1. Matter & Energy – Gases Aim: What is the relationship between vapor pressure & temperature?
Do Now:
Under which conditions will a real gas behave most like an ideal gas?
Low temperature & high pressure
Low temperature & low pressure
High temperature & high pressure
High temperature & low pressure
HW: graphing wkst. Mini-test on gas laws Fri.

2. Gases are subjected to various changing conditions…
Pressure (P)  kPa or atm
Volume (V)  L or mL
Temperature (T)  ALWAYS IN K WHEN DEALING WITH GASES!

3. Vapor Pressure
pressure exerted by gas particles of a substance in a closed system at constant T & V
Particles exist in L & G phase (equilibrium)

4. Vapor Pressure, cont.
If T is increased, what happens to the KE of the particles of the substance?
Move faster
When they move faster, what happens to the frequency of their collisions?
Collide more frequently
What happens to the forces holding the mlcls to one another in the liquid phase?
Knock more mlcls into vapor phase/break the forces
More vapor mlcls = higher vapor pressure
See Table H

5. Reference Table H
Shows relationship between T & vapor pressure of 4 liquids
As T ↑, vapor pressure ↑
Steeper curve means it vaporizes quickly (at a lower T)
This is because of weak intermolecular forces (forces that hold mlcls together)  Red Rover, Red Rover…

6. Table H
Dotted line – kPa – standard pressure (from STP Table A!)
This is STANDARD ATMOSPHERIC PRESSURE
When the curve crosses that line, the corresponding TEMP is the substance’s BOILING POINT
Note H2O as example

7. Example
Think about it - would you put a sealed container in the microwave?
No! As the substance heats up and mlcls move faster, more and more of them will escape into the vapor phase, increasing the pressure these mlcls are exerting on the walls of the container.

8. Check Understanding of Table H
What is the vapor pressure in kPa of water at 100oC?
What is the vapor pressure in kPa of ethanoic acid at 120oC?
What is the vapor pressure in kPa of propanone at 75oC?
Compare the vapor pressure of the four liquids at 70oC.

9. More Sample Questions
When the external pressure is kPa, water will boil at what temperature? 12.8°C, 14.5°C, 100°C, or 18°C

10. Interesting to note…
If the external pressure on the substance is lower (e.g. at higher altitudes), mlcls are able to vaporize more easily, so the vapor pressure increases more rapidly (there are more mlcls in the vapor phase).
…so, for example, water boils (vapor pressure reaches atmospheric pressure) faster at higher altitudes.