Chapter 13 Gases 1

Pressure Pressure is defined as force per unit area Pressure is exerted equally in all directions Pressure decreases with increasing altitude 2

Units for pressure 1 atm= 760 mm Hg=760.0 torr 3

Pressure conversions Use dimensional analysis to convert from one unit of pressure to another. Together: Convert 3.2 atm into mm Hg On your own: Convert 15 mm Hg into atm 4

Boyle’s Law Experiments with pressure were first performed by Robert Boyle Increase pressure means a decreased volume pressure and volume have an inverse relationship 5

Boyle’s Law (changing conditions) P 1 V 1 =P 2 V 2 (Must occur at constant temperature “Boyle’s at a constant temp.” Freon-12 was once widely used in refrigeration systems, but now has been replaced by another compound that does not lead to breakdown of the ozone layer. Consider a 1.5 L sample of freon at a pressure of 56 torr. If the pressure is changed to 150 torr at constant temp. what is the new volume? 6

Charles’ Law (changing conditions) Scientists continued to study gases after Boyle’s discovery. Jacques Charles showed that the volume of a gas was proportional to the temperature at constant pressure. This law illustrates a direct relationship between volume and temperature V 1 /T 1 = V 2 /T 2 7

Charles’ Law problem The volume of a sample of gas is 2.0 L when the temp is 11 o C, the temperature is changed which makes the new volume 3.0L. What is the new temperature? 8

Avogadro’s Law (changing conditions) Avogadro showed a direct relationship between moles and volume. V 1 /n 1 = V 2 /n 2 V= volume n=# of moles 9

Ideal gas Law (static conditions) 3 laws combine to form the ideal gas law: PV=nRT P= pressure(atm) V=Volume(L) n=# of moles R= Universal gas constant=.082 Latm/Kmol T = temp (K) 10

ideal gas law problem A sample of H 2 gas has a volume of 8.56 L at a temp of 0 o C and a pressure of 1 atm. Calculate the number of moles of H 2 gas 11

On your own Your hot air balloon is cube shaped and has the following dimensions: 1.2 m high 1 m wide 1 m deep. Knowing that 1 m 3 = 1L a.calculate the volume of your hot air balloon b.The pressure outside is 1 atm and the temp of air in your hot air balloon is 50 o C. c.How many moles of air is in your hot air balloon? 12

STP STP stands for standard temperature and pressure Standard temp is 0 o C Standard pressure is 1 atm Together: You have 11.2 liters of an ideal gas at STP. The number of moles of gas in this sample is…? 13

Answer = 5.33L 14

Daltons Law of partial pressures This law deals with mixtures of gas in a container For a mixture of gases, the total pressure is the sum of all the partial pressures of each gas in a mixture. For example if a tank held helium that had a pressure of 1 atm, H 2 that has a pressure of 2 atm and N 2 that has a pressure of 3 atm. The total pressure in the tank will be = 6 atm 15

Dalton’s Law of partial pressures Partial pressure= pressure a gas would exert if it were alone in a container P total =P 1 + P 2 + … 2 gas tanks hold 2 different gases. Tank 1 has Ar at a pressure of 1 atm. Tank 2 holds He at a pressure of 6 atm. What is the pressure of a new tank if both gases are combined in the new tank? 16

Stoichiometry Just as we did Stoich problems before (using balanced chemical equations to predict amounts of cmpds), we can also do stoich with gases 17

Solving Gas stoich 1.Figure out equation and balance 2.Write What’s given under cmpd. 3.Put ? & units of what we want under desired cmpd. 4.Make conversions into moles 5.Use mole ratios to determine moles of ? Cmpd. 18

Helpful shortcuts to step 4 4. Make conversions into moles L  mole (divide by 22.4) g  mole (divide by molar mass) 19

Stoich problem Calcium carbonate decomposes at high temperatures to form carbon dioxide and calcium oxide. How many grams of calcium carbonate will I need to form 3.45 liters of carbon dioxide? 20

STP cont. Answer = 0.5 moles The volume of a sample of gas at STP is 9.75 liters. What volume will this gas occupy at 100 o C and a pressure of 2.50 atm 21

Combined gas law (changing conditions) P 1 V 1 = P 2 V 2 T 1 T 2 Combined gas law is used when more than 1 variable is changing Be sure units are the same across equal sign!!! Ex. A gas balloon has a volume of liters when the temperature is 45.0 °C and the pressure is mm of mercury. What will its volume be at 20.0 °C and mm of mercury pressure? 22

Answer = 92.7L 23

Kinetic Molecular theory Explains behavior of ideal gases 1.Gases consist of tiny particles (atoms) 2.Particles are smal compared to distances between them 3.Particles are in constant random motion 4.Particles do not interact with each other 5.Avg KE of particles are proportional to temp. in kelvin 24