13.2 The Nature of Liquids

Describe the particles in a liquid

13.2 The Nature of Liquids

• Liquid particles are attracted to each other

• The particles in a gas are not

Intermolecular Forces• Dipole-Dipole

• Dispersion

• Hydrogen Bonds

Dipole – Dipole Forces

London Dispersion Forces

Hydrogen Bonding

• Particles of a liquid spin and vibrate while they move, adding to their average kinetic energy

• Most of the particles do not have enough energy to escape into the gaseous state; must overcome their intermolecular attractions

• The intermolecular attractions reduce the amount of space between particles of a liquid

• Liquids are more dense than gases

• Increasing pressure on liquid has hardly any effect on it’s volume

• Vaporization -the conversion of a liquid to a gas or vapor

• Evaporation -when this occurs at the surface of a liquid that is not boiling

• Some of the particles break away and enter the gas or vapor state; but only those with certain minimum kinetic energy escape from the surface of the liquid

• Why does a liquid evaporate faster when heated?

• Explain how evaporation is a cooling process

• What about the particles left behind?

Evaporation of a liquid in a closed container

• Fig. 13.6b on page 391 shows that no particles can escape into the outside air

• When some particles do vaporize, these collide with the walls of the container producing vapor pressure

A measure of the force exerted by a gas above a liquid is called the vapor pressure.

• Dynamic equilibrium • Rate of evaporation = rate of condensation

• An increase in temperature of a contained liquid increases the vapor pressure

• the particles have an increased kinetic energy, and more minimum energy to escape

Boiling Point (bp) • The temperature at which the

vapor pressure of the liquid is just equal to the external pressure on the liquid

• Since the boiling point is where the vapor pressure equals external pressure, the bp changes if the external pressure changes

• Normal boiling point- the bp of a liquid at a pressure of 101.3 kPa (or standard pressure)

• Water boils at 100° C• in Denver = ?• Denver is 1600 m above sea

level and average atmospheric pressure is about 85.3 kPa