GAses

1. GAses

2. Gas Properties • Four properties determine the physical behavior of any gas: • Amount of gas • Gas pressure • Gas volume • Gas temperature

3. Gas pressure • Gas molecules exert a force on the walls of their container when they collide with it

4. Atmospheric pressure • Torricelli barometer • In 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 tube Standard atmospheric pressure (1 atm) is 760 mm Hg

5. Units for pressure • In this course we usually convert to atm

6. Let’s practice… • Standard atmospheric pressure: • 1 atm = 760 mmHg • Convert 625 mmHg into atm • Convert 2.5 atm into mmHg

7. Gas pressure • A 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 gas • Ptotal = PA + PB • Each gas occupies the entire container volume, at its own pressure (the partial pressure of that gas)

9. Mixtures of Gases • When a gas is collected over water, it is always “wet” (mixed with water vapor). • Ptotal = Pbarometric = Pgas + Pwater vapor • Example: 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 Decrease • Pressure Decreases, Volume Increases • PV = 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 pressure and temperature is always linear • all gases reach P = 0 at same temperature, –273.15 °C Pressure (psi) • this temperature is ABSOLUTE ZERO temperature (°C)

13. Let’s practice…

14. Gas Laws: Charles • In 1787, Jacques Charles discovered the same relationship between gas volume and temperature: • relationship between volume and temperature is always linear • all gases reach V = 0 at same temperature, –273.15 °C volume (mL) • this temperature is ABSOLUTE ZERO temperature (°C)

15. Let’s Practice…

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 zero • The new temperature scale was named the Kelvin or absolute temperature scale • K = °C + 273.15

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 faster • Heavy molecules will move slower • All molecules at the same temp. have the same kinetic energy. • As temp. changes the velocity (speed) changes: • Increasing temp = increasing velocity • Decreasing temp = decreasing velocity • At 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 molecules • In 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 constant • R = 0.08206 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 law • PV = nRT • Two sets of conditions: general gas equation • P1V1 = R = P2V2 n1T1 n2T2

23. Examples • What 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 2.14 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 mmHg • 0°C or 273 K • Standard molar Volume (for any gas) • 1 mole = 22.4 L • At 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 L • Convert 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-1800s • KMT is based on the concept of an ideal or perfect gas

27. Ideal gas • Composed of tiny particles in constant, random, straight-line motion • Gas molecules are point masses, so gas volume is just the empty space between the molecules • Molecules collide with each other and with the walls of their container • The 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.