Matter & Energy – GasesAim: What is the relationship between vapor pressure & temperature?

Do Now:Under which conditions will a real gas behave

most like an ideal gas?1. Low temperature & high pressure2. Low temperature & low pressure3. High temperature & high pressure4. High temperature & low pressureHW: graphing wkst. Mini-test on gas laws Fri.

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!

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)

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

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…

Table H Dotted line – 101.3

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

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.

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.

More Sample Questions

When the external pressure is 101.3 kPa, water will boil at what temperature?12.8°C, 14.5°C, 100°C, or 18°C

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.