Kinetic-Molecular Theory and an Introduction to Pressure & Gas Stoich Gases Kinetic-Molecular Theory and an Introduction to Pressure & Gas Stoich

The Kinetic-Molecular Theory of Gases based on five assumptions. . . 1. Gases consist of large numbers of tiny particles that are far apart relative to their size this means that most of the space occupied by a gas is empty this is why gases can be compressed

2. Collisions between gas particles and between particles and container walls are elastic. elastic collisions mean that there is no net loss of kinetic energy in other words, kinetic energy is never lost, it is just transferred between the colliding particles

3. Gas particles are in constant, rapid, random motion. the particles are always moving around 4. There are no forces of attraction or repulsion between gas particles no intermolecular force exists between gas molecules I would assume that London Dispersion forces are negligible

5. The average kinetic energy of gas particles depends on the temperature of the gas. temperature is just a measure of the average kinetic energy

some additional characteristics of gases. . . gases will expand to fill any container that you put them in gases have the ability to flow, just like a liquid gases have low density by applying pressure, gases can be compressed to a smaller volume gases spread out via diffusion due to the positive change in entropy associated with the process some gases do not behave as ideal gases because the gas molecules are polar and attracted to one another

Pressure pressure is defined as the force per unit area on a surface for example. . . the air in a balloon exerts a force on the inner walls of the balloon the SI unit for force is the Newton (N)

the atmosphere exerts a pressure on the surface of the earth this is due to the weight of the gas molecules between the earth and the edge of the atmosphere

The air has mass and therefore exerts a force on the surface of the earth due to gravity As a result, we feel pressure AIR would someone on a mountain feel as much pressure as someone at sea level? Earth NO, because they don’t have as much air above them Atmosphere

Can anyone guess the average pressure at sea level? 1 atmosphere (atm) the device used to measure atmospheric pressure is called a barometer barometers generally consist of a tube filled with mercury pressure is measured based on the distance the pressure causes the mercury to move

Units of Pressure 1. millimeters of mercury (mm Hg) the average atmospheric pressure at sea level is 760 mm Hg 2. atmospheres of pressure (atm) 1 atmosphere is equal to 760 mm Hg 3. the SI unit for pressure is the pascal (Pa) one pascal is the same as 1 N/m2 1 atm= 1.01325 x 105 Pa=101.325 kPa 4. 1 torr = 1 mm Hg Inches of mercury 1 atm = 29.92 in Pounds per square inch 1 atm = 14.7 lb/in2

some equations we will use for gases will require certain units, so you will usually have to convert between various units of pressure for example. . . How many millimeters of mercury are in 3.6 atm? 1 atm = 760 mm Hg 3.6 atm X 760 mm Hg 2736 mm Hg = 1 atm

How many kPa are in 905 mm Hg? 905 mm Hg X 101.325 kPa 121 kPa = 760 mm Hg

PV = n R T Review of Gas Stoichiometry Remember a mole of gas at STP occupies 22.4 L So, 2 moles would occupy 44.8 L at STP 11.2 L of a gas would have 0.500 moles at STP When the conditions are not standard, you must use the ideal gas law P = pressure in atm V= volume in L n = number of mols R= Gas Constant : 0.0821 atm L/ mol K T = Temperature in Kelvin PV = n R T

Carbon dioxide decomposes to carbon monoxide and oxygen gas Carbon dioxide decomposes to carbon monoxide and oxygen gas. How many liters of oxygen gas will be made from 80.0 g of carbon dioxide at 755 mm Hg and 25 oC? 1. Write a balanced equation 2CO2 (g) 2CO (g) + O2 (g) 2. Calculate the moles of CO2 #mol= mass(g)/molar mass #mol= 80.0 g/44.01 g/mol = 1.818 mol CO2

3. Calculate the moles of O2 made 1 mol O2 1.818 mol CO2 X = 2 mol CO2 0.909 mol O2 4. Rearrange the equation PV=nRT V=nRT/P 5. Solve the equation V=(0.909 mol)(0.0821)(298 K)/(0.9934 atm) V= 22.4 L

in the previous example, you had to use the ideal gas law because the conditions were not standard At STP, you could have multiplied the moles of oxygen gas by 22.4 L/mol to find the total volume using the ideal gas law is always acceptable, just more work than necessary at STP