Chemistry SM-1232Week 2 Lesson 1

Dr. Jesse ReichAssistant Professor of Chemistry

Massachusetts Maritime AcademyFall 2008

Class Today• Pressure• Pressure Units• Boyle’s Law: Pressure and Volume• Charles’s Law: Volume and Temperature• Ideal Gas Law• Practice Problems

Definitions

• Pressure: Force / Area• Volume the amount of space something takes

up• Temperature: the amount of stored heat• N= the number of atoms (moles) in the system

Pressure

• Pressure = Force area• The force increases as more atoms are

introduced. The force decreases as atoms leave the system.

• If the force increases what happens to the pressure?

Conversion table• 1 atm = • 1.01325 bar• 101, 325 Pa• 101.325 kPa• 760 mm Hg• 760 Torr• 14.7 psi• 29.92 in. Hg• Convert 12.3 atm into each unit. Write the

conversion factor down as you solve these.

Boyle

• When you looked at the simulation you should have found that when you increase Pressure you decrease volume. When you increase volume you decrease pressure. The first person to recognize this relationship was Robert Boyle, and so the law get’s called Boyle’s Law. We call this an inverse relationship cause they move in opposite directions.

Boyle’s Law

• Chemists typically refer to the following equation as Boyle’s Law

• P1V1=P2V2

Solve Boyle’s Law

• If you play with an air pump, but keep the bottom part closed so the air can’t escape you have a good example of Boyle’s law. If the volume is 0.4L and 1 atm when it’s full, what is the pressure when you’ve pushed down on it and now the volume is 0.12 L?

Charles’s Law

• Volume and Temperature• Temperature in this case is measured in Kelvin.• Temp C + 273 = Temp in Kelvin

Charles’s Law

• Temp and Volume are directly proportional• This is a system where pressure isn’t changing• Like a hot air balloon, but not like a helium

balloon because the pressure would change.• This could be like a gas spreading through a

classroom, but not a gas in a closed vial.

Charles’s Law

• Assuming the pressure is constant and the amount of gas is constant here’s the rule:

• V1/T1 = V2/T2• Solve for V1, T1, and T2

Celcius

• What happens to that math if we calculate using 0C instead of 273 Kelvin?

Charles’s Law

• A chemist has run an experiment 15 times and is now positive that it makes the same number of nitrogen gas molecules every time. While trying to write up his results he realizes he forgot to write down a temperature that he ran the reaction. If the gas produced took up 2L of space when he ran it at 298 K (room temperature), and it took up 1.8L of space for the time when he didn’t note the temperature what was the missing temperature?

Gay-Lussac

• P and T are also a direct relationship when number of atoms and volume are constant.

• This is like a “pressure cooker”• The chemical apparatus is also called a bomb

calimeter.

Gay-Lussac

• P1/T1=P2/T2

• Solve for T1, solve for T2

Gay-Lussac

• A chemist is working with an unstable gas and wants to make sure it won’t blow up under too high a pressure or temperature. She puts the gas in a bomb calimeter. She sets the temperature to 300K and sees the gas at 20atm of pressure. She sets the temperature to 425K. What will the resulting pressure be?

Three Laws

• P1V1=P2V2 Boyle• P1/T1=P2/T2 Gay Lussac• V1/T1=V2/T2 Charles

• HEY! These three laws look like they could all be related to one bigger one! We call it the combined gas law.

Combined Gas Law

• P1V1/T1= P2V2/T2

• Solve for V2• Solve for T1

• How many different variables would you need numbers for to actually solve this problem.

Combined Gas Law

• D. Reich is in his lab working on his new device to take over the world. He makes 15L of a new chemical gas at STP (standard temp and pressure, 273K and 1 atm). He then cools it down to 200K and puts it under 8 atm of pressure. What volume will it take up now? Could he conceal it in a 1L pepsi bottle?

Avogadro’s Law

• Avogadro’s number is 6.02e23• It’s the number of atoms in 1 mole• 1 mole of atoms turns it’s atomic mass into

grams. Very handy when weighing chemicals.• He also found that the volume of gas at

constant pressure and temperature becomes dependant on the number of molecules of gas.

Avogadro’s Law

• V1/n1=V2/n2• Where n is the number of moles of a gas.

• So, if it takes up 40L when N = 2 moles. How many Liters will it take up when N = 3.5 moles?

Combining Everything

• Let’s figure out what V is equal to…• V ~ 1/P• V ~ T• V ~ n• Therefore, V ~ nT/P• Throw in a constant that can account for units

and we get V= RnT/P aka• PV= nRT

The Ideal Gas Law

• PV=nRT• This describes how ideal gasses behave.• If you know 4 variables you can always figure

out the last one.

Ideal Gas Constant

• R is the ideal gas constant• It’s value changes when we have different

units in the problem• Typically R = 0.082 L atm/ mol K

• That’s right, R has 4 different units associated with it. You’ll see why in a sec.

Problem

• In lab you produce 3 L of a gas at 1 atm and at 298K. How many moles of gas did you create?

• Make sure all the units cancel.

To Do

• Read 373 – 381• Copy your notes over• Quiz on chapter 11 in one week (this coming

Friday) 10 bonus points.• HW problems due March 16. Complete 1 HW

set for 2-3 people. 15 bonus points.• HW problems listed on the syllabus.