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1 Chapter 5 The Gas Laws

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2 Pressure n Force per unit area. n Gas molecules fill container. n Molecules move around and hit sides. n Collisions are the force. n Container has the area. n Measured with a barometer.

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3 Barometer n The pressure of the atmosphere at sea level will hold a column of mercury 760 mm Hg. n 1 atm = 760 mm Hg 1 atm Pressure 760 mm Hg Vacuum

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4 Manometer Gas h n Column of mercury to measure pressure. n h is how much lower the pressure is than outside.

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5 Manometer n h is how much higher the gas pressure is than the atmosphere. h Gas

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6 Units of pressure n 1 atmosphere = 760 mm Hg n 1 mm Hg = 1 torr n 1 atm = 101,325 Pascals = kPa n Can make conversion factors from these. n What is 724 mm Hg in kPa? n in torr? n in atm?

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7 The Gas Laws n Boyles Law n Pressure and volume are inversely related at constant temperature. n PV= k n As one goes up, the other goes down. n P 1 V 1 = P 2 V 2 n Graphically

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8 V P (at constant T)

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9 V 1/P (at constant T) Slope = k

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10 PV P (at constant T) CO 2 O2O L atm

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11 Examples n 20.5 L of nitrogen at 25ºC and 742 torr are compressed to 9.8 atm at constant T. What is the new volume? n 30.6 mL of carbon dioxide at 740 torr is expanded at constant temperature to 750 mL. What is the final pressure in kPa?

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12 Charles Law n Volume of a gas varies directly with the absolute temperature at constant pressure. n V = kT (if T is in Kelvin) n V 1 = V 2 T 1 = T 2 n Graphically

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13 V (L) T (ºC) H2OH2O He CH 4 H2H ºC

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14 Examples n What would the final volume be if 247 mL of gas at 22ºC is heated to 98ºC, if the pressure is held constant?

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15 Examples n At what temperature would 40.5 L of gas at 23.4ºC have a volume of 81.0 L at constant pressure?

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16 Avogadro's Law n Avagadros n At constant temperature and pressure, the volume of gas is directly related to the number of moles. n V = k n (n is the number of moles) n V 1 = V 2 n 1 = n 2

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17 Gay- Lussac Law n At constant volume, pressure and absolute temperature are directly related. n P = k T n P 1 = P 2 T 1 = T 2

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18 Combined Gas Law n If the moles of gas remains constant, use this formula and cancel out the other things that dont change. n P 1 V 1 = P 2 V 2. T 1 T 2

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19 Examples n A deodorant can has a volume of 175 mL and a pressure of 3.8 atm at 22ºC. What would the pressure be if the can was heated to 100.ºC? n What volume of gas could the can release at 22ºC and 743 torr?

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20 Ideal Gas Law n PV = nRT n V = L at 1 atm, 0ºC, n = 1 mole, what is R? n R is the ideal gas constant. n R = L atm/ mol K n Tells you about a gas is NOW. n The other laws tell you about a gas when it changes.

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21 Ideal Gas Law n An equation of state. n Independent of how you end up where you are at. n Does not depend on the path. n Given 3 you can determine the fourth. n An Empirical Equation - based on experimental evidence.

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22 Ideal Gas Law n A hypothetical substance - the ideal gas n Think of it as a limit. n Gases only approach ideal behavior at low pressure (< 1 atm) and high temperature. n Use the laws anyway, unless told to do otherwise. n They give good estimates.

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23 Examples n A 47.3 L container containing 1.62 mol of He is heated until the pressure reaches 1.85 atm. What is the temperature? n Kr gas in a 18.5 L cylinder exerts a pressure of 8.61 atm at 24.8ºC What is the mass of Kr? n A sample of gas has a volume of 4.18 L at 29ºC and 732 torr. What would its volume be at 24.8ºC and 756 torr?

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24 Gas Density and Molar Mass n D = m/V Let M stand for molar mass Let M stand for molar mass M = m/n M = m/n n n= PV/RT M = m PV/RT M = m PV/RT M = mRT = m RT = DRT PV V P P M = mRT = m RT = DRT PV V P P

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25 Examples n What is the density of ammonia at 23ºC and 735 torr? n A compound has the empirical formula CHCl. A 256 mL flask at 100.ºC and 750 torr contains.80 g of the gaseous compound. What is the molecular formula?

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26 Gases and Stoichiometry n Reactions happen in moles n At Standard Temperature and Pressure (STP, 0ºC and 1 atm) 1 mole of gas occupies L. n If not at STP, use the ideal gas law to calculate moles of reactant or volume of product.

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27 Examples n Mercury can be achieved by the following reaction What volume of oxygen gas can be produced from 4.10 g of mercury (II) oxide at STP? n At 400.ºC and 740 torr?

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28 Examples n Using the following reaction calculate the mass of sodium hydrogen carbonate necessary to produce 2.87 L of carbon dioxide at 25ºC and 2.00 atm. n If 27 L of gas are produced at 26ºC and 745 torr when 2.6 L of HCl are added what is the concentration of HCl?

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29 Examples n Consider the following reaction What volume of NO at 1.0 atm and 1000ºC can be produced from 10.0 L of NH 3 and excess O 2 at the same temperature and pressure? n What volume of O 2 measured at STP will be consumed when 10.0 kg NH 3 is reacted?

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30 Examples n What mass of H 2 O will be produced from 65.0 L of O 2 and 75.0 L of NH 3 both measured at STP? n What volume Of NO would be produced? n What mass of NO is produced from 500. L of NH 3 at 250.0ºC and 3.00 atm?

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31 Daltons Law n The total pressure in a container is the sum of the pressure each gas would exert if it were alone in the container. n The total pressure is the sum of the partial pressures. n P Total = P 1 + P 2 + P 3 + P 4 + P 5... n For each P = nRT/V

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32 Dalton's Law n P Total = n 1 RT + n 2 RT + n 3 RT +... V V V n In the same container R, T and V are the same. n P Total = (n 1 + n 2 + n )RT V n P Total = (n Total )RT V

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33 The mole fraction n Ratio of moles of the substance to the total moles. symbol is Greek letter chi symbol is Greek letter chi = n 1 = P 1 n Total P Total = n 1 = P 1 n Total P Total

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34 Examples n The partial pressure of nitrogen in air is 592 torr. Air pressure is 752 torr, what is the mole fraction of nitrogen? n What is the partial pressure of nitrogen if the container holding the air is compressed to 5.25 atm?

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35 Examples 3.50 L O L N atm n When these valves are opened, what is each partial pressure and the total pressure? 4.00 L CH atm atm

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36 Vapor Pressure n Water evaporates! n When that water evaporates, the vapor has a pressure. n Gases are often collected over water so the vapor pressure of water must be subtracted from the total pressure to find the pressure of the gas. n It must be given.

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37 Example n N 2 O can be produced by the following reaction what volume of N 2 O collected over water at a total pressure of 94 kPa and 22ºC can be produced from 2.6 g of NH 4 NO 3 ? ( the vapor pressure of water at 22ºC is 21 torr)

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38 Kinetic Molecular Theory n Theory tells why the things happen. n explains why ideal gases behave the way they do. n Assumptions that simplify the theory, but dont work in real gases. The particles are so small we can ignore their volume. The particles are so small we can ignore their volume. The particles are in constant motion and their collisions cause pressure. The particles are in constant motion and their collisions cause pressure.

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39 Kinetic Molecular Theory The particles do not affect each other, neither attracting or repelling. The particles do not affect each other, neither attracting or repelling. The average kinetic energy is proportional to the Kelvin temperature. The average kinetic energy is proportional to the Kelvin temperature. n Appendix 2 shows the derivation of the ideal gas law and the definition of temperature. n We need the formula KE = 1/2 mv 2

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40 What it tells us n (KE) avg = 3/2 RT n This the meaning of temperature. n u is the particle velocity. n u is the average particle velocity. n u 2 is the average of the squared particle velocity. the root mean square velocity is u 2 = u rms the root mean square velocity is u 2 = u rms

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41 Combine these two equations n For a mole of gas n N A is Avogadro's number n n

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42 Combine these two equations n n m is kg for one particle, so N a m is kg for a mole of particles. We will call it M n Where M is the molar mass in kg/mole, and R has the units J/Kmol. n The velocity will be in m/s

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43 Example n Calculate the root mean square velocity of carbon dioxide at 25ºC. n Calculate the root mean square velocity of hydrogen at 25ºC. n Calculate the root mean square velocity of chlorine at 250ºC.

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44 Range of velocities n The average distance a molecule travels before colliding with another is called the mean free path and is small (near ) n Temperature is an average. There are molecules of many speeds in the average. n Shown on a graph called a velocity distribution

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45 number of particles Molecular Velocity 273 K

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46 number of particles Molecular Velocity 273 K 1273 K

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47 Velocity n Average increases as temperature increases. n Spread increases as temperature increases.

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48 Effusion n Passage of gas through a small hole, into a vacuum. n The effusion rate measures how fast this happens. n Grahams Law the rate of effusion is inversely proportional to the square root of the mass of its particles.

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49 Effusion n Passage of gas through a small hole, into a vacuum. n The effusion rate measures how fast this happens. n Grahams Law the rate of effusion is inversely proportional to the square root of the mass of its particles.

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50 Deriving n The rate of effusion should be proportional to u rms n Effusion Rate 1 = u rms 1 Effusion Rate 2 = u rms 2

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51 Deriving n The rate of effusion should be proportional to u rms n Effusion Rate 1 = u rms 1 Effusion Rate 2 = u rms 2

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52 Diffusion n The spreading of a gas through a room. n Slow considering molecules move at 100s of meters per second. n Collisions with other molecules slow down diffusions. n Best estimate is Grahams Law.

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53 Examples n Helium effuses through a porous cylinder 3.20 time faster than a compound. What is its molar mass? n If mol of NH 3 effuse through a hole in 2.47 min, how much HCl would effuse in the same time? n A sample of N 2 effuses through a hole in 38 seconds. what must be the molecular weight of gas that effuses in 55 seconds under identical conditions?

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54 Diffusion n The spreading of a gas through a room. n Slow considering molecules move at 100s of meters per second. n Collisions with other molecules slow down diffusions. n Best estimate is Grahams Law.

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55 Real Gases n Real molecules do take up space and they do interact with each other (especially polar molecules). n Need to add correction factors to the ideal gas law to account for these.

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56 Volume Correction n The actual volume free to move in is less because of particle size. n More molecules will have more effect. n Bigger molecules have more effect n Corrected volume V = V - nb n b is a constant that differs for each gas. P = nRT (V-nb) P = nRT (V-nb)

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57 Pressure correction n Because the molecules are attracted to each other, the pressure on the container will be less than ideal gas n Depends on the type of molecule n depends on the number of molecules per liter. n since two molecules interact, the effect must be squared.

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58 Pressure correction n Because the molecules are attracted to each other, the pressure on the container will be less than ideal n depends on the number of molecules per liter. n since two molecules interact, the effect must be squared.

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59 Altogether n n Called the Van der Waals equation if rearranged n Corrected Corrected Pressure Volume

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60 Where does it come from n a and b are determined by experiment. n Different for each gas. n Look them up n Bigger molecules have larger b. n a depends on both size and polarity. n once given, plug and chug.

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61 Example n Calculate the pressure exerted by mol Cl 2 in a L container at 25.0ºC n Using the ideal gas law. n Van der Waals equation –a = 6.49 atm L 2 /mol 2 –b = L/mol

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