Gases expand, diffuse, exert pressure, and can be compressed because they are in a low-density state consisting of tiny, constantly moving particles. Section 1: Gases K What I Know W What I Want to Find Out L What I Learned

9(C) Describe the postulates of kinetic molecular theory. 4(C) Compare solids, liquids, and gases in terms of compressibility, structure, shape, and volume. 9(A) Describe and calculate the relations between volume, pressure, number of moles, and temperature for an ideal gas as described by Boyle's law, Charles' law, Avogadro's law, Dalton's law of partial pressure, and the ideal gas law. 11(A) Understand energy and its forms, including kinetic, potential, chemical, and thermal energies. 2(H)Organize, analyze, evaluate, make inferences, and predict trends from data. Gases Copyright © McGraw-Hill Education

Essential Questions How is the kinetic-molecular theory used to explain the behavior of gases? Why does mass affect the rates of diffusion and effusion? How is gas pressure measured and how is the partial pressure of a gas calculated? Gases Copyright © McGraw-Hill Education

Review kinetic energy New kinetic-molecular theory elastic collision temperature diffusion Graham’s law of effusion pressure barometer pascal atmosphere Dalton’s law of partial pressures Gases Copyright © McGraw-Hill Education Vocabulary

The Kinetic-Molecular Theory Kinetic-molecular theory explains the different properties of solids, liquids, and gases. Atomic composition affects chemical properties. Atomic composition also affects physical properties. The kinetic-molecular theory describes the behavior of matter in terms of particles in motion. Gases Copyright © McGraw-Hill Education Gases consist of small particles separated by empty space. Gas particles are too far apart to experience significant attractive or repulsive forces. Gas particles are in constant random motion. Collisions between gas particles are elastic. An elastic collision is one in which no kinetic energy is lost.

The Kinetic-Molecular Theory Kinetic energy of a particle depends on mass and velocity. Temperature is a measure of the average kinetic energy of the particles in a sample of matter. Gases Copyright © McGraw-Hill Education

Explaining the Behavior of Gases Great amounts of space exist between gas particles. Compression reduces the empty spaces between particles. Gases Copyright © McGraw-Hill Education

Explaining the Behavior of Gases Gases easily flow past each other because there are no significant forces of attraction. Diffusion is the movement of one material through another. Effusion is a gas escaping through a tiny opening. Graham’s law of effusion states that the rate of effusion for a gas is inversely proportional to the square root of its molar mass. Graham’s law also applies to diffusion. Gases Copyright © McGraw-Hill Education

Gases Copyright © McGraw-Hill Education GRAHAM’S LAW EVALUATE THE ANSWER A ratio of roughly 1.5 is logical because molecules of ammonia are about half as massive as molecules of hydrogen chloride. Because the molar masses have three significant figures, the answer also does. Note that the units cancel, and the answer is stated correctly without any units. Use with Example Problem 1. Problem Ammonia has a molar mass of 17.0 g/mol; hydrogen chloride has a molar mass of 36.5 g/mol. What is the ratio of their diffusion rates? Response ANALYZE THE PROBLEM You are given the molar masses for ammonia and hydrogen chloride. To find the ratio of the diffusion rates for ammonia and hydrogen chloride, use the equation for Graham’s law of effusion. KNOWNUNKNOWN molar mass HCl = 36.5 g/molratio of diffusion rates = ? molar mass NH3 = 17.0 g/mol

Gas Pressure Pressure is defined as force per unit area. Gas particles exert pressure when they collide with the walls of their container. The particles in the earth’s atmosphere exert pressure in all directions called air pressure. There is less air pressure at high altitudes because there are fewer particles present, since the force of gravity is less. Torricelli invented the barometer. Barometers are instruments used to measure atmospheric air pressure. Gases Copyright © McGraw-Hill Education

Gas Pressure Manometers measure gas pressure in a closed container. Gases Copyright © McGraw-Hill Education

Units of Pressure The SI unit of force is the newton (N). One pascal(Pa) is equal to a force of one Newton per square meter or N/m 2. One atmosphere is equal to 760 mm Hg or kilopascals. Gases Copyright © McGraw-Hill Education

Dalton’s Law of Partial Pressure Dalton’s law of partial pressures states that the total pressure of a mixture of gases is equal to the sum of the pressures of all the gases of the mixture. The partial pressure of a gas depends on the number of moles, size of the container, and temperature and is independent of the type of gas. At a given temperature and pressure, the partial pressure of 1mol of any gas is the same. Partial pressure can be used to calculate the amount of gas produced in a chemical reaction. P total = P 1 + P 2 + P P n Gases Copyright © McGraw-Hill Education

Gases Copyright © McGraw-Hill Education THE PARTIAL PRESSURE OF A GAS Use with Example Problem 2. Problem A mixture of oxygen (O 2 ), carbon dioxide (CO 2 ), and nitrogen (N 2 ) has a total pressure of 0.97 atm. What is the partial pressure of O 2 if the partial pressure of CO 2 is 0.70 atm and the partial pressure of N 2 is 0.12 atm? Response ANALYZE THE PROBLEM You are given the total pressure of a mixture and the partial pressure of two gases in the mixture. To find the partial pressure of the third gas, use the equation that relates partial pressures to total pressure. KNOWNUNKNOWN P N2 = 0.12 atmP O2 = ? atm P CO2 = 0.70 atm P total = 0.97 atm SOLVE FOR THE UNKNOWN State Dalton’s law of partial pressures. P total = P N2 + P CO2 + P O2 Solve for P O2. P O2 = P total - P CO2 - P N2 Substitute P N2 = 0.12 atm, P CO2 = 0.70 atm, and P total = 0.97 atm. P O2 = 0.15 atm

Gases Copyright © McGraw-Hill Education The Partial Pressure of a Gas EVALUATE THE ANSWER Adding the calculated value for the partial pressure of oxygen to the known partial pressures gives the total pressure, 0.97 atm. The answer has two significant figures to match the data.

Gases Copyright © McGraw-Hill Education Review Essential Questions How is the kinetic-molecular theory used to explain the behavior of gases? Why does mass affect the rates of diffusion and effusion? How is gas pressure measured and how is the partial pressure of a gas calculated? Vocabulary kinetic-molecular theory elastic collision temperature diffusion Graham’s law of effusion pressure barometer pascal atmosphere Dalton’s law of partial pressures