Gases & Atmospheric Chemistry

1. Gases & Atmospheric Chemistry Unit 5

2. States of Matter

3. Solid, Liquid & Gas

4. Forces Holding Solids Together • The forces that are holding a solid together are very strong • Forces: • Ionic • Covalent • Some intermolecular forces in some substances

5. Forces Holding Solids Together Explains why solids: • Have a definite shape • Strong bonds holding molecules together (rigid) • Do not flow readily • In order to be able to flow particles have to slip past one another, strong bonds do not allow this • Cannot be compressed • Strong bonds mean that there are few empty spaces between the particles

6. Forces Holding Liquids Together • The forces that are holding a liquid together are not as strong as ionic or covalent bonds • Forces: intermolecular • Bonds hold molecules closely together but do not lock them into place • Liquids can spread out and take the shape of the container while keeping a constant volume

7. Gases – Lack of Forces • Because gases have NO definite shape or volume there appears to be an absence of forces between the molecules in a gas • No limit to the diffusion of gas molecules into the atmosphere (a very large container)

8. The Kinetic Molecular Theory • Kinetic Molecular Theory = the idea that all substances contain particles that are in constant, random motion • Particles are continually moving & colliding Explains: • Diffusion • Evaporation

9. Diffusion • Example: food colouring is added to water will slowly spread out • Explanation from Kinetic Molecular Theory: molecules of food colouring and molecules of water are moving and colliding with each other which causes them to mix

10. Evaporation • Example: water in an open container slowly decreases as some of the water evaporates • Explanation from Kinetic Molecular Theory : some water molecules in the open container obtain sufficient energy from collision to escape from the liquid

11. 3 Types of Motion • A particle an exhibit 3 type of motion: • Vibrational = back-and-forth motion of atoms within a molecule • Rotational = spinning • Translational = straight line

12. 3 Types of Motion

13. Motion in Relation to State • Solid – mainly vibration due to restriction of the strong bonds • Particles stay together in a relatively ordered state • Liquid – some of all 3 types of motion • Less orderly state than solid • Gas – rotate and vibrate but translational (straight-line) motion is the most significant • Most disordered state with no organization

15. Properties of Gases • Gases are compressible: When pressure is increased, the volume of a gas decreases. When pressure is decreased, the volume of a gas increases. The volume of a liquid and a solid remain constant during changes in temperature because their particles cannot move independently of one another like the gas particles can. • Gases expand as the temperature increases (much more than water and solid). • Gases have very low viscosity (they flow fast). • Gases have much lower densities than solids or liquids. • ALL Gases are miscible (some liquids are miscible yet some are immiscible).

16. Earth’s Leaky Atmosphere? • Many of the gases that make up Earth’s atmosphere and those of the other planets are slowly leaking into space. • Hot gases, especially light ones, evaporate away • chemical reactions and particle collisions eject atoms and molecules • and asteroids and comets occasionally blast out chunks of atmosphere

17. The Atmosphere

18. Measurement of Gas Pressure • Pressure = force exerted on an object per unit of surface area • Unit = Pa (pascal) • Atmospheric Pressure = the force per unit area exerted by air on all objects • Standard Atmospheric Pressure: • 101.3 kPa or 760 mm Hg or 760 torr • one standard atmosphere (1atm)

19. Units of Pressure Conversion: 1 atm = 760 mm of Hg = 101.325 kPa = 1.01325 bar = 760 torr = 14.7 psi

20. Pressure Conversions A. What is 475 mm Hg expressed in atm? 760 mm Hg = 1 atm 475 mm Hg = x x = 475/760 = 0.625 atm B. The pressure of a tire is measured as 10 kPa. What is this pressure in mm Hg?

21. Pressure Conversions A. What is 2 atm expressed in torr?

22. STP & SATP • STP = Standard Temperature & Pressure • Exactly 0°C (273K) • 1atm or101.325kPa • SATP = Standard Ambient Temperature and Pressure • exactly 25°C (298K) • 100kPa

23. Gases Moving • Gases naturally move from areas of high pressure to low pressure, because there is empty space to move into • Examples of Spray Cans: whipped cream, hair spray, paint • a propellant forces the product out

24. Gas Law – Boyle’s LawRelationship: Pressure & Volume • As pressure on a gas increases, the volume of the gas decreases

25. Pressure and Volume Relationship • As pressure increases volume decreases

26. Gas Law – Boyle’s LawRelationship: Pressure & Volume • Boyle’s Law = as the pressure on a gas increase, the volume of the gas decreases proportionally p1v1 = p2v2 • Provided that the temperature and amount of gas are constant • The volume and pressure of a gas are inversely proportional Robert Boyle (1627-1691). Son of Early of Cork, Ireland. Graphically: P 1 (inverse relationship) V

28. Boyle’s Law Sample Problems:  • A 350 mL sample of air at 125 kPa is reduced to a volume of 250 mL. Calculate the new pressure. (ans: 180 kPa) • A 55 mL sample of helium at 525 torr is compressed to a certain volume at 2.5 atm. What is the new volume, in litres? (ans: 0.015 L)