1. Vapour Pressure

2. C11-1-12 MOLECULAR PROPERTIES
OUTCOME QUESTION(S):
C MOLECULAR PROPERTIES
Define normal boiling point temperature in terms of vapour pressure.
Include: volatility, dynamic equilibrium
Interpolate and extrapolate information from pressure versus temperature graphs and heating/cooling curves.
Explain colligative properties like freezing-point depression and boiling-point elevation at the molecular level.
Vocabulary & Concepts
Closed system  

3. Standard atmospheric pressure (at sea level):
Gas Pressure:
Gas particles are in constant motion and exert a force through collisions.
Air particles exert atmospheric pressure on everything
Let’s review…
Standard atmospheric pressure (at sea level):
1 atmosphere = 760 mmHg = kPa
Our bodies are built to withstand the force of air pressure pushing on us and so we don’t notice it…

4. Why is a closed container necessary to measure vapour pressure?
Vapor Pressure:
(l) (g)
Endothermic - particles on the surface that have enough energy to overcome IMFs
A measure of the natural evaporation rate of a substance
Why is a closed container necessary to measure vapour pressure?
Vapour Pressure:
The measured pressure of vapour at equilibrium above the liquid in a closed system.

5. “Dynamic equilibrium”
Vapour Pressure and Equilibrium:
Begin: molecules vapourize filling the space.
As more molecules evaporate, it becomes more likely for molecules to condense.
End: evaporation rate = condensation rate.
Amounts of each state remain constant
VAPOUR
“Dynamic equilibrium”
changing balance
Microscopically molecules are in flux but macroscopically quantities are constant.
LIQUID

6. Vapour pressure is a characteristic property
Vapour pressure can be measured by a manometer:
Compares vapour pressure to normal atmospheric pressure
Measures difference in height on both sides
Pvap = Patm + h (vapour pressure > air pressure)
Pvap = Patm – h (vapour pressure < air pressure)
Vapour pressure is a characteristic property

7. Vapour pressure is influenced by:
Vapour Pressure Equilibrium will only be reached with:
Sealed container
Constant condition ([ ], T, P, V)
closed system
Vapour pressure is influenced by:
Intermolecular forces
- stronger attraction restricts phase change
Temperature
- energy is required to phase change
Pressure
- pressure forces particles into liquid state
Vapour Pressure:
Measured pressure of vapour at equilibrium in a closed system.
Consider the equilibrium of a can of Coke – before you open it…

8. Vapour pressure is influenced by:
Intermolecular forces
Follow the logic arrows…
Weaker intermolecular forces
 less energy needed to break IMF
 easier for all particles to phase change (regardless of temperature)…
 increasing rate of vapourization
 increasing vapour pressure
The opposite for stronger IMF…
Compounds that easily vapourize (have low IMF)
are volatile (volatility)
low vapour pressure
high vapour pressure

9. Remember: high energy particles go first
Vapour pressure is influenced by:
Temperature
Increasing temperature
 increases the kinetic energy of all particles
 increasing the number of particles with the energy to break IMF…
 increasing rate of vapourization
 increasing vapour pressure
Of course the opposite happens with cooling…
Remember: high energy particles go first
low vapour pressure
high vapour pressure

10. Vapour pressure is influenced by:
Increasing pressure
 added force restricting particle motion
 harder for all particles to phase change (regardless of temperature)...
 decreasing rate of vapourization
 decreasing vapour pressure
An open system must contend with atmospheric pressure - air molecules colliding and pushing on the surface of the liquid making vaporization more difficult

11. Boiling: Boiling Point: (l) (g)
Endothermic – rapid vaporization of a liquid
Energized particles near the heat source spread out, forming bubbles that rise and enter the atmosphere
Increased vaporization at the surface pushes against the air particles reducing the pressure on the liquid - making it easier for vapour to form inside the liquid state - bubbles
low vapour pressure
high vapour pressure
Boiling Point:
Point (temperature) where the vapour pressure produced equals air pressure.
boiling

12. The opposite for stronger IMF…
Boiling Point and IMF:
Weaker intermolecular forces
 less energy needed to break free
 increased vaporization pushes against atmospheric pressure…
 easier for internal vapour to form
 lower boiling point (temperature)
The opposite for stronger IMF…
strong IMF
weak IMF
80oC
80oC - boiling

13. Where do we see changes in atmospheric pressure?
Boiling Point and Atmospheric Pressure:
Any change in air pressure will produce a change in the boiling point
Less atmospheric pressure
 less force pushing on the surface
 easier for surface vapour to equalize the air pressure…
 less energy needed for internal vaporization
 Lower boiling point
high air pressure
low air pressure
Where do we see changes in atmospheric pressure?
80oC
80oC - boiling

14. Atmospheric pressure decreases with altitude:
Less pressure, less energy needed to vaporize
“Boils” (vapour bubbles form) at lower temperature
Boiling Point of Water
Outer Space ? ft
90oC
7500 ft
92oC
5000 ft
95oC
2000 ft
98oC
Sea level
100oC
Food takes longer to cook at high altitudes – water isn’t as hot when it “boils”
Since there is NO atmosphere in space the lack of pressure massively reduces the boiling point of liquids - your gruesome fate: your blood, your eyeballs will boil furiously…

15. If air pressure is reduced boiling point is reduced…
Vapour Pressure Graphs and Boiling Point:
Graph of vapour pressure vs. temperature
Standard air pressure is kPa
Substance boils at point where vapour pressure equals atmospheric pressure
If air pressure is reduced boiling point is reduced…
Boiling Point

16. Pick any temperature to compare the compounds
From the vapour pressure graph below:
Which compound has the highest vapour pressure?
Which compound might have the strongest IMF?
What is the boiling point for ethanol at an air pressure of 85 kPa?
Pick any temperature to compare the compounds
a. chloroform
b. acetic acid
c. 73oC

17. Boil – when vapour pressure
A heating curve test of Acetic Acid was done and the results are graphed below:
Using the vapour pressure graph information as well, at what pressure must the heating test have been conducted?
The test was done at 90 kPa
Boil – when vapour pressure
equals air pressure

18. C11-1-12 MOLECULAR PROPERTIES
CAN YOU / HAVE YOU?
C MOLECULAR PROPERTIES
Define normal boiling point temperature in terms of vapour pressure.
Include: volatility, dynamic equilibrium
Interpolate and extrapolate information from pressure versus temperature graphs and heating/cooling curves.
Explain colligative properties like freezing-point depression and boiling-point elevation at the molecular level.
Vocabulary & Concepts
Closed system