Gases Implications of the Kinetic Molecular Theory

Pressure Produced by Gases Pressure is the force per unit area on a surface Collisions of gas molecules with surfaces produce pressure

Atmospheric Pressure You could not drink from a straw if there were no atmospheric pressure.

Atmospheric Pressure We feel the effects of atmospheric pressure on Earth. This pressure varies due to elevation and weather conditions.

Units for Pressure A barometer is a device used to measure pressure. This barometer measures the atmospheric pressure by determination of the column height of mercury that can be supported in an evacuated tube. 1 atm = 760 mm mercury = kPa = 760 torr

Ideal Gases Ideal gas is a hypothetical gas, a “perfect” gas. Assumptions made when performing calculations are that the gas is ideal meaning: – Particles are far apart relative to their size – Particles have elastic collisions – Particles are in continuous rapid, random motion – Particles don’t attract one another – Temperature of a gas depends on the average kinetic energy of the particles

Properties of Gases Expand to fill their container Fluid Low Density Compressible Diffusion and Effusion – Diffusion – spreading out without being stirred – Effusion – process in which gas particles pass through a tiny opening (smaller particles move faster and effuse faster)

Ideal Gas Law Describes the relationships between pressure, volume, temperature, and moles in a mathematical formula for gases. PV=nRT R is a constant used in calculations. Units for all variables in a gas law problem must conform to units of the R value used.

Solving Ideal Gas Law Problems R =.0821 L. atm mol. K What is the pressure in atmospheres exerted by a mol sample of nitrogen gas in a 10.0 L container at 298 K?

Solution to Problem PV = nRT P (10.0 L) = (0.500 mol) ( L. atm/mol. K) ( 298 K) P = (0.500 mol) ( L. atm/mol. K) ( 298 K) 10.0 L P = 1.22 atm

Practice Problems What pressure, in atmospheres, is exerted by mol of hydrogen gas in a 4.08 L container at 35°C? A gas sample contains 8.77 L at 20°C. What is the pressure, in atmospheres, given that there are 1.45 mol of gas in the sample?

Standard Temperature and Pressure For purposes of comparison, scientists have agreed on standard conditions of exactly 1 atm pressure and 0°C. These conditions are called standard temperature and pressure and are abbreviated STP. Note: In gas law problems, only Kelvin temperature are used. 0°C = 273 K

Problems How many moles are contained in the following at STP? A L N 2 ? B L Ne?

Try This Find the mass in grams of 2.80 L CO 2 at standard temperature and pressure.

Other Gas Laws Boyle’s Law: Pressure-Volume relationship When we compare the same sample of gas at two different pressures, we can calculate the change in volume. We can also calculate a new pressure from two different volume conditions. P 1 V 1 = P 2 V 2

Boyle’s Law A balloon filled with helium gas has a volume of 500 mL at a pressure of 1 atm. The balloon is released and reaches an altitude of 6.5 km, where the pressure is 0.5 atm. If the temperature has remained the same, what volume does the gas occupy at this height?

Charles’s Law: Volume-Temperature Relationship This formula is used to predict a new set of volume-temperature conditions when the pressure is held constant. V 1 = V 2 T 1 T 2 A sample of neon gas has a volume of 752 mL at 25.0° C. What will the volume be at 100.0°C if pressure is held constant?

Gay-Lussac’s Law: Pressure-Temperature Relationship When volume is held constant, the pressure is directly proportional to the average kinetic energy or temperature. P 1 =P 2 T 1 T 2 The gas in a container is at a pressure of 3.00 atm at 25°C. Directions on the container warn the user not to keep it in a place where the temperature exceed 52°C. What would the gas pressure in the container be at 52°C?

The Combined Gas Law When a gas undergoes changes in temperature, pressure, and volume at the same time, this formula is used. P 1 V 1 /T 1 =P 2 V 2 /T 2 A helium balloon has a volume of 50.0 L at 25°C and 1.08 atm. What volume will it be have at atm and 10.0 °C?

Avogadro’s Law In 1811 Amadeo Avogadro proposed that equal volumes of ALL gases under the same conditions have the same number of particles

STP: 1 mole gas = 22.4 L

According to Avogadro’s Law 1 L of H 2 and 1 L of CO 2 gases a.Have the same mass b.Have unequal volumes c.Contain 1 mol of gas each d.Contain equal numbers of molecules