Unit 6 Chapters 10, 5, & 11

1. Unit 6 Chapters 10, 5, & 11 Gases And Intro to Thermochemistry

2. Properties of Gases • Three phases of matter solid liquid gas Definite shape and volume Definite volume, shape of container Shape and volume of container

3. Gases A gas consists of small particles that • move rapidly in straight lines. • have essentially no attractive (or repulsive) forces. • are very far apart. • have very small volumes compared to the volume of the container they occupy. • have kinetic energies that increase with an increase in temperature.

4. Properties of Gases • A gas is a collection of molecules that are very far apart on average. In air, gas molecules occupy only 0.1% of the total volume. In liquids, molecules occupy ~ 70% of the total space. • Gases are highly compressible. Volume decreases when pressure is applied.

5. Properties of Gases • Chemical properties of gases vary depending on their composition. • Air: ~ 78% N2 and ~ 21% O2 • CO2: colorless, odorless • CO: colorless, odorless, highly toxic • CH4: colorless, odorless, flammable • NO2: toxic, red-brown, irritant • N2O: colorless, sweet odor (laughing gas) • Cl2: yellow/green, toxic

6. Properties That Describe a Gas Gases are described in terms of four properties: pressure (P), volume(V), temperature(T), and amount(n).

7. F A P = Pressure • Pressure is the amount of force applied to an area. • Atmospheric pressure is the weight of air per unit of area.

8. Altitude and Atmospheric Pressure Atmospheric pressure • is about 1 atmosphere at sea level. • depends on the altitude and the weather. • is lower at high altitudes where the density of air is less. • is higher on a rainy day than on a sunny day.

9. Barometer A barometer • measures the pressure exerted by the gases in the atmosphere. • indicates atmospheric pressure as the height in mm of the mercury column.

10. Pressure Many different units used to report pressure. • millimeters of Hg (mm Hg) • inches of Hg (in. Hg) • pounds per square inch (psi) • atmosphere (atm) • torr (torr) • pascal (Pa) = SI base unit • kilopascal (kPa) Must know units and abbreviations!!

11. Pressure Relationships between different pressure units: 1 atm = 760 mm Hg = 760 torr = 29.92 in. Hg = 14.7 psi = 1.01325 x 105 Pa = 101.325 kPa Must be able to interconvert between units. Memorize the ones in red You must know that 1 kPa = 1000 Pa

12. Learning Check • What is 475 mm Hg expressed in atm? 1) 475 atm 2) 0.625 atm 3) 3.61 x 105 atm 475 mm Hg x 1 atm = 0.625 atm 760 mm Hg

13. Learning Check B. The pressure in a tire is 2.00 atm. What is this pressure in mm Hg? 1) 2.00 mm Hg 2) 1520 mm Hg 3) 22 300 mm Hg 2.00 atm x 760 mm Hg = 1520 mm Hg 1 atm

14. Conversion factor: 1 atm = 760 torr More practice The measured pressure inside the eye of a hurricane was 669 torr. What was the pressure in atm? Given: 669 torr Find: P (atm) P (atm) = 669 torr x 1 atm = 0.880 atm 760 torr

15. Kinetic-Molecular Theory • The behaviorof gases can be described and explained using kinetic molecular theory. • the “theory of moving molecules” • You must know the basic ideas that are part of kinetic molecular theory.

16. Kinetic Molecular Theory • Gases consist of large numbers of molecules that are in continuous, random motion. • The combined volume of all the molecules of the gas is negligible compared to the total volume in which the gas is contained. • i.e. the molecules are very far apart on average

17. Kinetic Molecular Theory • Attractive and repulsive forces between gas molecules are negligible. • Energy can be transferred between molecules during collisions, but the average kinetic energy of the molecules does not change as long as the temperature remains constant. • Collisions are perfectly elastic.

18. Kinetic Molecular Theory • The average kinetic energy of the molecules is proportional to the absolute temperature. • At any given temperature all molecules of a gas have the same average kinetic energy. • As T (in K) increases, KE increases.

19. Gas Laws • Four variables are needed to define the physical condition or state of any gas: • Temperature (T) • Pressure (P) • Volume (V) • Amount of gas (moles: n) • Equations relating these variables are known as the Gas Laws

20. Boyle’s LawPressure and Volume Boyle’s Law states that • the pressure of a gas is inversely related to its volume when T and n are constant. • if volume decreases, the pressure increases.

21. PV Constant in Boyle’s Law In Boyle’s Law, the product P x V is constant as long as T and n do not change. Boyle’s Law can be stated as PV=constant P1V1 = P2V2 (T, n constant) P1V1 = 8.0 atm x 2.0 L = 16 atm L P2V2 = 4.0 atm x 4.0 L = 16 atm L P3V3 = 2.0 atm x 8.0 L = 16 atm L

22. Solving for a Gas Law Factor The equation for Boyle’s Law can be rearranged to solve for any factor. P1V1 = P2V2 Boyle’s Law Can find the change in volume if pressure changes: To solve for V2 , divide both sides by P2. P1V1 = P2V2 P2 P2 V1 x P1 = V2 P2

23. Learning Check For a cylinder containing helium gas indicate if cylinder A or cylinder B represents the new volume for the following changes (n and T are constant). 1) pressure decreases B (volume increases) 2) pressure increases A (volume decreases)

24. Learning Check If a sample of helium gas has a volume of 120 mL and a pressure of 850 mm Hg, what is the new volume if the pressure is changed to 425 mm Hg ? • 60 mL 2) 120 mL 3) 240 mL Known: P1 = 850 mm Hg P2 = 425 mm Hg V1 = 120 mL V2 = ?? P1V1=P2V2 V2 = V1 x P1 = 120 mL x 850 mm Hg = 240 mL P2 425 mm Hg Pressure ratio increases volume

25. Learning Check A sample of helium gas in a balloon has a volume of 6.4 L at a pressure of 0.70 atm. At 1.40 atm (T constant), is the new volume represented by A, B, or C?

26. Charles’ Law:Temperature and Volume In Charles’ Law, • the Kelvin temperature of a gas is directly related to the volume. • P and n are constant. • when the temperature of a gas increases, its volume increases.

27. Charles’ Law: V and T • Charles’ Law is written V T Or: V1V2 (P and n constant) T1 T2 • Rearranging Charles’ Law to solve for V2 T2 x V1 = V2 x T2 T1 T2 V2= V1x T2 T1 = constant =

28. Calculations Using Charles’ Law A balloon has a volume of 785 mL at 21°C. If the temperature drops to 0°C, what is the new volume of the balloon (P constant)? 1. Set up data table: Conditions 1Conditions 2 V1 = 785 mL V2 = ? T1 = 21°C = 294 K T2 = 0°C = 273 K Be sure to use the Kelvin (K) temperature in gas calculations.

29. Calculations Using Charles’ Law (continued) 2. Solve Charles’ law for V2: V1 = V2 T1 T2 V2 = V1 x T2 T1 V2 = 785 mL x 273 K = 729 mL 294 K

30. Learning Check Use the gas laws to complete sentence with 1) increases 2) decreases. A. Pressure _______, when V decreases. B. When T decreases, V _______. C. Pressure _______ when V changes from 12 L to 24 L D. Volume _______when T changes from 15 °C to 45°C increases decreases decreases increases

31. Think! Does the volume of a fixed quantity of gas decrease to half its original value when the temperature is lowered from 100C to 50C?

32. Avogadro’s hypothesis • Equal volumes of gases at the same temperature and pressure contain equal numbers of molecules.

33. Avogadro's Law: Volume and Quantity (Moles) In Avogadro’s Law • the volume of a gas is directly related to the number of moles (n) of gas. • T and P are constant.

34. Avogadro's Law: Volume and Quantity (Moles) V = constant n or V1 V2 = n1 n2

35. Learning Check If 0.75 mole helium gas occupies a volume of 1.5 L, what volume will 1.2 moles helium occupy at the same temperature and pressure? 1) 0.94 L 2) 1.8 L 3) 2.4 L

36. Solution STEP 1 Conditions 1 Conditions 2 V1 = 1.5 L V2 = ??? n1 = 0.75 mol He n2 = 1.2 moles He STEP 2 Solve for unknown V2 V2 = V1 x n2 n1 STEP 3 Substitute values and solve for V2. V2 = 1.5 L x 1.2 moles He = 2.4 L 0.75 mol He