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Unit 7 “The Behavior of Gases”

Unit 7 “The Behavior of Gases”. Chemistry CDO High School. Variables that describe a Gas. The four variables and their common units: 1. pressure (P) in kilopascals 2. volume (V) in Liters 3. temperature (T) in Kelvin 4. amount (n) in moles. 1. Pressure of Gas.

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Unit 7 “The Behavior of Gases”

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  1. Unit 7“The Behavior of Gases” Chemistry CDO High School

  2. Variables that describe a Gas • The four variables and their common units: 1. pressure (P) in kilopascals 2. volume (V) in Liters 3. temperature (T) in Kelvin 4. amount (n) in moles

  3. 1. Pressure of Gas • a measure of the force exerted by the gas on the walls of a container • The greater the number of collisions between gas particles and the wall the greater the pressure

  4. Pressure Conversions • 1 atm = 101.3 kPa = 760 mmHg = 760 torr • The pressure in Tucson 668 mmHg, what is that pressure in: • atm • kPa • torr

  5. 2. Amount of Gas • Increasing the number of gas particles increases the number of collisions • thus, the pressure increases

  6. Pressure and the number of molecules are directly related • Gases naturally move from areas of high pressure to low pressure, because there is empty space to move into

  7. 3. Volume of Gas • As volume decreases, pressure increases. • Thus, volume and pressure are inversely related to each other

  8. 4. Temperature of Gas • Raising the temperature of a gas increases the pressure, if the volume is held constant. (Temp. and Pres. are directly related)

  9. The Gas Laws

  10. #1. Boyle’s Law Gas pressure is inversely proportional to the volume, when temperature is held constant. • Pressure x Volume = a constant • Equation: P1V1 = P2V2 (T = constant) • P1 = initial pressure • V1 = initial volume • P2 = final pressure • V2 = final volume

  11. #2. Charles’s Law • The volume of a fixed mass of gas is directly proportional to the Kelvin temperature, when pressure is held constant.

  12. Converting Celsius to Kelvin • Gas law problems involving temperature will always require that the temperature be in Kelvin. Kelvin = C + 273 and °C = Kelvin - 273

  13. #3. Gay-Lussac’s Law • The pressure and Kelvin temperature of a gas are directly proportional, provided that the volume remains constant. V is constant

  14. #4. Avogadro’s Law • States that under equal conditions of temperature and pressure, equal volumes of gases contain an equal number of molecules. P, T are constant n = moles of gas

  15. #5. The Combined Gas Law The combined gas law expresses the relationship between pressure, volume and temperature of a fixed amount of gas.

  16. The combined gas law contains all the other gas laws! • If the temperature remains constant... P1 V1 P2 x V2 x = T1 T2 Boyle’s Law

  17. The combined gas law contains all the other gas laws! • If the pressure remains constant... P1 V1 P2 x V2 x = T1 T2 Charles’sLaw

  18. The combined gas law contains all the other gas laws! • If the volume remains constant... P1 V1 P2 x V2 x = T1 T2 Gay-Lussac’s Law

  19. #6. The Ideal Gas Law #1 • Equation: PV = nRT • Ideal Gas Constant (R) • R = 8.314 (L kPa) / (molK) • The other units must match the value of the constant, in order to cancel out.

  20. #7. Ideal Gas Law 2 • PVmm = gRT • g = mass, in grams • mm = molar mass, in g/mol

  21. Ideal Gas Equation #3 • Density is mass divided by volume Pmm = dRT d = density

  22. #8 Dalton’s Law of Partial Pressures For a mixture of gases in a container, PTotal = P1 + P2 + P3 + . . . • P1 represents the “partial pressure”, or the contribution by that gas. • Dalton’s Law is particularly useful in calculating the pressure of gases collected over water.

  23. Connected to gas generator Collecting a gas over water – one of the experiments in this unit involves this.

  24. If the first three containers are all put into the fourth, we can find the pressure in that container by adding up the pressure in the first 3: 2 atm + 1 atm = 6 atm + 3 atm 4 3 2 1 Sample Problem 14.6, page 434

  25. Real Gases and Ideal Gases

  26. Ideal Gases don’t exist, because: • Molecules do take up space • There are attractive forces between particles - otherwise there would be no liquids formed

  27. Real Gases behave like Ideal Gases... • When the molecules are far apart. • The molecules do not take up as big a percentage of the space • We can ignore the particle volume. • This is at low pressure

  28. Real Gases behave like Ideal Gases… • When molecules are moving fast • This is at high temperature • Collisions are harder and faster. • Molecules are not next to each other very long. • Attractive forces can’t play a role.

  29. Diffusion is: • Molecules moving from areas of high concentration to low concentration. • Example: perfume molecules spreading across the room. • Effusion: Gas escaping through a tiny hole in a container. • Both of these depend on the molar mass of the particle, which determines the speed.

  30. Diffusion:describes the mixing of gases. The rate of diffusion is the rate of gas mixing. • Molecules move from areas of high concentration to low concentration. • Fig. 14.18, p. 435

  31. Effusion: a gas escapes through a tiny hole in its container -Think of a nail in your car tire… Diffusion and effusion are explained by the next gas law: Graham’s

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