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HL Chemistry

HL1-8.ppt Gas Laws. HL Chemistry. Review – Avogadro’s Law. Avogadro’s Law: Equal volumes of gases at the same T & P contain the same number of molecules . (which means that coefficients in a balanced equation can be ‘read’ as volumes of gases). Review – Molar Volume.

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HL Chemistry

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  1. HL1-8.ppt Gas Laws HL Chemistry

  2. Review – Avogadro’s Law Avogadro’s Law: Equal volumes of gases at the same T & P contain the same number of molecules. (which means that coefficients in a balanced equation can be ‘read’ as volumes of gases)

  3. Review – Molar Volume 1 mole of any gas takes up 22.4 dm3 of spaceat STP Standard Temperature (273K or 0oC) & Pressure (101 kPa or 1 atm) VSTP = 22.4 dm3 / mol Which can be rearranged… n = VSTP / 22.4 dm3 mol

  4. Review – Molar Volume 1 mole of any gas takes up 22.4 dm3 of spaceat STP Standard Temperature (273K or 0oC) & Pressure (101 kPa or 1 atm) VSTP = 22.4 dm3 / mol Which can be rearranged… n = VSTP / 22.4 dm3 mol

  5. Other applications of Avagadro’s law &/or Molar Volume: • Determine number of moles (or g) of a given volume of gas at STP. (1.4.2 # 3, part of 5, part of 7) • Determine volume of a known quantity (mol or g) of a gas at STP (1.4.2 # 4, part of # 10) • Calculate molar mass given information about moles (or g) and volume… and knowing molar volume (1.4.2 # 8) • Determine density of a gas at STP (1.4.2 # 9)

  6. The Ideal Gas Law • Q – Can all gases be considered ‘ideal’? • Most real gases act ideally under normal conditions. • They tend to act less ideally when: • They have high molar masses Why? • They are at very high pressures Why? • They are at very low temperatures Why?

  7. Which of these gases would act the least ideally?

  8. The Ideal Gas Law • PV=nRT where • P = pressure in kPa • V = volume in dm3 (Litres) • n = moles of gas • R = universal gas constant • (8.314 kPa dm3 mol-1 K-1) • (0.0821 if you’re using atm, which you won’t in IB…) • T = temperature in K

  9. Pressure - You want pressure in kPa… • What do you do if it is given in mm Hg? • What do you do if it is given in atm?

  10. The Ideal Gas Law • As long as you can remember PV=nRT, you can derive all of the other gas laws. • If the number of moles of gas is not changing, we can just solve for n. • n=PV/RT • Since the moles aren’t changing, we can set the right side of the equation equal to itself. • P1V1/RT1= P2V2/RT2 and because R is the same… • P1V1/T1= P2V2/T2 (combined gas law)

  11. If you have a problem where one of these variables is held constant, you can simplify the combined gas law by eliminating that variable: P1 V1 / T1 = P2 V2 / T2 P1 V1 = P2 V2 V1 / T1 = V2 / T2 P1 / T1 = P2 / T2

  12. We now have this collection of gas laws: • P1V1/T1=P2V2/T2 (Combined Gas Law) • P1V1=P2V2 (Boyle’s Law) • V1/T1=V2/T2 (Charles’ Law) • P1/T1=P2/T2 (Gay Lussac’sLaw)

  13. Gas Laws TIP #1: Always use K for temperatures because it is an “absolute” scale (no negatives)TGas Laws TIP #2: If P or V are on both sides, the units don’t matter as long as they are the same on both sides.Gas Laws TIP #3: You must be VERY careful with your units when using the Ideal Gas Law!

  14. Your Assignment: • Read pp. 21-26 • Do Ex 1.4.3 • on pp. 25-26 • # 1-10 • Identify which gas law you are using to solve the problem… then solve the problem . Show your work…. (at minimum include the set up using some form of a gas law)

  15. Charles Law • Charles's law is an experimental gas law which describes how gases tend to expand when heated. It was first published by Frenchnatural philosopherJoseph Louis Gay-Lussac in 1802, although he credits the discovery to unpublished work from the 1780s by Jacques Charles.

  16. Charles Law • T  V • The following is a more useful way of expressing this law.

  17. Create a hypothesis for what happened here.

  18. Here’s what really happened. • Workers used hot pressurized water to clean the tanker car. • Tank was drained. • Door was closed and tanker was left overnight. • Temperature inside the car plummeted overnight. • V/T=V/T (Temp goes down, Volume must go down also)

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