B. As the gas particles move and strike a surface, they push on that surface 1. If we could measure the total amount of force exerted by gas

Transcription

1 Chapter 5: Gases I. The Structure of a Gas A. Gases are composed of particles that are flying around very fast in their container(s). 1. The particles travel in straight lines until they encounter either the container wall or another particle; then they bounce off. 2. There is a lot of empty space between particles. 224

2 B. As the gas particles move and strike a surface, they push on that surface 1. If we could measure the total amount of force exerted by gas molecules hitting a surface at any one instant, we would know the pressure the gas is exerting. 225

3 II. Measuring Air Pressure A. We measure the effect of particle collisions with the walls of the container or on a surface. B. We measure air pressure with a barometer. The image cannot be displayed. Your computer may not have enough memory to open the image, or the image may have been corrupted. Restart your computer, and then open the file again. If the red x still appears, you may have to delete the image and then insert it again. 226

4 C. What would happen if the column of mercury were replaced by a column of water? 227

5 D. What happens to the height of the column of mercury in a mercury barometer as you climb to the top of a mountain? 228

6 E. Units of Pressure 229

7 F. Ex: A high-performance bicycle tire has a pressure of 132 psi. What is the pressure in mmhg? G. Ex: Convert 45.5 psi into kpa 230

8 H. Manometers 1. Manometers are U-shaped tubes, partially filled with a liquid, connected to the gas sample on one side and open to the air on the other. 2. Two types: Open ended manometer is open to the atmosphere and measures the pressure difference between a the pressure of a gas and atmospheric pressure. 231

9 III. Gas Laws A. Boyle s Law 1. Experiment 232

10 2. Data 233

11 234

12 3. Boyle s Law: Molecular View a. Pressure is caused by the molecules striking the sides of the container. 235

13 4. Ex: A cylinder with a movable piston has a volume of 7.25 L at 4.52 atm. What is the volume at 1.21 atm? 236

14 5. Ex: A balloon is put in a bell jar and the pressure is reduced from 782 torr to atm. If the volume of the balloon is now 2.78 x 10 3 ml, what was it originally? 237

15 B. Charles Law 1. Experiment 238

16 B. Charles Law 2. Data Height of an Air Bubble vs. Temperature Height of Gas Bubble (mm) y = 0.158x R² = T ( C) 239

17 B. Charles Law 3. Data: different axes Height of an Air Bubble vs. Temperature 60 Height of Gas Bubble (mm) y = 0.158x R² = T ( C) 240

18 4. Ex: A gas has a volume of 2.57 L at 0.00 C. What was the temperature at 2.80 L? 241

19 5. Ex: The temperature inside a balloon is raised from 25.0 C to C. If the volume of cold air was 10.0 L, what is the volume of hot air? 242

20 6. Combined Gas Law 243

21 C. Ideal Gas Law 1. By combining the gas laws we can write a general equation 2. n is the number of moles of gas 3. R is called the gas constant 4. When P is in atm and V is in Liters: R = L atm mol K 244

22 4. Ex: How many moles of gas are in a basketball with total pressure 24.3 psi, volume of 3.24 L at 25 C? 245

23 D. Standard Conditions 1. Because the volume of a gas varies with pressure and temperature, chemists have agreed on a set of conditions to report our measurements so that comparison is easy we call these standard conditions: 2. STP = Standard Temperature and Pressure: 246

24 3. Ex: What volume will 1.00 mol of O 2 occupy at 1.00 atm and 273 K? 247

25 4. Ex: A gas occupies 10.0 L at 44.1 psi and 27 C. What volume will it occupy at 1.00 atm and 273 K? 248

26 5. Ex: Calculate the volume occupied by 637 g of SO 2 (MM 64.07) at 6.08 x 10 4 mmhg and 23 C. 249

27 E. Mixtures of Gases 1. When gases are mixed together, their molecules behave independently of each other a. all the gases in the mixture have the same volume all completely fill the container b. all gases in the mixture are at the same temperature 250

28 2. Therefore, in certain applications, the mixture can be thought of as one gas 251

29 3. Dalton s Law of Partial Pressures a. The pressure of a single gas in a mixture of gases is called its partial pressure. b. The sum of the partial pressures of all the gases in the mixture equals the total pressure: 252

30 c. The partial pressure of any one gas, P A, is equal to its mole fraction, χ A, times the total pressure: 253

31 4. Ex: Determine the mass of Ar in a mixture of He, Ne and Ar with: P He =341 mmhg, P Ne = 112 mmhg, P T = 662 mmhg, V = 1.00 L, T = 298 K. 254

32 5. Ex: Find the partial pressure of neon in a mixture with total pressure 3.9 atm, volume = 8.7 L, temperature = 598 K, and 0.17 moles Xe. 255

33 6. Ex: Find the mole fractions and partial pressures in a 12.5 L tank with 24.2 g He and 4.32 g O 2 at 298 K. 256

34 7. Ex: Find the mole fraction of neon in a mixture with total pressure 3.9 atm, volume 8.7 L, temperature 598 K, and 0.17 moles Xe. 257

35 IV. Collecting A Gas A. Mostly done in lab over water. 1. Ex: Zn(s) + 2 HCl(aq) H 2 (g) + ZnCl 2 (aq) 258

36 2. The partial pressure of the water vapor, called the vapor pressure, depends only on the temperature. 259

37 3. Ex: 1.02 L of O 2 collected over water at 293 K with a total pressure of mmhg. Find the mass of O

38 4. Ex: 0.12 moles of H 2 is collected over water in a 10.0 L container at 323 K. Find the total pressure. 261

39 V. Gas Problems with Stoichiometry A. The ideal gas law allows us to convert from the volume of the gas to moles: then we can use the coefficients in the equation as a mole ratio P, V, T of Gas A mole A mole B P, V, T of Gas B 262

40 1. Ex: What volume of H 2 is needed to make 35.7 g of CH 3 OH at 738 mmhg and 355 K? CO(g) + 2 H 2 (g) CH 3 OH(g) 263

41 2. Ex: How many grams of H 2 O form when 1.24 L H 2 reacts completely with O 2 at 1.00 atm and 273 K? O 2 (g) + 2 H 2 (g) 2 H 2 O(g) 264

42 3. Ex: What volume of O 2 at atm and 313 K is generated by the thermolysis of 10.0 g of HgO? 2 HgO(s) 2 Hg(l) + O 2 (g) 265

43 V. The Kinetic Molecular Theory of Ideal Gases A. Gases are composed of particles that are flying around very fast in their container(s). B. The size of a particle is negligible compared to the volume of the gas

44 C. The collision of one particle with another (or with the walls of the container) is elastic. 267

45 D. The average kinetic energy of a particle is proportional to the temperature in Kelvin. 268

46 4. Gas particles have a distribution of Kinetic Energy values called the Boltzmann Distribution that is the same for all particles at the same temperature. 269

47 5. But all gases have different masses and therefore different velocities even at the same temperature 270

48 6. Ex: Draw a Boltzmann Distribution using molecular speed on the x-axis for methane at T = 300 K and at T = 500 K. 271

49 7. Ex: Draw a Boltzmann Distribution using molecular speed on the x-axis for methane and neon at the same temperature. 272

50 VI. Diffusion and Effusion A. The process of a collection of molecules spreading out from high concentration to low concentration is called. 273

51 B. The process by which a collection of molecules escapes through a small hole into a vacuum is called. 274

52 C. Diffusion and effusion both occur more rapidly for molecules with higher velocities. D. Ex: Which will diffuse more rapidly: oxygen gas or argon gas? 275

53 VII. Real Gases A. At STP, most gases are pretty close to ideal: 1. Ideal gas law assumes 276

54 B. At low temperatures and high pressures, gases act less ideal. 1. At low temperatures, the speed of the particles is lower. 2. At high pressures, the particles are forced closer together so that IMF are more important. 277