Physical Characteristics of Gases Chapter 10

Kinetic-molecular theory Particles of matter are always in motion Chemistry Chapter 10

Ideal gas An imaginary gas that perfectly fits all the assumptions of the kinetic-molecular theory. We can often treat real gases as ideal gases and still get good results. Chemistry Chapter 10

Assumptions of KMT of gases Large numbers of tiny particles that are far apart compared to their size Low density Easily compressed Elastic collisions No kinetic energy is lost when gas particles collide with each other or their container It can be transferred between particles, but the total kinetic energy remains the same Chemistry Chapter 10

Gas particles are in continuous, rapid, random motion There are no attractive or repulsive forces between gas particles When they hit, they don’t stick together The average kinetic energy of gas particles depends on the temperature of the gas Direct relationship Chemistry Chapter 10

Expansion Gases have indefinite shape and volume. They completely fill any container they are in. They also take the shape of that container. Because: they move rapidly in all directions and don’t stick together Chemistry Chapter 10

Fluidity Gas particles slide past each other. They can flow Fluid: something that can flow (can be gas or liquid) Chemistry Chapter 10

Diffusion Spontaneous mixing of the particles of two substances caused by their random motion. Gas particles spread out to fill their new container Chemistry Chapter 10

Effusion When gas particles pass through a small opening Particles leak out of the container Chemistry Chapter 10

Real gases Do not completely follow kinetic-molecular theory Especially deviant at high pressures and low temperatures Noble gases are closest to ideal Very polar gases are farthest from ideal Chemistry Chapter 10

Discuss Describe the conditions under which a real gas is most likely to behave ideally. Explain the following properties of gases using the kinetic-molecular theory: expansion, fluidity, low density, compressibility, and diffusion. Chemistry Chapter 10

Describing gases Needed: They are mathematically related. Volume Temperature Number of molecules Pressure They are mathematically related. Chemistry Chapter 10

Pressure balloon The force per unit area on a surface. Chemistry Chapter 10

Force A push or a pull Measured in newtons (N). At the Earth’s surface, 1 kg of mass exerts 9.8 N of force due to gravity. Chemistry Chapter 10

Chemistry Chapter 10

Pressure of gases Gases exert pressure on any surface with which they collide. Depends on volume, temperature, and number of molecules Chemistry Chapter 10

Atmospheric pressure Air around Earth exerts a pressure on it’s surface and everything on it. Like the weight of all the molecules pressing down. Chemistry Chapter 10

Barometer Used to measure atmospheric pressure. Height of liquid (usually mercury) in tube can be used to express atmospheric pressure. At sea level, the average is 760 mm Hg. h Chemistry Chapter 10

Manometer Used to measure the pressure of gases. The height difference between the two arms is the pressure. Chemistry Chapter 10

Pressure Units Chemistry Chapter 10

STP Standard temperature and pressure. 0 °C and 1 atm Used to compare volumes of gases. Chemistry Chapter 10

Example A weather report gives a current atmospheric pressure of 745.8 mm Hg. Convert this to Atmospheres 0.9813 atm Torr 745.8 torr Kilopascals 99.43 kPa Chemistry Chapter 10

Discuss Define pressure What is STP? Convert 151.98 kPa to atmospheres Chemistry Chapter 10

Boyle’s Law Fixed: mass and temperature The volume varies inversely with pressure Less volume, means the particles hit the walls more often. This increases the pressure Chemistry Chapter 10

Boyle’s Law Mathematically: Each sample of gas has its own k. Chemistry Chapter 10

Example A helium-filled balloon contains 125 mL of gas at a pressure of 0.974 atm. What volume will the gas occupy at standard pressure, assuming constant temperature? 122 mL Chemistry Chapter 10

You try A weather balloon with a volume of 1.375 L is released from Earth’s surface at sea level. What volume will the balloon occupy at an altitude of 20.0 km, where the air pressure is 10.0 kPa, assuming constant temperature? 13.9 L Chemistry Chapter 10

Charles’s Law Fixed: mass and pressure Volume varies directly with temperature. As temperature goes up, the particles have more energy, so they hit the walls more often and with more force This pushes the walls outward. Chemistry Chapter 10

Charles’s Law Mathematically Chemistry Chapter 10

Kelvin Scale Charles’s law works more elegantly on the Kelvin Scale than the Celsius Scale. If you double the temperature, the volume doubles. Not true with Celsius We must use Kelvin for Charles’s Law. Chemistry Chapter 10

Kelvin Scale Absolute zero: lowest possible temperature All particle motion stops 0 K, -273.15 °C Often rounded to 273 Chemistry Chapter 10

Example A balloon filled with oxygen gas occupies a volume of 5.5 L at 25 °C. What volume will the gas occupy at 100. °C, assuming constant pressure? 6.9 L Chemistry Chapter 10

You try A sample of nitrogen gas is contained in a piston with a freely moving cylinder. At 0.0 °C, the volume of the gas is 375 mL. To what temperature must the gas be heated to occupy a volume of 500. mL, assuming constant pressure? 91 °C Chemistry Chapter 10

Gay-Lussac’s Law Fixed: mass and volume Pressure varies directly with temperature (in Kelvin) As temperature goes up, energy of particles goes up. They go faster and hit the walls harder. If the walls can’t move, the pressure goes up. Chemistry Chapter 10

Gay-Lussac’s Law Mathematically: Chemistry Chapter 10

You try The temperature within an automobile tire at the beginning of a long trip is 25 °C. At the conclusion of the trip, the tire has a pressure of 1.80 atm. What is the final Celsius temperature within the tire if its original pressure was 1.75 atm? Assume constant volume. 34 °C Chemistry Chapter 10

Combined gas law Expresses the relationship between pressure, volume, and temperature of a fixed amount of gas. Chemistry Chapter 10

You try The volume of a gas at 27.0 °C and 0.200 atm is 80.0 mL. What volume will the same gas sample occupy at standard conditions? 14.6 mL Chemistry Chapter 10

Dalton’s Law The total pressure in a container is the sum of the partial pressures of all the gases in the container. Chemistry Chapter 10

Application We can collect gases by displacing water. When we do this, Read Patm from the barometer. Look up Pwater in table A-8 in the appendix. Chemistry Chapter 10

Example A student has stored 100.0 mL of neon gas over water on a day when the temperature is 27.0 °C. If the barometer in the room reads 743.3 mm Hg, what is the pressure of the neon gas in its container? 716.6 mm Hg Chemistry Chapter 10

You try A sample of nitrogen gas is collected over water at a temperature of 23.0 °C. What is the pressure of the nitrogen gas if atmospheric pressure is 785 mm Hg? 764 mm Hg Chemistry Chapter 10