Liquids and Solids

Ch 11

Comparison of Liquids and Solids to Gases

Liquids & solids are much more dense than gases Inorganic liquids and solids have

densities ranging from 1 – 8 g/cm3, some up to 20 g/cm3

Most organic liquids & solids have densities ranging from 0.7 – 2.0 g/cm3

Gas densities are usually between 10-2 and 10-4 g/c,3

Comparison of Liquids and Solids to Gases

Gases expand to fill all available space & must be kept in enclosed containers

Liquids fills any container from the bottom up to a level dictated on by the mass of the liquid present

Liquids conform to the shape of their container

Solids maintain shape without a container

Comparison of Liquids and Solids to Gases

Gases lack significant attractive forces

Liquids and Solids- significant attractive forces!

Intermolecular Forces

3 Types to be aware of:

Dipole-Dipole Forces

London Dispersion Forces

Hydrogen Bonding

Dipole-Dipole Forces

Molecular compounds share electrons in a covalent bond, usually not equally! e- congregate at 1 end of the

molecule, giving it polarity, creating a dipole.

Polar molecules are attracted to each other.

Attractive forces are represented by the equation: Force = (δ+)(δ-)

r2

Dipole-Dipole Forces

For gases to become a liquid – the attractive forces must overcome the KE of the moving gas molecule. Decreasing the distance b/w

molecules increases the attractive force.

Increasing P on a gas forces the molecules closer together

Cooling a gas reduces its avg KE

Force = (δ+)(δ-)

r2

Dipole-Dipole Forces

Boiling Pt (Condensation Pt) – indicator of the attractive forces b/w molecules Measure of how much KE has to be

increased so it overcomes the attractive forces in the liquid.

Low BP – low attractive forces High BP – higher attractive forces

Highly polar molecules have higher BPs.

London Forces of Attraction

Explain how nonpolar gases develop the forces necessary to condense into liquids. Nonpolar atoms & molecules may

become momentarily polar when an unsymmetrical distribution of their e- results in instantaneous dipoles. Sometimes called dispersion

forces, instantaneous dipole forces, or induced dipole forces.

London Forces of Attraction

Very weak attractive forces, leading to very low BPs.

The halogens, like the noble gases, don’t have permanent dipoles, but…Iodine is a solid and bromine is a liquid at room T.

London Forces of Attraction

What’s up with I2 and Br2?

Polarizability of e- clouds! The ease with which the e- cloud

around an atom or molecule can be deformed into a dipole. Small atoms/molecules have e-

clouds held tightly to nucleus- low polarizability.

Large atoms/molecules, w/ loosely held e- have high polarizability

London Forces of Attraction

These forces can explain the behavior or many molecules. The more e- in a molecule, the more

opportunity to form instantaneous dipoles- so… increase in attractive forces means higher BPs.

Aklanes – CnH2n+2

Called normal alkanes, n-alkanes, because the vary in a regular way. (homologous series)

Hydrogen Bonding

Extraordinarily large dipole-dipole forces attributed to the large electronegativity difference between H and the other atom on the next molecule (F, N, or O).

Physical Properties of Liquids

Surface tension

Viscosity

Evaporation

Vapor Pressure

Boiling Pt.

Heat of Vaporizaiton

Surface Tension

Caused by an increase in the attractive forces b/w molecules at the surface of a liquid compared to the forces b/w molecules in the center (bulk) of the liquid. Causes fluids to minimize their

surface area… Small droplets form spheres

Surface Tension

Look at a molecule on the interior… The solvent molecule is surrounded by

other solvent molecules on all sides.

Look at a molecule on the surface… Some of the molecules surrounding

the the solvent molecules have been removed so the surface molecules will compensate by attracting neighboring molecules more strongly to reduce added potential energy. Causes surface molecules to be

closer to each other.

Surface Tension

Cohesive forces – attractions b/w identical molecules in the liquid

Adhesive forces – attractions b/w different molecules, like a liquid and a flat surface

If cohesive forces are stronger than adhesive forces…

If adhesive forces are stronger than cohesive forces…

Viscosity

A liquid’s resistance to flow.

Attractive forces are responsible for viscosity. Molecules move more freely in solutions

with low attractive forces Liquid alkanes have lower viscosities

because they only have London forces Water is more viscous because it has

hydrogen bonding Syrup is very viscous because all the

bulky sugar molecules have lots of –OH groups, which hydrogen bond to the water in the mixture.

Low viscosity

High viscosity

Viscosity

Decreases as the liquid’s T is increased. Molecules have higher KE, weakens

intermolecular forces (IMFs).

Evaporation

The process in which a liquid in an open container is slowly converted into a gas at the surface of the liquid. Some liquids evaporate more rapidly than

others.

Reverse of condensation, must have enough sufficient KE to escape the attractive forces

Evaporation

Factors that affect evaporation Surface area of the liquid – the

greater the surface area, the greater the evaporation

Evaporation

Factors that affect evaporation Temperature – Increasing

the T increases the # molecules with enough KE to escape as a gas

Evaporation

Boiling – when T is increased enough, boiling occurs. Molecules do not have to reach the

surface to enter the gas phase.

Vapor Pressure

Pressure that develops in the gas phase above a liquid when the liquid is placed in a closed container.

Dynamic equilibrium – occurs when the rate the liquid evaporates equals the rate the gas condenses

Vapor Pressure

Rate a liquid evaporates – dependent on T

Rate a gas condenses – dependent on the frequency the gas molecules collide with the liquid “wall” of the container.

Therefore – vapor pressure depends only one the nature of the liquid (attractive forces) & the temperature (KE) If T increases, Vapor Pressure

increases.

Boiling Point

Boiling occurs when the vapor pressure of the liquid is equal to atmospheric pressure Normal boiling point- refers to the

boiling point when atmospheric pressure is 760 mHg

Heat of Vaporization

ΔHvap - the energy needed to convert 1 gram of liquid into 1 gram of gas at a temperature equal to the normal boiling point of the liquid. Units are J/g or J/mol (if using molar

heat of vaporization)

ΔHvap = -ΔHcond

Heat of Vaporization

There are differences in the heats of vaporization that can be related to the IMFs For similar-size molecules, hydrogen-

bonded substances have largest ΔHvap.

Polar substances have higher ΔHvap than similar shape nonpolar substances

Increasing London forces increases ΔHvap

Heat of VaporizationCompound

Formula ΔHvap

(kJ/mol)Attractive force

Water H2O +43.9 H bonding

Ammonia NH3 +21.7 H bonding

Hydrogen fluoride

HF +30.2 H bonding

Hydrogen chloride

HCl +15.6 Dipole-dipole

Hydrogen sulfide

H2S +18.8 Dipole-dipole

Fluorine F2 +5.9 London

Chlorine Cl2 +10.0 London

Bromine Br2 +15.0 London

Methane CH4 +8.2 London

Ethane C2H6 +15.1 London

Propane C3H8 +16.9 London

The amount of heat needed to vaporize a liquid is very large. This explains why water can be quickly raised to its boiling point, but a long time is needed to boil away all the water.