GASES Chapter 12 in your text book

KINETIC-MOLECULAR THEORY OF GASES Gas particles are in constant random and rapid motion. The space between gas molecules is very large compared to the actual size of a molecule/atom of gas. Gas molecules collide with each other and the sides of the container that holds them. Collisions with other gas particles are completely elastic and result in no net change in kinetic energy. Collisions with the sides of the container result in the creation of force per unit area of a surface =pressure. Pressure exerted by a gas is the result of the collisions of gas molecules against the walls of its container.

Properties of Gases are fluids = any substance that can flow have low density are highly compressible completely fill a container have mass exert pressure move thru each other rapidly pressure depends on the temperature and volume of container

Atmospheric Pressure Pressure exerted on objects on surface of earth by the atmospheric gases in response to gravitational pull STP (Standard Temperature & Atmospheric Pressure) – Amount of force applied on objects at sea level

UNITS OF PRESSURE atm = atmospheric pressure mmHg = millimeters of mercury Pa = pascals kPa = kilopascals torr = Torr

Pressure Conversions 1 atm = 760 mmHg 1 atm = 760 torr 1 atm = kPa 1 atm = 101,325 Pa 1atm = lb/in 2 (psi) 1bar = 100,000 pascals 1N/m 2 = 1 pascal So, 1 atm = 760 mmHg = 760 torr = kPa = 101,325 Pa

How does elevation affect atmospheric pressure? High elevations = less atmospheric pressure (less dense air exerts less pressure) EX. Denver Colorado Low elevations = greater atmospheric pressure (atmosphere is denser as you move closer to the earth’s surface) EX. Death Valley

How is pressure measured? Barometer = an instrument that measures atmospheric pressure (unit = mmHg). Manometer = instrument that measures the pressure of a confined (trapped) gas against atmospheric pressure.

Open Manometers When gas pressure is less than atmospheric pressure, the mercury is pushed toward the gas reservoir. The balance is between the air on the left and the gas plus mercury on the right: P air = P gas + P Hg The weight of the mercury is measured as the height difference: P Hg =  h So P air = P gas +  h Or P gas = P air -  h P Hg P air

Open Manometers When gas pressure is greater than atmospheric pressure, the mercury is pushed toward the open end. The balance is between the gas on the right, and the air plus mercury on the left. P air + P Hg = P gas The weight of the mercury is measured as the height difference: P Hg =  h So P gas = P air +  h P air P Hg

Sample Problems P AIR = 765 mm  h = 27 mm  h = 13 mm Pgas = 753 mm  h = 20 mm P air = 790 mm Find the pressure of the gas in each manometer. Pay attention to whether the manometer is open or closed!

Sample Problem Answers 1. P gas +  h = P air P gas = 790 mm - 20 mm = 770 mm Hg 2. P gas = vacuum + 13 mm = 13 mm Hg 3. P gas = P air +  h P gas = = 792 mm Hg

Measureable Properties of Gases P = pressure exerted by gas V = volume (L) occupied by the gas T = temperature (K) of the gas n = number of moles of the gas

Boyle’s Law P 1 x V 1 = P 2 x V 2 When the temperature and the amount of gas is constant, the pressure & volume have an indirect relationship. if V increases, the P decreases if V decreases, the P increases

Example A syringe holds 2.0ml of oxygen gas at a pressure of 672 mmHg. The volume of the syringe is reduced to 1.5ml by pushing in the plunger. If temperature and moles remain constant what is the new pressure in atmospheres?

Charles’s Law V 1 = V 2 T 1 T 2 When the pressure and amount of gas are constant, temperature and volume have a direct relationship: if V increases, T increases if V decreases, T decreases

EXAMPLE If 1,235mL of nitrogen gas is placed in a sealed container at a temperature of 57°C, what will its volume (in liters) be when the temperature of the gas reaches 150°C?

62.7 mL of helium is at a pressure of 842 mmHg. The volume of the gas increased to 96.5 mL, what is the new pressure?

Question of the Day If 254mL of nitrogen gas is placed in a sealed container at a temperature of 52°C, what will its volume (in liters) be when the temperature of the gas reaches 140°C?

Gay-Lussac’s Law When the volume and amount of gas are constant, the pressure and temperature have a direct relationship: if P increases, T increases if P decreases, T decreases (P= same unitT = Kelvin)

Avogadro’s Law When pressure and temperature are constant, volume & the number of moles have a direct relationship if V increases, moles increases if V decreases, moles decreases (V = litersn = moles)

Gay-Lussac’s Law A can of gas is at 30. kpa and 20°C. What is its pressure at 48°C

Avogadro’s Law If 0.105mol of helium gas occupies a volume of 2.35L at a certain temperature & pressure, what volume would 0.432mol of helium occupy under the same conditions?

Combined Gas Law Combination of Boyle’s, Charles’s, & Gay-Lussac’s (Moles are constant) P = same unit V = liters T = Kelvin

Combined Gas Law A sample of methane gas that has a volume of 3.8L at 8°C and 362kPa is heated to 84°C and the pressure is raised to 573kPa. Calculate its new volume

Dalton’s Law of Partial Pressures Each gas in a mixture creates pressure as if the other gases were not present. The total pressure is the sum of the pressures created by the gases in the mixture The pressure each gas exerts in the mixture is called its partial pressure P total = P 1 + P 2 + P 3 + …….P 4

Dalton’s Law A mixture of neon and argon gases exert a total pressure of 2.39atm. The partial pressure of neon alone is 1.84atm. What is the partial pressure of argon gas?

Ideal Gas Law PV = nRT P must be in atm V must be in liters R = T must be in Kelvin n must be in moles

Ideal Gas Law A weather balloon contains 1.10 x 10 5 mol of helium and has a volume of 2.37 x 10 6 L at 1.00 atm. Calculate the temperature of the helium in the balloon in Kelvin and °C.