Gases Section 1 – Properties of Gases Section 2 – Gas Laws, and Gas Stoichiometry Section 3 – Kinetic Molecular Theory

Gases 1. To learn about atmospheric pressure and how barometers work 2. To learn the units of pressure 3. To understand how the pressure and volume of a gas are related 4. To do calculations involving Boyle’s Law 5. To learn about absolute zero 6. To understand how the volume and temperature of a gas are related 7. To do calculations involving Charles’s Law 8. To understand how the volume and number of moles of a gas are related 9. To do calculations involving Avogadro’s Law Objectives Section 1 – Properties of Gases

Gases Properties of Gases In organized soccer, a ball that is properly inflated will rebound faster and travel farther than a ball that is under-inflated. If the pressure is too high, the ball may burst when it is kicked. You will study variables that affect the pressure of a gas.

Gases Compressibility Why are gases easier to compress than solids or liquids are?

Gases Compressibility Compressibility is a measure of how much the volume of matter decreases under pressure. When a person collides with an inflated airbag, the compression of the gas absorbs the energy of the impact.

Gases Compressibility Gases are easily compressed because of the space between the particles in a gas. The distance between particles in a gas is much greater than the distance between particles in a liquid or solid. Under pressure, the particles in a gas are forced closer together.

Gases Factors Affecting Gas Pressure What are the three factors that affect gas pressure?

Gases Factors Affecting Gas Pressure The amount of gas, volume, and temperature are factors that affect gas pressure.

Gases Factors Affecting Gas Pressure Four variables are generally used to describe a gas. The variables and their common units are pressure (P) in kilopascals volume (V) in liters temperature (T) in kelvins the number of moles (n).

Gases Factors Affecting Gas Pressure Amount of Gas You can use kinetic theory to predict and explain how gases will respond to a change of conditions. If you inflate an air raft, for example, the pressure inside the raft will increase.

Gases Factors Affecting Gas Pressure Collisions of particles with the inside walls of the raft result in the pressure that is exerted by the enclosed gas. Increasing the number of particles increases the number of collisions, which is why the gas pressure increases.

Gases Factors Affecting Gas Pressure If the gas pressure increases until it exceeds the strength of an enclosed, rigid container, the container will burst.

Gases Factors Affecting Gas Pressure Aerosol Spray Paint

Gases Factors Affecting Gas Pressure Volume You can raise the pressure exerted by a contained gas by reducing its volume. The more a gas is compressed, the greater is the pressure that the gas exerts inside the container.

Gases Factors Affecting Gas Pressure When the volume of the container is halved, the pressure the gas exerts is doubled.

Gases Factors Affecting Gas Pressure Temperature An increase in the temperature of an enclosed gas causes an increase in its pressure. As a gas is heated, the average kinetic energy of the particles in the gas increases. Faster-moving particles strike the walls of their container with more energy.

Gases Factors Affecting Gas Pressure When the Kelvin temperature of the enclosed gas doubles, the pressure of the enclosed gas doubles.

Gases Pressure Barometer – device that measures atmospheric pressure Invented by Evangelista Torricelli in 1643 Measuring Pressure

Gases Pressure Changing weather conditions Atmospheric Pressure

Gases Pressure Changing altitude Atmospheric Pressure

Gases Pressure 1 standard atmosphere = atm = mm Hg = torr = 101,325 Pa Units of Pressure

Gases Pressure A manometer measures the pressure of a gas in a container. Units of Pressure

Gases Pressure and Volume: Boyle’s Law Robert Boyle’s experiment

Gases Pressure and Volume: Boyle’s Law

Gases Pressure and Volume: Boyle’s Law Graphing Boyle’s results

Gases Pressure and Volume: Boyle’s Law This graph has the shape of half of a hyperbola with an equation PV = k Volume and pressure are inversely proportional. –If one increases the other decreases.

Gases Boyle’s Law: Pressure and Volume

Gases Pressure and Volume: Boyle’s Law Another way of stating Boyle’s Law is P 1 V 1 = P 2 V 2 (constant temperature and amount of gas)

Gases As the temperature of the water increases, the volume of the balloon increases. Volume and Temperature: Charles’s Law

Gases Volume and Temperature: Charles’s Law Graphing data for several gases

Gases Volume and Temperature: Charles’s Law It is easier to write an equation for the relationship if the lines intersect the origin of the graph. –Use absolute zero for the temperature

Gases Volume and Temperature: Charles’s Law These graphs are lines with an equation V = bT (where T is in kelvins) Volume and temperature are directly proportional. –If one increases the other increases. Another way of stating Charles’s Law is V 1 = V 2 T 1 T 2 (constant pressure and amount of gas)

Gases Volume and Moles: Avogadro’s Law

Gases Volume and Moles: Avogadro’s Law Volume and moles are directly proportional. –If one increases the other increases. –V = an –constant temperature and pressure Another way of stating Avogadro’s Law is V 1 = V 2 n 1 n 2 (constant temperature and pressure)

Gases 1. To understand the ideal gas law and use it in calculations 2. To understand the relationship between the partial and total pressure of a gas mixture 3. To do calculations involving Dalton’s law of partial pressures 4. To understand the molar volume of an ideal gas 5. To learn the definition of STP 6. To do stoichiometry calculations using the ideal gas law Objectives Section 2 – Gas Laws, and Gas Stoichiometry

Gases The Ideal Gas Law Boyle’s Law V = k (at constant T and n) P Charles’s Law V = bT (at constant P and n) Avogadro’s Law V = an (at constant T and P) We can combine these equations to get

Gases The Ideal Gas Law Rearranging the equation gives the ideal gas law PV = nRT R = L atm mol K

Gases Dalton’s Law of Partial Pressures What happens to the pressure of a gas as we mix different gases in the container?

Gases Dalton’s Law of Partial Pressures Dalton’s law of partial pressures For a mixtures of gases in a container, the total pressure exerted is the sum of the partial pressures of the gases present. P total = P 1 + P 2 + P 3

Gases The partial pressure of oxygen must be kPa or higher to support respiration in humans. The climber below needs an oxygen mask and a cylinder of compressed oxygen to survive. Dalton’s Law of Partial Pressures

Gases Dalton’s Law of Partial Pressures The pressure of the gas is affected by the number of particles. The pressure is independent of the nature of the particles.

Gases Dalton’s Law of Partial Pressures Two crucial things we learn from this are: The volume of the individual particles is not very important. The forces among the particles must not be very important.

Gases Dalton’s Law of Partial Pressures Collecting a gas over water Total pressure is the pressure of the gas + the vapor pressure of the water.

Gases Dalton’s Law of Partial Pressures Collecting a gas over water How can we find the pressure of the gas collected alone?

Gases C. Gas Stoichiometry Molar Volume Molar volume of an ideal gas at STP 22.4 L Standard temperature and pressure (STP) –0 o C and 1 atm For one mole of a gas at STP

Gases Graham’s Law Diffusion is the tendency of molecules to move toward areas of lower concentration until the concentration is uniform throughout.

Gases Graham’s Law Bromine vapor is diffusing upward through the air in a graduated cylinder.

Gases Graham’s Law After several hours, the bromine has diffused almost to the top of the cylinder.

Gases Graham’s Law During effusion, a gas escapes through a tiny hole in its container. Gases of lower molar mass diffuse and effuse faster than gases of higher molar mass.

Gases Graham’s Law Thomas Graham’s Contribution Graham’s law of effusion states that the rate of effusion of a gas is inversely proportional to the square root of the gas’s molar mass. This law can also be applied to the diffusion of gases.

Gases Graham’s Law Comparing Effusion Rates A helium filled balloon will deflate sooner than an air-filled balloon.

Gases Graham’s Law Helium atoms are less massive than oxygen or nitrogen molecules. So the molecules in air move more slowly than helium atoms with the same kinetic energy.

Gases Graham’s Law Because the rate of effusion is related only to a particle’s speed, Graham’s law can be written as follows for two gases, A and B.

Gases Graham’s Law Helium effuses (and diffuses) nearly three times faster than nitrogen at the same temperature.

Gases 1. To understand the relationship between laws and models (theories) 2. To understand the postulates of the kinetic molecular theory 3. To understand temperature 4. To learn how the kinetic molecular theory explains the gas laws 5. To describe the properties of real gases Objectives Section 3 – Kinetic Molecular Theory

Gases Laws and Models: A Review

Gases Laws and Models: A Review A model can never be proved absolutely true. A model is an approximation and is destined to be modified.

Gases The Kinetic Molecular Theory of Gases

Gases The Implications of the Kinetic Molecular Theory Meaning of temperature – Kelvin temperature is directly proportional to the average kinetic energy of the gas particles Relationship between Pressure and Temperature – gas pressure increases as the temperature increases because the particles speed up Relationship between Volume and Temperature – volume of a gas increases with temperature because the particles speed up

Gases Real Gases Gases do not behave ideally under conditions of high pressure and low temperature. Why?

Gases Real Gases At high pressure the volume is decreased Molecule volumes become important Attractions become important