2 Gas PropertiesFour properties determine the physical behavior of any gas:Amount of gasGas pressureGas volumeGas temperature
3 Gas pressureGas molecules exert a force on the walls of their container when they collide with it
4 Standard atmospheric pressure (1 atm) is 760 mm Hg Torricelli barometerIn the closed tube, the liquid falls until the pressure exerted by the column of liquid just balances the pressure exerted by the atmosphere.Patmosphere proportional to height of liquid in tubeStandard atmospheric pressure (1 atm) is 760 mm Hg
5 Units for pressureIn this course we usually convert to atm
6 Let’s practice… Standard atmospheric pressure: 1 atm = 760 mmHgConvert 625 mmHg into atmConvert 2.5 atm into mmHg
7 Gas pressureA manometer compares the pressure of a gas in a container to the atmospheric pressure
8 Mixtures of Gases Each gas contributes to the total pressure The pressure caused by each gas is the partial pressure of that gasPtotal = PA + PBEach gas occupies the entire container volume, at its own pressure (the partial pressure of that gas)
9 Mixtures of GasesWhen a gas is collected over water, it is always “wet” (mixed with water vapor).Ptotal = Pbarometric = Pgas + Pwater vaporExample: If 35.5 mL of H2 are collected over water at 26 °C and a barometric pressure of 755 mm Hg, what is the pressure of the H2 gas? The water vapor pressure at 26 °C is 25.2 mm Hg.
10 Relationships between gas properties: pressure, volume, and temp 1660 Robert Boyle investigates P and V:Indirect Relationship:Pressure Increases, Volume DecreasePressure Decreases, Volume IncreasesPV = constant or P1V1 = P2V2
11 Let’s Practice…A sample of gas occupies 10 L at .800 atm. What will the volume be if the pressure decreases to .750 atm?A sample of gas occupies 25 L at 1.5 atm. What will the new pressure be, if the volume increases to 30 L?
12 Gas Pressure & Temperature Gas pressure is proportional to gas temperature:• relationship between pressureand temperature is always linear• all gases reach P = 0 at sametemperature, – °CPressure (psi)• this temperature isABSOLUTE ZEROtemperature (°C)
14 Gas Laws: CharlesIn 1787, Jacques Charles discovered the same relationship between gas volume and temperature:• relationship between volumeand temperature is always linear• all gases reach V = 0 at sametemperature, – °Cvolume (mL)• this temperature isABSOLUTE ZEROtemperature (°C)
16 A temperature scale for gases: the Kelvin scale 1860 English physicist, William Thomson (Lord Kelvin), suggests a relationship between kinetic energy and temperature.A new temperature scale was invented that has zero = absolute zeroThe new temperature scale was named the Kelvin or absolute temperature scaleK = °C
17 Let’s Practice… K = °C + 273.15 Convert 98.6 °C into Kelvin Convert 125 K into °C
18 Temperature and Kinetic Energy The absolute (Kelvin) temperature of a substance is directly proportional to the kinetic energy of its molecules.Kinetic energy is the energy an object has because of its motion.KE = 1/2mv2
19 Temperature and Kinetic Energy Light molecules will move fasterHeavy molecules will move slowerAll molecules at the same temp. have the same kinetic energy.As temp. changes the velocity (speed) changes:Increasing temp = increasing velocityDecreasing temp = decreasing velocityAt absolute zero = velocity of zero (motion stops)
20 Gas laws: Avogadro Avogadro’s hypothesis is Equal volumes of gases at the same temperature and pressure contain equal numbers of moleculesIn mathematical terms, the ratio of gas volume to moles is constant, if pressure and temperature do not change
21 Putting it all together: Ideal Gas Equation Combining Boyle’s Law, Charles’ Laws, and Avogadro’s Law give one equation that includes all four gas variables:R is the ideal or universal gas constantR = atm L/mol K (most useful)If P is in units of mmHg, multiply by 760 mmHg then use 62.4 mmHg L/mol K
22 Using the Ideal Gas Equation Ideal gas equation may be expressed two ways:One set of conditions: ideal gas lawPV = nRTTwo sets of conditions: general gas equationP1V1 = R = P2V n1T n2T2
23 ExamplesWhat is the volume occupied by 20.2 g NH3 gas at –25 °C and 752 mm Hg?How many moles of He gas are in a 5.00 L tank at 10.5 atm pressure and 30.0 °C?A 1.00 mL sample of N2 gas at 36.2 °C and atm is heated to 37.8 °C while the pressure is changed to 1.02 atm. What volume does the gas occupy at this temperature and pressure?
24 Standard Molar volume and Stoichiometry Scientists have chosen a set of standard conditions (standard temp. and pressure) STP:1 atm or 760 mmHg0°C or 273 KStandard molar Volume (for any gas)1 mole = 22.4 LAt STP, 22.4 L of any gas contains one mole of gas molecules (6.02 x 1023 molecules)
25 Let’s Practice…Convert .5 moles of gas into LConvert 12 L into moles
26 A Model for Gas Behavior The gas laws describe what gases do, but they do not explain why.The Kinetic Molecular Theory of Gases is the model that explains gas behavior.KMT was developed by Maxwell and Boltzmann in the mid-1800sKMT is based on the concept of an ideal or perfect gas
27 Ideal gasComposed of tiny particles in constant, random, straight-line motionGas molecules are point masses, so gas volume is just the empty space between the moleculesMolecules collide with each other and with the walls of their containerThe molecules are completely independent of each other, with no attractive or repulsive forces between them.Individual molecules may gain or lose energy during collisions, but the total energy of the gas sample depends only on the absolute temperature.