Measuring pressure • Force per unit area • Units in use by • Force is measured in chemists include Newtons • Pascals (Pa) or KPa (Kg*m/sec2) • Millimeters of • Area can be mercury (mmHg) measured in meters • Atmospheres (atm) • Non-SI units were • Torrs (torr – no PSI, or pounds per abbreviation) square inch
Converting • 1 atmosphere = 760 mmHg • 1 atmosphere = 101 325 Pa • 1 atmosphere = 101.325 kPa • 1 atmosphere = 760 torrs • You can write conversion factors between any two of these units • Let’s practice some!
From HMC, p. 365 Use Dimensional Analysis!! • The average atmospheric pressure in Denver, Colorado is 0.830 atm. Express this pressure in mm Hg and in kPa • Convert a pressure of 1.75 atm to kPa and to mm Hg. • Convert a pressure of 570. torr to atmospheres and to kPa.
STP • Standard temperature and pressure • 1atmosphere • 0 degrees Celsius • STP is an important concept because it gives you a point of reference. • STP is also a motor oil. That’s cool, if irrelevant.
3 labs at once! • Put a balloon over the mouth of an Erlenmeyer flask containing about 40 mL of water. Place the flask on a hot plate for 10 minutes. Plunge the flask into an icewater bath. Make observations. • Set up the syringe in the clamps as shown. Record the volume. Record subsequent volumes with one book on top of the set-up, then two, three, and 4. Graph volume as a function of number of books. • Heat 100 mL of water on a hotplate until it is about 75C. Pour the water into a 2L bottle, swirl it, dump it and cap it tightly. Observe.
What will you turn in? • A graph showing the relationship between books and volume of gas • A paragraph describing any relationships you have observed between pressure, temperature and volume of a gas.
P1V1=P2V2 • If a particular sample of gas is in a container, pressure changes that occur at constant temperature will result in volume changes. • Which of the experiments you did deals with this? • When you add pressure, what happens to volume? • Can you graph data from your lab to quantify the relationship?
V1/T1=V2/T2 • If a particular sample of gas is in a container, temperature changes that occur at constant pressure will result in volume changes. • Which of the experiments you did deals with this? • When you increase the temperature of a gas, what happens to volume?
Gay-Lussac’s Law: Relating pressure and temperature
Of course, these laws only work on a containedvolume of air. Coupla holes in the ‘chute, allbets are off...
P1/T1=P2/T2 • If a particular sample of gas is in a container, temperature changes that occur at constant volume will result in pressure changes. • Which of the experiments you did illustrates this? • When you increase the temperature of a gas, what happens to pressure?
Chemistry Homework due next time • Page 348, #2, 4, 5, 7 (don’t write a book about number 7!!) • Page 353, 1, 3, 6, 16, 17, 18 • Read chapter 11 so you get practice with the new vocabulary!
There’s just one catch... • We can’t use the Fahrenheit or the Celsius scale for these calculations because they both allow the possibility of negative values. • You can’t have a volume of -12 liters, or -3.4 atmospheres of pressure. For V and P, zero is the lowest it goes. So temperature has to be based on a system with absolute zero.
Kelvin • Theoretical explanation • Empirical exploration • The value? • Converting to Kelvins • Annotating Kelvins • Calvin vs. Kelvin!
Putting it all together • The combined gas law • Before and after scenarios on one sample of gas • Doing Boyle, Charles, and Gay-Lussac problems with the combined gas law • Using algebra on the formula before including values • Including labels
Homework End of chapter 11; 9, 11, 18-32 EVEN Can we talk about your homework for a minute…?
Ideal Gases and the Kinetic Molecular Theory: a review • Gases are composed of tiny particles that are far apart from each other. • Particles are in constant motion. (They have kinetic energy.) • We interpret their kinetic energy as temperature. • There is no attraction or repulsion between particles. • Collisions are elastic.
What does this mean? • The size of a particular molecule of gas does not determine the volume of the gas sample. • Higher temperature is actually just faster molecules. • Gas particles that are “sticky” (polar) are less ideal than particles that are non-sticky (like noble gases) • Low pressure and high temperature helps gases act more ideally, too, because they have fewer and faster collisions.
Please Note! • When heated, molecules of a gas do not expand! They go faster, they crash into their container more and into each other more. Either this means more pressure, or, if the pressure is held constant, it means these molecules become more spread out. • Gases expand when heated. Individual molecules don’t expand!
KE = ½ mv2 • KE is, essentially, temperature • If 2 gas samples have the same temperature, they have the same KE. • If the mass of one of these gases is higher than the mass of the other, at the same temperature, the lighter gas has a higher velocity.
Did you notice...? • You were able to find the number of moles of gas by converting to STP! • We know that 1.000 moles of a gas at 273.15K and 1.000 atmospheres has a volume of exactly 22.414 Liters • PV/T is directly related to the number of moles OR • PV/T = constant “R” x number of moles • Solve for R!
PV=nRT • Clapyron, Father of the ideal gas law • The value you obtain for R depends on the label you use for pressure. • The label for R is... Atmosphere Liters per mole Kelvin Or... atm·L/Mol·K
The avanT-garde r • Homework: Create some form of artistic expression of the value of R and its label. • Purpose: To assist in the memorization of the value and the label. To allow students with different talents to excel. • Secondary purpose: to lighten up and have a little fun amidst all the math. • Observe examples of work from previous victims...um, students.
Developing the rubric as a class? • The rubric Criterion 3 2 1 0 should include Creativity Exemplary Adequate Limited None creativity, ? ? ? ? effort, and what else? Effort • How do we compare Content “pairs projects” with Presentation individuals?
Molar mass is grams per mole • We can calculate the mass of a sample of gas by weighing its container before removing the gas and after • We can calculate number of moles by finding the volume of a sample, the temperature of the sample, the dry gas pressure, and using PV=nRT. Solve for n, number of moles. • Grams divided by moles = molar mass!
Full Formal lab report… • No hypothesis required • Don’t forget to subtract out the water vapor pressure • Show calculations by hand • Calculate molar mass and percent error • Describe any sources of error in your conclusion • Explain the water vapor issue in your conclusion
In summary… • Pressure comes in various units which you can convert. • KMT is the model we use to explain the behavior of gas particles. • Boyle, Charles, and Gay-Lussac’s laws together make the combined gas law. • Kelvin scale must always be used in gas law problems • PV=nRT • STP provides a link between measureable quantities and number of particles in a sample. • We can use gas laws to calculate molar mass of a gas.