Chapter 6 Gases

Kinetic Molecular Theory of Gases Small particles moving continually and randomly with rapid velocities in straight lines Attractive forces among particles are negligible Volume of occupied by particles is very small compared to volume of gas. Average kinetic energy of particles is proportional to Kelvin Temperature

Ideal Gas Assumptions of kinetic molecular theory of gases are the same as for “Ideal Gas” The only added assumption is that collisions are elastic – they bounce off each other like billiard balls. Ideal gases are well represented by ordinary gases at the usual conditions of temperature and pressure at which we exist.

Properties and Units Pressure (P): 1 atmosphere = 760 mm Hg = 760 torr = 101,325 Pascal = kiloPascal. Volume (V) – liters Temperature (T) – degrees Kelvin K = C Number of Moles of gas (n) Standard temperature and pressure: 273 K and 1 atmosphere.

Relationships among properties Boyle’s Law: P 1 V 1 = P 2 V 2 at fixed T, constant amount (n) Charles Law: V 1 / T 1 = V 2 / T 2 at fixed P, constant amount (n) Gay-Lussac’s Law: P 1 / T 1 = P 2 / T 2 at fixed V, constant amount (n) Combined: at constant n,

Ideal Gas Law Called an “equation of state” – there is plenty of research in discovering these equations for “real” substances. Purpose is to predict a missing property given other properties of a substance. Includes pressure, volume, temperature, number of moles. PV = nRT R is the universal gas constant, L atm deg -1 mole -1 Avogadro’s Law: One mole of any gas occupies 22.4 liters at STP (standard temperature and pressure, 760 torr, 273 K)

Boyle’s Law Given 7.24 liters of gas at 680 torr. Compress the volume to 5.89 liters keeping temperature constant. What will the pressure of the gas become?

Charles Law You have a 1.96 liter sample of gas at 18 deg Celsius. Keeping the pressure constant, increase the temperature to 59 deg Celsius. What will its new volume become?

Gay-Lussac’s Law A tire has a pressure of 3.45 atm at 20 degrees Celsius. After some driving, the tire heats up to 45 degrees Celsius. What would be the corresponding new pressure in the tire?

Combined gas law Suppose you have moles of gas confined at standard temperature and pressure. What is its volume at standard temperature and pressure? Now, change the temperature to 20 degrees Celsius and the pressure to 635 torr. What will its new volume become?

Ideal Gas Law Suppose you react 2.45 grams of zinc with excess hydrochloric acid to produce hydrogen gas. What volume of hydrogen gas will you make at 735 torr and 22 degrees Celsius?

Dalton’s Law of Partial Pressures Consistent with ideal gas model. All molecules are essentially alike as gases when combined. Chemically, they differ. However, this difference is unimportant compared to their behavior as ideal gases. Total pressure is the sum of the partial pressures of the constituent gases.

Dalton’s law of Partial pressures Return to the reaction of zinc with hydrochloric acid to produce hydrogen: The hydrogen is collected over water so that the gas collected is really a mixture of hydrogen and water vapor (at saturation). The temperature of the laboratory is 24 degrees Celsius and the partial pressure of water vapor at this temperature is 22.4 mmHg. The atmospheric pressure is mmHg. What is the volume of dry hydrogen gas at this temperature and pressure produced by the complete reaction of 3.02 grams of zinc?