Chapter 11 The Behavior of Gases 1
Section 11.1 The Properties of Gases Objectives: Explain why gases are easier to compress than solids or liquids are. Describe the three factors that affect gas pressure Describe the relationships among the temperature, pressure, and volume of a gas. Use the combined gas law to solve problems. 2
Compressibility Gases can expand to fill its container, unlike solids or liquids The reverse is also true: They are easily compressed, or squeezed into a smaller volume Compressibility is a measure of how much the volume of matter decreases under pressure 3
Gas Particle Compression in an Air Bag This is the idea behind placing air bags in automobiles Particles of gas (in the airbag) compress more than particles of solid (in the steering wheel or dash board) The impact forces the gas particles closer together, because there is a lot of empty space between them 4
Variables that describe a Gas The four variables and their common units: 1. Pressure (P) in kilopascals 2. Volume (V) in Liters 3. Temperature (T) in Kelvin 4. Amount (n) in number of particles The amount of gas, volume, and temperature are factors that affect gas pressure. 5
Amount of Gas Before pumping When we add air to a bicycle tire, we are adding gas molecules. More gas particles means more collisions thus, the pressure increases This is why the tire feels harder after pumping it up After pumping Relationship: # gas particles, Pressure 6
Pressure Gradient Gas particles naturally move from areas of high pressure low pressure Example whipped cream, hair spray, spray paint Stored with air under high pressure When you press the nozzle, the gas is free to move and it move out of the can because atmospheric pressure is lower than the pressure inside the can. 7
Volume of Gas In a smaller container, the molecules have less room to move. The particles hit the sides of the container more often. As volume decreases, pressure increases 8
Temperature of Gas Raising the temperature of a gas increases the pressure, if the volume is held constant. Remember, temperature is a measure of energy Gases at higher temp. have more kinetic energy: The molecules hit the walls harder, and more frequently What would happen if you threw an aerosol can into a fire? Heat from fire adds energy, as energy, temp. and pressure. When can can t withstand pressure, it explodes. What happens to a helium filled balloon on a cold day? When should your car tire pressure be checked? 9
Objectives Section 11.2 The Gas Laws Describe the relationships among the temperature, pressure, and volume of a gas Use the combined gas law to solve problems 10
The Gas Laws The relationships we talked about in section 1 are described by three gas laws: Boyle s Law Pressure and Volume If temperature is constant, as the volume of a gas is decreased, the pressure increases. Volume of a gas varies inversely with pressure V, P 11
The Gas Laws Charles s Law Temperature and Volume If pressure is constant, as temperature of an enclosed gas increases, the volume increases Volume of gas is directly proportional to Kelvin temperature T, V 12
The Gas Laws Gay-Lussac s Law Pressure and Temperature If volume is constant, as temperature of an enclosed gas increases, the pressure increases Pressure of gas is directly proportional to Kelvin temperature T, P 13
The Gas Laws GREAT NEWS! All three laws can be combined into one called the combined gas law! Describes the relationship among pressure, temperature and volume of an enclosed gas Allows us to do calculations for situations in which only the amount of gas is constant There is a catch All temperature measurements MUST be in Kelvin Remember K = o C + 273 14
The Combined Gas Law The combined gas law expresses the relationship between pressure, volume and temperature of a fixed amount of gas. P V 1 T 1 1 P 2 T V 2 2 15
Section 11.3 Ideal Gases Objectives Compute the value of an unknown using the ideal gas law Compare and contrast real and ideal gases 16
Ideal Gas Law The combined gas law solves problems with three variables assuming that the amount of gas does not vary Cannot use the combined gas law to calculate the number of moles of a gas in a fixed volume at a known temperature and pressure 17
Ideal Gas Law PV = nrt R = ideal gas constant 8.31 (L. kpa)/(k. mol) Allows us to calculate the number of moles of a gas in a fixed volume at a known temperature and pressure 18
Practice Problem A deep underground cavern contains 2.24 x 10 6 L of methane gas (CH 4 ) at a pressure of 1.50 x 10 3 kpa and a temperature of 315K. How many kilograms of CH 4 does the cavern contain? 19
Ideal Gases An ideal gas is a gas that obeys the gas laws under ALL conditions of temperature and pressure. It does this because: The particles themselves have no volume There are no attractive forces between the particles However an ideal gas does not really exist! 20
Ideal Gases don t exist, because: 1. Gas particles do take up space - In other words, they do have volume 2. There are attractive forces between particles - Otherwise there would be no liquids formed 21
However, Real Gases behave like Ideal Gases... When the particles are very far apart and do not take up as big a percentage of the space We can ignore the particle volume. This happens at low pressure 22
Real Gases behave like Ideal Gases When particles are moving fast This is at high temperature Collisions are harder and faster. Particles are not next to each other very long. Attractive forces can not play a role. 23
Why we work with Ideal Gases Although ideal gases do not really exist, real gases can behave like ideal gases at low pressure and high temperature, and so If we ASSUME a gas is ideal: it is much easier to predict how it will behave, the math is much easier and in reality it is a very close approximation for how the real gas will behave. 24