Week 2 Ideal Gases Liceo Alfano

Reminder Last week: Heat propagation Conduction Convection Radiation This week: Ideal gases

Concept Question The molecules in a gas are 1) Close together 2) Far apart

Concept Question The molecules in a gas are 1) Close together 2) Far apart

Concept Question As you add heat to a gas, the molecules move 1) Faster 2) Slower

Concept Question As you add heat to a gas, the molecules move 1) Faster 2) Slower

What is an Ideal Gas? The movements of each molecule in a gas are so random that it’s impossible to model them exactly An ideal gas is a “perfect gas”– a gas where we simplify the movements so we can make calculations In an ideal gas, we pretend the molecules are very small and that they don’t interact with each other This is true of most situations, so it is a valid assumption

What is Pressure? Pressure is a measure of how much individual atoms are “pushing” on the sides of a container in a gas More collisions = higher pressure If you pump air into a container that can’t get any bigger, would the pressure increase or decrease?

What is Pressure? Pressure = Force/Area We control the pressure of a gas with a piston Measured in atmospheres (atm) or Pascales (Pa)

Concept Question If I decrease the volume of an ideal gas container (at constant temperature), the pressure will 1) Increase 2) Decrease

Concept Question If I decrease the volume of an ideal gas container (at constant temperature), the pressure will 1) Increase 2) Decrease

Video As pressure decreases, volume does what?

Scuba Diving Think of a scuba diver coming up from the bottom of the ocean As he gets closer to the surface, what happens to the pressure? As the pressure decreases, what happens to the volume of the air in his lungs? If he comes up too quickly, his lungs might explode!

Concept Question Now let’s look at pressure and temperature. If I increase the temperature of an ideal gas (at constant volume), the pressure will 1. Increase 2. Decrease

Concept Question Now let’s look at pressure and temperature. If I increase the temperature of an ideal gas (at constant volume), the pressure will 1. Increase 2. Decrease

Video When the temperature goes down after the fire stops, what happens to the pressure in the bottle?

Car Tires Think about the tires on a car If they get very hot, what will happen to the pressure of the air inside? The tires can’t get too hot, or they’ll explode!

Concept Question Now let’s look at volume and temperature. If I increase the temperature of an ideal gas (at constant pressure), the volume will 1. Increase 2. Decrease

Concept Question Now let’s look at volume and temperature. If I increase the temperature of an ideal gas (at constant pressure), the volume will 1. Increase 2. Decrease

Video As the temperature drops, what happens to the volume of the air in the balloons?

Hot Air Balloon Think about when they are inflating a hot air balloon. What are they doing? They make the air inside the balloon hot, making it get bigger!

Football If you inflate a football inside then go play outside on a cold day, what happens to the volume of the football? The football actually gets a bit smaller!

Combined Gas Law THE EQUATION: P 1 V 1 /T 1 = P 2 V 2 /T 2 This is for ONE GAS in TWO STATES Does this agree with the conditions we just established? Let’s keep temperature constant Now let’s keep volume constant Now let’s keep pressure constant

Ideal Gas Law Taking the combined gas law and plugging in STP (standard temperature and pressure) conditions, we get a new equation THE EQUATION: PV = nRT P = pressure (atm) V = volume (L) n = number of moles of the gas R = ideal gas constant ( L*atm/mol*K or 8.31m 3 *Pa/K*mol) T = temperature (K)

Experiments Collapsing can (collasso della lattina) Dancing cap (Il tappo ballerino) Boiling syringe (La siringa ad ebollizione) “Pee-pee” boy

Practice Problems Remember, PV = nRT (R= L*atm/mol*K) and P 1 V 1 /T 1 = P 2 V 2 /T 2 If I have 21 mol of nitrogen gas at a pressure of 78 atm and a temperature of 900K, what is the volume of the gas? A container filled with 0.25 mol of oxygen gas has a volume of 2.35L at a temperature of 45C. What is the pressure of the gas?

Practice Problems Remember, PV = nRT (R= L*atm/mol*K) and P 1 V 1 /T 1 = P 2 V 2 /T 2 An air bubble at the bottom of a lake, at a pressure of 120kPa, has a volume of 1cm 3. If the temperature at the bottom of the lake is 5.5C and the temperature at the top is 21C, what is the volume of the bubble just before it reaches the surface? 1 atm = 101kPa 1L = 1000cm 3

Practice Problems Remember, PV = nRT (R= L*atm/mol*K) and P 1 V 1 /T 1 = P 2 V 2 /T 2 I have 21.6kg of neon at a pressure of 3.65 atm. What will the pressure be if the nitrogen is replaced by 21.6kg of carbon gas? (volume and temperature don’t change) Molecular mass of neon= kg/mole Molecular mass of carbon= kg/mole