practice  Calculate the volume occupied by 1.0 kg of hydrogen gas at STP?  Ans 1.1x10 4 L

Stoichiometry Of Reacting Gases  Gay Lussac was the first to summarize the relation between volumes of reacting gases in his law of combining volumes.  N 2(g) +2O 2(g) -  2NO 2(g)  In which 1 mol nitrogen reacts with 2 mol of oxygen to form 2 mol of nitrogen dioxide.

Class practice  Calculate the volume of ethyne (acetylene) C 2 H 2 produced at 25°C and 1.00atm when 10 g of calcium carbide reacts completely with water in the reaction  CaC 2 (s) +2H 2 O(l) -  Ca(OH) 2 (s) +C 2 H 2(g)  Ans 3.8 L

Gas Density  The densities of gases increase with increasing pressure and decreasing temperature and are proportional to the molar mass.

Determining molar mass from gas density  The volatile organic compound geraniol a component of oil of roses is used in perfumes.The density of the vapor at 260°C is 0.480gL when the pressure is 103 Torr. What is the molar mass of geraniol?

Diffusion and Effusion  The gradual dispersal of one substance through another substance such as the odor of perfume,spreading through the air is called as diffusion.  The escape of one substance through a small hole into a vacuum, like the escape of air through a small hole in a spacecraft is called effusion.

 Diffusion Effusion

 Rate Of Effusion ∞ √(1/molar mass)  Or rate ∞ √1/M  Effusion time ∞ √molar mass or  t effuse ∞√M  Effusion time of A/Effusion time of B = √(molar mass of A/molar mass of B) or t effuse (A)/t effuse (B)= √ (M A /M B )  The time it takes the molecules of a gas to effuse through an opening or diffuse through another gas is directly proportional to the square root of it’s molar mass

 If it takes a certain amount of helium atoms 10 s to effuse through a porous barrier, how long does it take the same amount of methane molecules CH 4 under the same conditions?

Home work  Page 211  5.28,5.32  Page 213  5.56,5.60