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1Chemistry: Atoms First Julia Burdge & Jason Overby Chapter 11GasesKent L. McCorkleCosumnes River CollegeSacramento, CACopyright (c) The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
2Properties of Gases11.1Gases differ from solids and liquids in the following ways:A sample of gas assumes both the shape and volume of the container.Gases are compressible.The densities of gases are much smaller than those of liquids and solids and are highly variable depending on temperature and pressure.Gases form homogeneous mixtures (solutions) with one another in any proportion.
4at constant temperature The Gas Laws11.4(a)(b)(c)P (mmHg)76015202280V (mL)1005033Boyle’s law states that the pressure of a fixed amount of gas at a constant temperature is inversely proportional to the volume of the gas.P1V1 = P2V2at constant temperature
5The Gas LawsCalculate the volume of a sample of gas at 5.75 atm if it occupies 5.14 L at 2.49 atm. (Assume constant temperature.)Solution:Step 1: Use the relationship below to solve for V2:P1V1 = P2V2
6Worked Example 11.3If a skin diver takes a breath at the surface, filling his lungs with 5.82 L of air, what volume will the air in his lungs occupy when he dives to a depth where the pressure of 1.92 atm? (Assume constant temperature and that the pressure at the surface is exactly 1 atm.)Strategy Use P1V1 = P2V2 to solve for V2.Solution P1 = 1.00 atm, V1 = 5.82 L, and P2 = 1.92 atm.V2 = = = 3.03 LP1 × V1P21.00 atm × 5.82 L1.92 atmThink About It At higher pressure, the volume should be smaller. Therefore, the answer makes sense.
7The Gas LawsCharles’s and Gay-Lussac’s law, (or simply Charles’s Law) states that the volume of a gas maintained at constant pressure is directly proportional to the absolute temperature of the gas.Higher temperatureLower temperature
8The Gas LawsCharles’s and Gay-Lussac’s law, (or simply Charles’s Law) states that the volume of a gas maintained at constant pressure is directly proportional to the absolute temperature of the gas.at constant pressure
9Worked Example 11.4A sample of argon gas that originally occupied 14.6 L at 25°C was heated to 50.0°C at constant pressure. What is its new volume?Strategy Use V1/T1 = V2/T2 to solve for V2. Remember that temperatures must be expressed in kelvin.Solution T1 = K, V1 = 14.6 L, and T2 = K.V2 = = = 15.8 LV1 × T2T114.6 L × KKThink About It When temperature increases at constant pressure, the volume of a gas sample increases.
10at constant temperature and pressure The Gas LawsAvogadro’s law states that the volume of a sample of gas is directly proportional to the number of moles in the sample at constant temperature and pressure.at constant temperature and pressure
11Worked Example 11.5If we combine 3.0 L of NO and 1.5 L of O2, and they react according to the balanced equation 2NO(g) + O2(g) → 2NO2(g), what volume of NO2 will be produced? (Assume that the reactants and products are all at the same temperature and pressure.)Strategy Apply Avogadro’s law to determine the volume of a gaseous product.Solution Because volume is proportional to the number of moles, the balanced equation determines in what volume ratio the reactants combine and the ratio of product volume to reactant volume. The amounts of reactants given are stoichiometric amounts.According to the balanced equation, the volume of NO2 formed will be equal to the volume of NO that reacts. Therefore, 3.0 L of NO2 will form.Think About It When temperature increases at constant pressure, the volume of a gas sample increases.
12The Gas LawsA sample of gas originally occupies 29.1 L at 0.0°C. What is its new volume when it is heated to 15.0°C? (Assume constant pressure.)Solution:Step 1: Use the relationship below to solve for V2: (Remember that temperatures must be expressed in kelvin.
13The Gas LawsWhat volume in liters of water vapor will be produced when 34 L of H2 and 17 L of O2 react according to the equation below:2H2(g) + O2(g) → 2H2O(g)Assume constant pressure and temperature.Solution:Step 1: Because volume is proportional to the number of moles, the balanced equation determines in what volume ratio the reactants combine and the ratio of product volume to reactant volume. The amounts of reactants given are stoichiometeric amounts.34 L of H2O will form.
14The Gas Laws Cooling at constant volume: pressure decreases Heating at constant volume: pressure increases
15The Gas Laws Cooling at constant pressure: volume decreases Heating at constant pressure: volume increases
16The Gas LawsThe presence of additional molecules causes an increase in pressure.
17The Gas LawsThe combined gas law can be used to solve problems where any or all of the variables changes.
18The Gas LawsThe volume of a bubble starting at the bottom of a lake at 4.55°C increases by a factor of 10 as it rises to the surface where the temperature is 18.45°C and the air pressure is atm. Assume the density of the lake water is 1.00 g/mL. Determine the depth of the lake.Solution:Step 1: Use the combined gas law to find the pressure at the bottom of the lake; assume constant moles of gas.
19Worked Example 11.6If a child releases a 6.25-L helium balloon in the parking lot of an amusement park where the temperature is 28.50°C and the air pressure is mmHg, what will the volume of the balloon be when it has risen to an altitude where the temperature is °C and the air pressure is mmHg?Strategy In this case, because there is a fixed amount of gas, we use P1V1/T1 = P2V2/T2. The only value we don’t know is V2. Temperatures must be expressed in kelvins. We can use any units of pressure, as long as we are consistent.Solution T1 = K, T2 = K.V2 = = = 10.2 LP1T2V1P2T1757.2 mmHg × K × 6.25 L366.4 mmHg × KThink About It Note that the solution is essentially multiplying the original volume by the ratio of P1 and P2, and by the ratio of T2 to T1. The effect of decreasing external pressure is to increase the balloon volume. The effect of decreasing temperature is to decrease the volume. In this case, the effect of decreasing pressure predominates and the balloon volume increases significantly.
20The Ideal Gas Equation11.5The gas laws can be combined into a general equation that describes the physical behavior of all gases.Boyle’s lawCharles’s lawAvogadro’s lawPV = nRTrearrangementR is the proportionality constant, called the gas constant.
21The Ideal Gas EquationThe ideal gas equation (below) describes the relationship among the four variables P, V, n, and T.PV = nRTAn ideal gas is a hypothetical sample of gas whose pressure-volume-temperature behavior is predicted accurately by the ideal gas equation.
22The Ideal Gas EquationThe gas constant (R) is the proportionality constant and its value and units depend on the units in which P and V are expressed.PV = nRT
23The Ideal Gas EquationStandard Temperature and Pressure (STP) are a special set of conditions where:Pressure is 1 atmTemperature is 0°C ( K)The volume occupied by one mole of an ideal gas is then L:PV = nRT
24(1 mol)(0.08206 L∙atm/K∙mol)(298.15 K) Worked Example 11.7Calculate the volume of a mole of ideal gas at room temperature (25°C) and 1 atm.Strategy Convert the temperature in °C to kelvins, and use the ideal gas equation to solve for the unknown volume.Solution The data given are n = 1 mol, T = K, and P = 1.00 atm. Because the pressure is expressed in atmospheres, we use R = L∙atm/K∙mol in order to solve for volume in liters.V = = = 24.5 LnRTP(1 mol)( L∙atm/K∙mol)( K)1 atmThink About It With the pressure held constant, we should expect the volume to increase with increased temperature. Room temperature is higher than the standard temperature for gases (0°C), so the molar volume at room temperature (25°C) should be higher than the molar volume at 0°C–and it is.
25Ptotal = PN2 + PO2 = 4.48 atm + 4.48 atm = 8.96 atm Gas Mixtures11.7When two or more gases are placed in a container, each gas behaves as though it occupies the container alone.1.00 mole of N2 in a 5.00 L container at 0°C exerts a pressure of 4.48 atm.Addition of 1.00 mole of O2 in the same container exerts an additional 4.48 atm of pressure.The total pressure of the mixture is the sum of the partial pressures (Pi):Ptotal = PN2 + PO2 = 4.48 atm atm = 8.96 atm
26Gas MixturesDalton’s law of partial pressure states that the total pressure exerted by a gas mixture is the sum of the partial pressures exerted by each component of the mixture:
27Gas MixturesDetermine the partial pressures and the total pressure in a 2.50-L vessel containing the following mixture of gases at 15.8°C: mol He, mol H2, and mol Ne.Solution:Step 1: Since each gas behaves independently, calculate the partial pressure of each using the ideal gas equation:
28Gas MixturesDetermine the partial pressures and the total pressure in a 2.50-L vessel containing the following mixture of gases at 15.8°C: mol He, mol H2, and mol Ne.Solution:Step 2: Use the equation below to calculate total pressure.Ptotal = atm atm atm = 2.17 atm
29Gas MixturesEach component of a gas mixture exerts a pressure independent of the other components. The total pressure is the sum of the partial pressures.
30Worked Example 11.12A 1.00-L vessel contains mole of N2 gas and mole of H2 gas at 25.5°C. Determine the partial pressure of each component and the total pressure in the vessel.Think About It The total pressure in the vessel can also be determined by summing the number of moles of mixture components ( = mol) and solving the ideal gas equation for Ptotal:(0.227 mol)( L∙atm/K∙mol)( K)1.00 L= 5.56 atmPtotal =Strategy Use the ideal gas equation to find the partial pressure of each component of the mixture, and sum the two partial pressures to find the total pressure.Solution T = KPtotal = PN2 + PH2 = 5.27 atm atm = 5.56 atm(0.215 mol)( L∙atm/K∙mol)( K)1.00 LPN2 == 5.27 atm( mol)( L∙atm/K∙mol)( K)1.00 LPH2 == atm
31Gas MixturesThe relative amounts of the components in a gas mixture can be specified using mole fractions.There are three things to remember about mole fractions:The mole fraction of a mixture component is always less than 1.The sum of mole fractions for all components of a mixture is always 1.Mole fractions are dimensionless.Χi is the mole fraction.ni is the moles of a certain componentntotal is the total number of moles.
32Worked Example 11.13In 1999, the FDA approved the use of nitric oxide (NO) to treat and prevent lung disease, which occurs commonly in premature infants. The nitric oxide used in this therapy is supplied to hospitals in the form of a N2/NO mixture. Calculate the mole fraction of NO in a L gas cylinder at room temperature (25°C) that contains mol N2 and in which the total pressure is atm.Think About It To check your work, determine χN2 by subtracting χNO from 1. Using each mole fraction and the total pressure, calculate the partial pressure of each component using χi = Pi/Ptotal and verify that they sum to the total pressure.Strategy Use the ideal gas equation to calculate the total number of moles in the cylinder. Subtract moles of N2 from the total to determine moles of NO. Divide moles NO by total moles to get mole fraction.Solution The temperature is K.total moles = =mol NO = total moles – N2 = – = mol NOχNO = = = 0.001PVRT(14.75 atm)(10.00 L)( L∙atm/K∙mol)( K)= molnNOntotal0.007 mol NO6.029 mol
33Reactions with Gaseous Reactants and Products 11.8What mass (in grams) of Na2O2 is necessary to consume 1.00 L of CO2 at STP?2Na2O2(s) + 2CO2(g) → 2Na2CO3(s) + O2(g)Solution:Step 1: Convert 1.00 L of CO2 at STP to moles using the ideal gas equation.PV = nRT(1 atm)(1.00 L) = n( Latm/mol K)( K)n = moles CO2
34Reactions with Gaseous Reactants and Products What mass (in grams) of Na2O2 is necessary to consume 1.00 L of CO2 at STP?2Na2O2(s) + 2CO2(g) → 2Na2CO3(s) + O2(g)Solution:Step 2: Determine the stoichiometric amount of Na2O2.
352Na2O2(s) + 2CO2(g) → 2Na2CO3(s) + O2(g) Worked Example 11.14Sodium peroxide (Na2O2) is used to remove carbon dioxide from (and add oxygen to) the air supply in spacecrafts. It works by reacting with CO2 in the air to produce sodium carbonate (Na2CO3) and O2.2Na2O2(s) + 2CO2(g) → 2Na2CO3(s) + O2(g)What volume (in liters) of CO2 (at STP) will react with a kilogram of Na2O2?Strategy Convert the given mass of Na2O2 to moles, use the balanced equation to determine the stoichiometric amount of CO2, and then use the ideal gas equation to convert moles of CO2 to liters.
36(12.82 mol CO2)(0.08206 L∙atm/K∙mol)(273.15 K) Worked Example (cont.)Solution The molar mass of Na2O2 is g/mol (1 kg = 1000 g). (Treat the specified mass of NaO2 as an exact number.)1000 g Na2O2 ×12.82 mol Na2O2 ×VCO2 =1 mol Na2O277.98 g Na2O2= mol Na2O22 mol CO22 mol Na2O2= mol Na2O2(12.82 mol CO2)( L∙atm/K∙mol)( K)1 atm= L CO2Think About It The answer seems like an enormous volume of CO2. If you check the cancellation of units carefully in ideal gas equation problems, however, with practice you will develop a sense of whether such a calculated volume is reasonable.
37Reactions with Gaseous Reactants and Products Although there are no empirical gas laws that focus on the relationship between n and P, it is possible to rearrange the ideal gas equation to find the relationship.PV = nRTrearrangementThe change in pressure in a reaction vessel can be used to determine how many moles of gaseous reactant are consumed:
382LiOH(aq) + CO2(g) → Li2CO3(s) + H2O(l) Worked Example 11.15Another air-purification method for enclosed spaces involves the use of “scrubbers” containing aqueous lithium hydroxide, which react with carbon dioxide to produce lithium carbonate and water:2LiOH(aq) + CO2(g) → Li2CO3(s) + H2O(l)Consider the air supply in a submarine with a total volume of 2.5×105 L. The pressure of atm, and the temperature 25°C. If the pressure in the submarine drops to atm as the result of carbon dioxide being consumed by an aqueous lithium hydroxide scrubber, how many moles of CO2 are consumed.Think About It Careful cancellation of units is essential. Note that this amount of CO2 corresponds to 162 moles or 3.9 kg of LiOH. (It’s a good idea to verify this yourself.Strategy Use Δn = ΔP×(V/RT) to determine Δn, the number of moles CO2 consumed.Solution ΔP = atm – atm = 7.9×10-3 atm, V = 2.5×105 L, and T = K.ΔnCO2 = 7.9×10-3 atm ×2.5×105 L( L∙atm/K∙mol) × ( K)= 81 moles CO2consumed
39Reactions with Gaseous Reactants and Products The volume of gas produced by a chemical reaction can be measured using an apparatus like the one shown below.When gas is collected over water in this manner, the total pressure is the sum of two partial pressures:Ptotal = Pcollected gas + PH2O
40Reactions with Gaseous Reactants and Products The vapor pressure of water is known at various temperatures.
41Gas MixturesCalculate the mass of O2 produced by the decomposition of KClO3 when 821 mL of O2 is collected over water at 30.0°C and atm.Solution:Step 1: Use Table 11.5 to determine the vapor pressure of water at 30.0°C.
42Reactions with Gaseous Reactants and Products Calculate the mass of O2 produced by the decomposition of KClO3 when 821 mL of O2 is collected over water at 30.0°C and atm.Solution:Step 2: Convert the vapor pressure of water at 30.0°C to atm and then use Dalton’s law to calculate the partial pressure of O2.Ptotal = PO2 + PH2OPO2 = Ptotal – PH2OPO2 = atm – atm = atm
43Reactions with Gaseous Reactants and Products Calculate the mass of O2 produced by the decomposition of KClO3 when 821 mL of O2 is collected over water at 30.0°C and atm.Solution:Step 3: Convert to moles of O2 using the ideal gas equation and then find mass.PV = nRT
44Ca(s) + H2O(l) → Ca(OH)2(aq) + H2(g) Worked Example 11.16Calcium metal reacts with water to produce hydrogen gas:Ca(s) + H2O(l) → Ca(OH)2(aq) + H2(g)Determine the mass of H2 produced at 25°Cand atm when 525 mL of the gas is collected over water as shown in FigureThink About It Check unit cancellation carefully, and remember that the densities of gases are relatively low. The mass of approximately half a liter of hydrogen at or near room temperature and 1 atm should be a very small number.Strategy Use Dalton’s law of partial pressures to determine the partial pressure of H2, use the ideal gas equation to determine moles of H2, and then use the molar mass of H2 to convert to mass. (Pay careful attention to units. Atmospheric pressure is given in atmospheres, whereas the vapor pressure of water is tabulated in torr.)Solution PH2 = Ptotal – PH2O = atm – atm = atmmoles of H2 =moles of H2 = (2.008×10-2 mol)(2.016 g/mol) = g H2( atm)(0.525 L)( L∙atm/K∙mol)( K)= 2.01×10-2 mol
45Experimentally measured pressure Real Gases11.6The van der Waals equation is useful for gases that do not behave ideally.Experimentally measured pressureContainer volumecorrectedpressure termcorrectedvolume term