1. Chemistry/Physical Setting
The Nature of Liquids
Br. Jabreal

2. Aim: What is the nature of liquids?
Do Now: Describe the arrangement of particles in a liquid, and draw a phase diagram of a liquid made up of molecules of the following substance:
Definite volume and take the
shape of their container.

3. A Model for Liquids
According to kinetic theory, particles of liquids also have kinetic energy
Allows liquid particles to flow past each other and allows liquids to conform to shape of their containers
However, unlike gases, liquid molecules DO have attractive forces between each other

4. Evaporation Evaporation: Process by which a liquid changes to a gas.
Would this be a physical or chemical change?

5. Evaporation
Evaporation tends to take place at surface of liquids at ALL temperatures.
During evaporation, only the molecules with a certain minimum kinetic energy can escape surface of the liquid
Liquids evaporate faster when heated because …
They are being supplied with more kinetic energy, allowing more particles to overcome the attractive forces keeping them in liquid state

6. Vapor Pressure
Vapor Pressure: Measure of force exerted by a gas above a liquid in a closed system.
Constantly, some particles are condensing and others are evaporating, but the force of the gas (vapor) particles colliding with the liquid’s surface is the vapor pressure
Vapor pressure increases as temperature increases
The increased supply of kinetic energy allows more particles to escape as gases, and more gas exerts more force

7. Equilibrium exists between the vapor (gas) and liquid
Rate of evaporation eventually equals rate of condensation, although no net change in number of particles of liquid or gas
Evaporation
Liquid
Vapor (Gas)
Condensation

8. At equilibrium, amount of vapor and amount of liquid NOT necessarily equal, although their rates of
conversion
are.

9. Boiling Point
When a liquid is heated to the point that particles throughout the entire sample have enough energy to escape as gas particles, the liquid is considered to be boiling.
Boiling point: Temperature at which the vapor pressure of the liquid equals external pressure on the liquid

10. Boiling Point As pressure changes, so does the boiling point.
Explain how/why based on the definition of the boiling point.
At higher altitudes, what do you think the effect would be on the boiling point? How does this affect cooking?
Normal boiling point refers to boiling point at standard pressure (1 atm or kPa).

11. Table H Vapor pressure of Four Liquids

12. Table H: Vapor Pressure and Temperature
Table H can be used to determine the boiling point of liquids at different pressures.
Point where dotted line intersects curve is normal boiling point
What’s the normal boiling point of ethanol?
What’s the boiling point of water when the pressure is 70 kPa?
What’s the boiling point of ethanoic acid at 85 kPa?

13. Practice
At which temperature is the vapor pressure of ethanol equal to the vapor pressure of propanone at 35 oC? 35 60 82 95

14. Homework Section 13.2 Assessment Page 409 Questions 9, 10, 12 and 13
Question 75 (a-c)