Chemistry 20 Unit 2 Gases FITB Notes General Outcome: Topic A Characteristics of Gases We use technologies that were designed with the knowledge of the visible characteristics ( ) of gases ex. SCUBA equipment, hot air balloons, jackhammers Gases have several distinct macroscopic (visible with the unaided eye) properties: 1. Gases are i.e. pressure = volume 2. Gases as temperature increases i.e. temperature = volume (soft container) temperature = pressure (rigid) 3. Gases have (viscosity) allows them to escape quickly through small openings (leak) 4. Gases have low 5. Gases mix evenly and completely; they are all (gases mix well!) 6. Gases have no shape or volume, they of the container they are in by expanding or compressing MEMORIZE these properties; they represent the properties of gases! Topic B Particle Theory We need models to describe how gases behave on the (particle) level The kinetic molecular theory says that all particles are in at all times KMT focuses on particle motion, which cannot be seen with the unaided eye ( )! An ideal gas (which is hypothetical) is defined by the following characteristics: 1. The gas molecules are in constant motion, where they move in a straight line until they with another particle or the wall of the container 2. The gas molecules are (they have mass but, and act like point spheres) 3. The only interaction between molecules of the gas and container are collisions collisions where kinetic energy is, not lost! Topic C Ideal Gas vs. Real Gas 1
gases correspond perfectly to equations gases do not behave this way however, they are behave much like ideal gases! For ideal gases we assume that there are no attractions (LD, DD, HB) at all between molecules of gas In real gases, however, there are between molecules of gas We don t have to worry about considering this in our calculations because at standard and conditions the ideal gas molecules are: 1. (lots of space) 2. are moving very (high energy) 3. not interacting ( ) much with each other Real gases behave like ideal gases at: temperatures (fast moving) pressures (very far apart) Real gases deviate from ideal gas behaviour at: temperatures (moving very slowly) pressures (molecules close together) Real gases also deviate from ideal gas behaviour if molecules have: size (more electrons) shape (branching, etc.) Topic D Atmospheric Pressure Although gas molecules in the atmosphere have little, Earth s gravitational pull keeps them near the surface (except H, He) Atmospheric pressure is the force that a column of air exerts on a particular area on the Earth s surface (force per ) Atmospheric pressure is exerted in all directions to the same extent Air is less compressed as altitude increases so pressure is exerted higher up Pascal and Perier used Hg(l) to prove that atmospheric pressure with altitude The work of Pascal, Perier and Torricelli all led to the development of the mercury There are several different units used to measure pressure: millimetres of mercury ( ) atmosphere ( ) kilopascal ( ) Pascal (Pa) 1 = 1000 (optional) bar 1 = 100 (optional) You will be using the standard unit of kpa in gas law calculations and therefore you must be able to convert and to kpa Memorize the following standard pressures: To convert other units of pressure to kpa, set up a 2
ex. 1: Convert 650 mmhg to kpa. ex. 2: Convert 2.5 atm to kpa. Topic E Boyle s Law Robert Boyle studied the mathematical relationship between the pressure and volume of a gas at temperature against the walls of a container is caused by collisions of the gas molecules with the walls As you the volume of a contained gas, there is less room for the gas particles A smaller volume means that the number of collisions Container pressure is proportional to the number of collisions against the walls (more collisions means a pressure) Boyle found a relationship between pressure and volume (MEMORIZE): During an inhale, lung volume to pressure and draw in more air Boyle s Law volume of a gas varies inversely with the pressure, at constant temperature and mass (MEMORIZE): ex. A balloon is filled with 30.0 L of helium gas at 100 kpa. What is the volume when the balloon rises to an altitude where the pressure is only 25.0 kpa? (constant T) ex. The pressure on 2.50 L of anesthetic gas is 100 kpa. If 6.25 L of gas is the required volume, what pressure must it be under, assuming constant temperature? Topic F Law of Combining Volumes 3
Gay-Lussac analyzed chemical that involved gases He studied the of the gaseous reactants and products and concluded that the gases combine in very simple The Law of Combining Volumes states: when gases react, the volumes of the gas reactants and products (at constant temperature and pressure) are always in ex. N 2 (g) + 3H 2 (g) 2NH 3 (g) ex. What volume of nitrogen is used up if 100 ml of ammonia is formed in a composition reaction? Topic G Kelvin Temperature Scale When volume and temperature of a gas are graphed, the plot is (as long as amount of gas and pressure were constant) It was also noticed that when these linear plots were extrapolated down to, all the lines converged at one point The temperature when the volume of a gas is zero is (zero kinetic energy!) Lord Kelvin (1848) suggested that this is the lowest possible temperature, (no particle movement whatsoever!) He established a new temperature scale which is called the scale in his honour t is used for temperature in T is used for temperature in -273.15 C = absolute zero = K We use (K), instead of Celsius, for calculations in chemistry: ex. Topic H Charles Law Jacques Charles (and Gay-Lussac) noticed a relationship between the and of a gas As temperature, so did the kinetic energy of the gas molecules (because gas particles move faster at temperature!) As the molecules move faster, they also exert higher on the inside of the container 4
Gas volume will expand under this pressure until the pressure inside equals the outside ( ) pressure Charles Law volume of a gas varies directly with the temperature, at constant pressure and mass (MEMORIZE): ex. A balloon was inflated at 27 C and has a volume of 4.0 L. If it is heated to 57 C, what is the new volume? (assume constant P) ex. A sample of gas occupies 6.8 L at 110 C. What will the final temperature be in C when the volume is decreased to 5.6 L? Topic I STP and SATP Two temperature and pressure conditions are on the back of your periodic table: STP (Standard Temperature, Pressure) conditions = C and kpa SATP (Standard Ambient Temperature, Pressure) conditions = C and kpa These two conditions will be used frequently Remember to convert temperature to Kelvin! Topic J Combined Gas Law I II We can combine Boyle s Law with Charles Law to get the Combined Gas Law: ex. A weather balloon is filled with H2 (g) at 20 C and 100 kpa. It has a volume of 7.50 L. It rises to an altitude where the air temperature is -36 C and the pressure is 28 kpa. What is the new volume of the balloon? 5
Topic K Ideal Gas Law I II Ideal Gas Law A gas law that combines moles with P, V and T (on periodic table): ex. A rigid steel vessel with a volume of 20.0 L is filled with nitrogen gas to a pressure of 20,000 kpa at 27 C. a) How many moles of nitrogen does the vessel contain? b) What is the mass of nitrogen? ex. What is the volume of 10.8 mol of oxygen gas at 100.00 kpa and 15.5 C? ex. What is the pressure exerted by 15.5 g of methane, CH4 (g), if it occupies a volume of 10.0 L at 25 C? Topic L Law of Partial Pressures Dalton s law of partial pressures states that in a mixture of gases that do not chemically, the total pressure is the of the partial pressures of each individual gas: ex. Two gases are pumped into a 32.0 L reaction vessel at 25.0 C, one after another. First, 6.20 mol O 2 (g) is pumped in. Then 8.30 mol of H 2 (g) is added. What would the pressure gauge read after each gas is pumped in? 6