1

Surface Tension

• Surface tension is a measure of the elastic force (strength) in the surface of a liquid.

• Also, surface tension is defined as the amount of energy required to stretch or increase the surface of a liquid by unit area.

• Therefore, as intermolecular forces are increased, surface tension also increases.

2

3

4

Surface tension and capillary

• When a liquid is placed in a capillary, a capillary action is observed where:

• The liquid may climb and rise inside the capillary above the liquid level. This occurs when the attractive forces between the liquid molecules and the glass surface (adhesion) is stronger than the intermolecular forces (cohesion). Water is an Example. The opposite is true for a liquid with very high intermolecular forces like mercury.

5

Cohesion and Adhesion

6

7

Viscosity

8

Viscosity is a measure of a fluids resistance to flow

9

Viscosity of some common liquids

LiquidViscosity (N s/m2)

Acetone3.16*10-4

Carbon tetrachloride9.69*10-4

Ethanol1.20*10-3

Glycerol1.49

Water1.01*10-3

10

Phase Changes

• A phase is a homogeneous part of a system in contact with other parts of the system, but separated from them by a well-defined boundary.

• Phase change is the transformation from one phase to another.

• Phase changes are physical processes

11

12

Liquid – vapor equilibrium

• Evaporation or vaporization:

At any given temperature, a certain number of molecules in a liquid possess sufficient kinetic energy to escape from the surface.

• When molecules evaporate they exert a vapor pressure. The vapor pressure can be measured using a U shaped manometer.

13

14

Condensation

• The opposite of evaporation.

• As the concentration of molecules in the vapor phase increases, some molecules return to the liquid phase (condense).

• Condensation takes place due to intermolecular forces since molecules are in contact with the liquid phase.

15

Dynamic Equilibrium

• The rate of vapor formation is the same as the rate of condensation.

• The vapor pressure measured at under dynamic equilibrium of condensation and evaporation is called “the equilibrium vapor pressure”. For simplicity we just say vapor pressure.

• The equilibrium vapor pressure is the maximum vapor pressure a liquid exerts at a given temperature.

16

Normal boiling point

• The vapor pressure increases with increasing temperature.

• For each liquid, there is a temperature at which a liquid starts to boil.

• The boiling point of a liquid is the temperature at which the vapor pressure of the liquid is equal to the external vapor pressure.

• The normal boiling point is reached when the external pressure is 1 atm.

17

18

Critical Temperature and Pressure• A gas can be made to liquefy by two techniques:• 1. Cooling a sample of the gas• 2. Apply a pressure to the gas• In both cases, gas molecules aggregate to form

liquid droplets.• A critical temperature (Tc) can be identified as

the temperature above which the gas can not be made to liquefy, regardless of the applied pressure.

• Tc is the highest temperature at which a substance can exist as a liquid.

19

Critical Pressure

• The minimum pressure that must be applied to bring about liquefaction at the critical temperature is called the critical pressure, Pc.

• Below Tc intermolecular forces are strong enough to hold molecules together.

• Above Tc molecular motion is so energetic that it is much stronger than intermolecular forces, therefore, no liquid formation.

20

Heat of vaporization

• The molar heat of vaporization is defined as the energy (usually in kJ) required to vaporize one mole of a liquid.

• Therefore, Hvap is directly proportional to strength of intermolecular forces.

• The vaporized molecules have a certain vapor pressure at the temperaure of interest.

21