Chapter 9 Objectives Intro to gases, pressure Boyle’s Law, Charles’ Law, Avogadro’s Law, Dalton’s Law, Ideal Gas Law, Graham’s Law Kinetic Molecular Theory Deviations from ideal behavior and the van der Waals equation
Intro to Gases Gases will expand to fill the volume of their containers Mixtures of gases will mix evenly and completely when confined in a container Gases are compressible and have lower densities than solids or liquids
Pressure Pressure is defined as F/A 1 Pascal = 1 N/m 2 Instruments for measuring pressure often use mercury because of its high density STP – standard temperature and pressure 1 atm and 0° C Hg Hg is a liquid at room temperature Atmospheric pressure
Units of Pressure 1 atm = 760 mm Hg 760 torr 101325 Pa 1.01325 bar 14.696 psi
Boyle’s Law The Pressure-Volume Relationship At constant T, the volume of a gas is inversely proportional to the pressure As volume decreases, pressure increases. As volume increases, pressure decreases Since P 1 V 1 = constant and P 2 V 2 = constant, P 1 V 1 = P 2 V 2 P constant V
A sample of Neon to be used in a neon sign has a volume of 1.51 L at a pressure of 635 torr. Calculate the volume of the gas after it is pumped into the glass tubes of the sign, where it shows a pressure of 785 torr. 2 nd : what information is given? 1 st : what are we looking for? 3 rd : Check for the same units of pressure: Example 1
4 th : Solve for V 2 using Boyle’s Law V 2 = (V 1 x P 1 )/P 2 = (1.51 L x 635 torr)/785 torr = 1.22 L
Temperature A measure of the average translational kinetic energy of gaseous molecules The faster molecules move on average, the greater their kinetic energy more kinetic energy means higher temperatures KE = ½ mv 2 The temperature of a gas can be related to the volume it occupies
Charles’s Law The Temperature-Volume Relationship At constant P, the volume of a gas increases linearly with temperature V = constant x T As temperature increases, volumes increases. As temperature decreases, volume decreases. Since V 1 = constant x T 1 and V 2 = constant x T 2 , 2 2 1 1 T V T V HAVE to use Kelvin temperatures!
Implications of Charles’s Law Data Plotting V vs. T gives a straight line Extrapolating the plots for different gases to V = 0 all gave the same x-intercept of -273.15°C Lord Kelvin named this absolute zero and based his temperature scale on it
Temperature is sometimes measured with a gas thermometer by observing the change in the volume of the gas as the temperature changes at constant pressure. The hydrogen in a particular hydrogen gas thermometer has a volume of 150.0 cm 3 when immersed in a mixture of ice and water (0.00 °C).
- Fall '19