1. GASES Chapter 10

2. Example: Air 78% nitrogen 21% oxygen Molecules only take up about 0.1% of total volume (the rest is empty space)

3. PRESSURE Pressure is the force that acts on a given area. Atmospheric pressure Due to gravity the atmosphere exerts a downward force and hence a pressure on the earth’s surface. Atmospheric pressure can be measured using a barometer.

4. Units: SI unit – Pascal = Pa (N.m-2) Other units: 1 atm = 760 torr = 760 mm Hg = 101.325 kPa 1 bar = 100 kPa

5. Atmospheric pressure

6. Mercury Barometer

7. THE GAS LAWS Charle’s Law (T-V Relationship) Boyle’s Law (P-V Relationship) Avogadro’s Law (n-V Relationship)

8. BOYLE’S LAW (P-V RELATIONSHIP) (T and n constant) The volume of a fixed amount of gas maintained at constant temperature is inversely proportional to pressure.

9. The volume of a fixed amount of gas maintained at constant pressure is directly proportional to it absolute temperature. CHARLE’S LAW (T-V RELATIONSHIP) (P and n constant)

10. The volume of a gas maintained at constant temperature and pressure is directly proportional to the number of moles of gas. AVOGADRO’S LAW (n-V RELATIONSHIP) (T and P constant)

11. IDEAL GAS EQUATION (T and n constant) (P and n constant) Thusor

12. R is the gas constant. R = 0.08206 L.atm.mol -1.K -1 R = 8.314 m 3.Pa.mol -1.K -1 R = 8.314 J.mol -1.K -1

13. If 1 mole of ideal gas at 1 atm and 0 o C (273.15 K), then: V = 22.41 L STP for gases: 1 atm and 0 o C (273.15 K) V = (1 mol)(0.08206 L.atm.mol -1.K -1 )(273.15 K) (1 atm)

14. Rearrange: DENSITY OF GASES PV = nRT E.g.:  (CO2) >  (O2)  used in fire extinguishers

15. An explosion is essentially a reaction which generates large amounts of gaseous products (increase in entropy) and is also associated with a release in energy (exothermic). EXPLOSIONS A knowledge of the properties of gases is important because gases are often reactants or products in chemical reactions.  G =  H – T  S

16. We also know: Thus the large pressure due to the large volumes of gas formed is also responsible for the destructive power of an explosion.

18. Example A miniature volcano can be made in the lab with ammonium dichromate. When ignited it decomposes in a fiery display. (NH 4 ) 2 Cr 2 O 7 (s)  N 2 (g) + 4H 2 O(g) + Cr 2 O 3 (s) If 5.0 g of ammonium dichromate is used, and if the gases from this reaction are trapped in a 3.0 L flask at 23 o C, what is the total pressure of the gas in the flask? (Ignore the air in the flask) Challenge: Do not ignore the air in the flask.