1 Chapter 11 The Behavior of Gases Objectives:  Analyze the changes of a gas while changing pressure, temperature, volume and number of particles  Measure atmospheric pressure, convert between pressure units and calculate gas laws

2 Basic Assumptions of Kinetic Theory of Gases: The Kinetic Theory of Gases: 1. Gases are composed of tiny particles (atoms or molecules), hard spheres with insignificant volume. In-between is empty space without attractive or repulsive forces between them. This explains the “Compressibility and Expandability of Gases”

3 Gas particles are in constant motion 2. Gas particles travel fast in a straight line until they collide with one another or with the container wall. 3. All collisions are perfectly elastic, which means that the total kinetic energy remains the same.

4 Gas pressure is a force exerted by a gas per surface area of the container wall. Gas pressure and Number of Gas Particles

5 Amount of Gas and Gas Pressure Twice the number of gas particles doubles the pressure. When a gas is pumped into a closed rigid con- tainer, the pressure increases proportional to the number of particles added. Twice the number of gas particles doubles the pressure.

6 The ball that contains more air has higher pressure and greater mass.

7 Doubling the kinetic energy of gas particles (Kelvin temperature scale) doubles the pressure. Temperature and Gas Pressure

8 Atmospheric Pressure results from collisions of air molecules with objects It decreases with elevation and also depends on the weather. (barometric pressure). Atmospheric pressure is quite strong 14.7 pounds per square inch

9 Barometers measure atmospheric pressure. Torricelli’s Barometer

10 The SI unit for pressure is Pascal (Pa) Other units are: mm Hgtorr millimeter of mercury (mm Hg) or torr, mb grams per square centimeter: millibar (mb), atm atmospheres (atm), psi pounds per square inch (psi), etc. A pressure gauge measures pressure in closed containers, for example tires.

11 STP Standard Temperature and Pressure (STP) = 0  C and 1 atm Standard Atmospheric Pressure kPa = 760 mm Hg = 1 atm = 1013 mb= 14.7 psi

12 Example problem: 17.6 psi How much is 17.6 psi expressed in kPa?Conversion factor kPa 14.7 psi known pressure Start with known pressure and cancel out old unit 17.6 psi101.3 kPa121 kPa 17.6 psi x kPa = 121 kPa 14.7 psi Converting Pressure Units kPa = 760 mm Hg = 1 atm = 1013 mb = 14.7 psi kPa = 1 1 atm = kPa = 1 1 atm = 1 etc. any combination 760 mm Hg 14.7 psi 760 mm Hg 14.7 psi because each refer to the same pressure

13 Volume and Gas Pressure Changing volume to change Pressure: 1/2 the volume doubles the pressure P 1 = 100 kPaP 2 = 50 kPaP 3 = 200 kPa V 1 = 1.0LV 2 = 2.0 LV 3 = 0.5 L

14 Boyle’s Law For a given mass of gas at constant temperature, the volume of the gas varies inversely with the pressure. P 1 x V 1 = P 2 x V 2 P 1 = 100 kPa P 2 = 50 kPa P 3 = 200 kPa V 1 = 1.0L V 2 = 2.0L V 3 = 0.5 L P 1 x V 1 = P 2 x V 2 = P 3 x V 3 = 100 (Lx kPa)

15 Increasing the temperature while holding the pressure constant will _____?______ the volume Why?

16 Charles’ Law Charles ( ) observed that at constant pressure the graph of gas volume versus temperature yields a straight line. The lines extended (extrapo- lated) to zero volume all intersect the temperature axis at the same point, C.

17 Absolute Zero Lord Kelvin identified C, as absolute zero, the lowest temperature, at which the average kinetic energy of gas particles would be zero. 0 K = C

18 Charles’ Law The volume of a gas is directly proportional to its Kelvin tempera- ture if pressure and mass are kept constant. V 1 = 1L T 1 = 300 K V 2 = 2 L T 2 = 600 K 100 kPa V 1 V 2 T 1 T 2 =

19 The Combined Gas Law Is a combination of Boyle’s and Charles’ laws, when pressure, temperature and volume all change at the same time. P 1 x V 1 =P 2 x V 2 T 1 T 2

20 Table 1: How does the pressure change when only one variable changes?

21 The law of combining gas volumes states that is chemical reactions involving gases, the ratio of the gas volumes is a small whole number, which reflect the coefficients in the balanced equation. (use coefficients of balanced equation for volumes) 22 2 H 2 (g) + O 2 (g)  2 H 2 O (g) 2 liters Hydrogen gas + 1 liter oxygen gas produce only 2 liters of water vapor

22 Avogadro’s Principle Equal volumes of gases at the same temperature and pressure contain equal number of particles. because particles are tiny compared to the amount of empty space around them For gases it makes no difference what kind of atom or molecule

23 Spray Cans Push button Gas propellant (High Pressure) Outside air (low Pressure) Product Pressure-resistant curved bottom