Ch. 5 Gases

1. Ch. 5Gases • Gases are defined by: • The particles are free to move (fluid). • The particles are far apart. • The particles move very fast. • The particles have no attraction nor repulsion for each other. • The fact that they have no definite shape or volume – they take the shape of the container. • The fact that they are compressible.

2. Variables • Gases have four measurable variables. • Pressure (P) of a gas. • Measured in a variety of units like mmHg, atm, Pascals, torr, bars, and psi. • Volume (V) of a gas, • Measured in mL or L. • Temperature (T) of a gas. • Measured in Celsius or Kelvin. • For all formulas, Kelvin temperatures must be used. • Quantity of a gas. • Measured in grams or moles (n). • Formulas will require moles.

3. Pressure • Pressure is the force per unit area. • As the molecules of a gas collide with the walls, they exert a force.

5. Atmospheric Pressure • The column of air above us exerts a force due to the gravitational force. • Atmospheric pressure has two main variables. What are they?

6. Atmospheric Pressure • A barometer is used to measure pressure. • A dish is filled with liquid mercury. • A glass tube that has been evacuated of all air molecules is then placed into the pool of mercury.

7. Atmospheric Pressure • The height of the mercury in the tube over the pool of mercury is measured with a ruler. • In weather, it is reported as inches of mercury – inHg. • We will measure ours in millimeters of mercury – mmHg. • On an average day, at sea level – the column would measure 760mmHg = 1 atmosphere (atm).

8. Atmospheric Pressure • As altitude increases, the pressure decreases. • Denver, CO • Death Valley, CA

9. Empirical Gas Laws • Look at the relationship between two of the four variables. • Boyle’s, Charles’, and Gay-Lussac’s Laws. • Direct (linear) relationship – as one variable increase, so does the other. • Inverse relationship – as one variable increase, the other decreases.

10. Boyle’s Law • Allows P and V to vary, while n and T are constant. • Boyle found that as P , the V  and vice versa. • What type of relationship – Direct or Inverse? • P x V = constant • Formula: P1V1 = P2V2 • LEP #1

11. Graph

12. Charles’ Law • Allows V and T to vary, while n and P are constant. • Charles’ found that as V , the T . • What type of relationship – Direct or Inverse? • Formula: • LEP #2

13. Graph

14. Gay-Lussac’s Law • Allows P and T to vary, while n and V are constant. • Gay-Lussac found that as P , the T . • Direct or Inverse? • Formula? • LEP #3

15. Combined Gas Law • All three empirical gas laws can be merged into a single formula. • This means that two of the three variables can be changed. • LEP #4

16. Avogadro’s Relationship • The quantity of a gas (moles) and volume are proportional. • STP = standard temperature (0oC) and pressure (1 atm). • At STP, one mole of any gas will occupy 22.4L. • LEP #5

17. Ideal Gas Law • The relationship between ALL four variables is found in the ideal gas law. • Formula: PV = nRT. • R is called the gas law constant and equals 0.08206 L atm / mol K. • Because of the units, each of the variables must be in the same units! • LEP #6

18. Ideal Gas Law • Other derived versions of the ideal gas law can be useful. • Since moles = grams (g) / molar mass (MW),

19. Ideal Gas Law • Density is grams / volume, so we can include this in our formula as well. • LEP #7 • LEP #8

20. Stoichiometry and Gases • Many reactions generate a gas as a product. • Grams of solid or liquid must be converted to moles of gas. • Ex) 2 KClO3(s) 2 KCl(s) + 3 O2(g) • In this example, when 2 moles of KClO3 decompose, 3 moles of O2 gas are generated. • LEP #8

21. Explosives • Explosives like dynamite, TNT, nitroglycerin, etc. are compounds that produce mostly gases when allowed to react. • Unlike hydrocarbons, most explosives _________ rather than burn. • A ____________ is a very rapid chemical reaction using oxygen that is contained in the material rather than in the air.

22. Explosives

23. TNT 2 C7H5N3O6 → 3 N2 + 5 H2O + 7 CO + 7 C

24. ANFO • Ammonium nitrate / fuel oil is commonly used in mining. • It was also used in the bombing of the Murrah building in Oklahoma City. • LEP #10

25. Dalton’s Law • Many gases we encounter are mixtures of two or more substances. • The total pressure exerted by a mixture is equal to the partial pressures of each of the gases. • Ptotal = Pa + Pb + Pc + … • The partial pressure of a gas is proportional to its mole fraction. • Pa = Ptotalca. • LEP #11, #12

26. Kinetic Molecular Theory • In the study of gases, five postulates describing them were published by Rudolf Clausius in 1857. • Gases consist of large numbers of molecules that are in continuous, random motion. • Pressure is uniform on any surface.

27. Kinetic Molecular Theory • The volume occupied by the molecules is negligible when compared to the volume of the container. • A 5.0L container is assumed to be the volume even though the gas molecules must occupy some of that space. • Attractive and repulsive forces between gas molecules are negligible.

28. Kinetic Molecular Theory • Collisions of gas molecules are elastic. Energy can be transferred between molecules, but cannot be lost. • The average kinetic energy of a gas is proportional to the temperature of the gas.

29. KE of a Gas

30. Root-mean-square Velocity • This is an approximation of the average speed of a gas molecule. • LEP #13

31. Real Gases • All gases deviate from “ideal” behavior under different conditions. • At higher pressures, gas molecules do occupy a portion of the volume.

32. Real Gases • At low temperatures, gas molecules can have attractive forces for each other. • Both of these factors can be corrected for in the van der Waals equation.

33. Real Gases