1. The Nature of Gases

2. All gases exhibit remarkably similar physical behavior Pure gases and mixtures behave the same! A mole of methane (CH 4 ) in a balloon behaves just like a mole of air in a balloon! Because all gases behave the same, regardless of their chemical identities, we refer to them all as particles. Particles include single atoms (ex: He), diatomic molecules (ex: O 2 ), and polyatomic molecules (ex: CO 2 ).

3. Properties and Characteristics of Gases 1. Gases have mass. 2. Gases are easy to compress. 3. Gases fill their containers completely.

4. Properties and Characteristics of Gases 4. Gases move through other gases easily and rapidly. 5. Gases exert pressure. 6. Gas pressure depends on temperature.

5. Kinetic Molecular Theory—esp. gasses! gases (and other matter) are made up of tiny particles (atoms and molecules) that have mass the distance between particles is very large the particles are in constant, random motion collisions between particles and with container walls are elastic (no energy is lost to friction) kinetic energy of a gas depends on the temperature gas particles don’t exert forces on each other (there is no attraction – no IMF)

6. https://twigcarolina.com/experiment/frozen-balloon-4210/

7. Measuring Gases To completely describe a gas and make predictions about behavior when conditions are changed, four variables must be examined: amount – usually measured and described in terms of moles relate mass to moles using molar mass relate particles to moles using Avogadro’s number From the periodic table!1 mole = 6.02x10 23 particles volume – usually measured in liters; volume of gas is simply the volume of its container

8. remember metric prefixes and conversions: 1000 L = 1 kL 100 L = 1 hL 10 L = 1 daL 1 L = 1 L 1 L = 10 dL 1 L = 100 cL 1 L = 1000 mL Others on the front of your data book

9. Measuring Gases temperature – the average kinetic energy of the gas; measured in lab on a Celsius thermometer, then converted to the Kelvin scale (recall that absolute zero is the temperature at which all molecular motion ceases) K = °C + 273 25°C = 298 K 100°C = 373 K 225 K = -48 °C pressure – from the collision of particles with the container walls; each collision results in an outward “push”

10. Pressure – air and atmospheric pressure Atmospheric pressure is a result of the fact that gases are pulled on by gravity and the gas particles are constantly crashing into each other and you (and everything else). There is higher pressure at lower elevation because the column of air above you is longer (and therefore heavier!).

11. Pressure Pressure is measured by a barometer (the height of the mercury column is balanced by the atmospheric pressure.)

12. Calculating Pressure Pressure = Force Area Common units: pascals, kilopascals, atmospheres, millimeters of mercury (mm Hg), bars, torrs Conversions: 1 N/m 2 = 1 Pa (N/m 2 = Newton per square meter) 1000 Pa = 1 kPa 1 atm = 101.3 kPa 1 atm = 101,325 Pa 1 atm = 760 mm Hg

13. Complete the following conversions using factor labeling: a. 2.2 atm = ________kPa b. 103,442 Pa = _________kPa c. 778 mm Hg = _________ atm 2.2 atm x 101.3 kPa = 222.86 kPa = 220 kPa 1 atm 103442 Pa x 1 kPa = 103.442 kPa 1000 Pa 778 mm Hg x 1 atm = 1.02368 atm = 1.02 atm 760 mm Hg

14. https://twigcarolina.com/film/gas-laws-3612/

15. Practice A researcher checks a barometer to find that atmospheric pressure is 1.05 atm at the beginning of an experiment involving gases. What is the pressure in millimeters of mercury?

16. Enclosed Gases Open container – gases can move freely, they will expand in response to added energy such that the pressure stays the same inside and out. Closed container – gases cannot move freely, in response to added energy the pressure inside the container will increase relative to the outside. How would you describe the pressure of a gas in an expandable container like a balloon?

17. Standard Temperature and Pressure Referred to as “STP”: Temperature = 0°C or 273 K Pressure = 1 atm or 760 mm Hg or 101.3 kPa What do you already know about gas at STP?

18. Avogadro’s Law Equal volumes of gases at the same temperature and pressure contain an equal number of particles. all gases show the same physical behavior a larger volume is due to a larger number of particles (assuming P and T are constant) 1 mole of gas at STP = 6.02 x 10 23 particles = 22.4 L Because of this….

19. Dalton’s Law of Partial Pressures The sum of the partial pressures of all the components in a gas mixture is equal to the total pressure of the gas mixture. P T = P a + P b + P c + …

20. Example Problems 1. What is the atmospheric pressure if the partial pressures of nitrogen, oxygen, and argon are 604.5 mm Hg, 162.8 mm Hg, and 0.5 mm Hg, respectively? (remember, P T = P a + P b + P c ) P T = 604.5 mm Hg + 162.8 mm Hg + 0.5 mm Hg = 767.8 mm Hg 2. A mixture of gases contains 3 moles of Neon and 5 moles of Argon. If the total pressure is 2.5 atm, what is the partial pressure of each component? (Hint: neon contributes 3/8 of the total pressure…) 2.5 atm is created by 8 moles of gas, so if Ne is 3/8 and Ar is 5/8… P Ne = (3/8) x 2.5 atm = 0.94 atm P Ar = (5/8) x 2.5 atm = 1.56 atm P T = 0.94 atm + 1.56 atm = 2.5 atm

21. Collecting a gas “over water” Because gases mix readily with other gases, they must be collected in an environment where mixing cannot occur. The easiest way to do this is with water because water displaces the air in, say, a test tube. So when a gas is collected “over water,” that means the container (test tube) is filled with water and the gas is bubbled through the water into the container. However, water vapor will exist above the surface of the water = water vapor Thus, the pressure inside the container is from the collected gas and the water vapor. This is where Dalton’s Law of Partial Pressures becomes useful.

22. Collecting a gas “over water”

23. Practice Hydrogen is collected over water in a collecting bottle. If the partial pressure of water vapor is 85 mm Hg when the total pressure of the gas is 665 mm Hg, what is the partial pressure of the hydrogen gas in the bottle? (remember, P T = P a + P b + P c )

24. Collecting a gas “over water” The vapor pressure from water depends on the temperature of the water. You can look it up if necessary. At room temperature (about 21°C) how much pressure would be due to water vapor?