REAL GASES

Deviations from ideal gas law

WHY?1. Molecules have volume

2. Molecules have attractive forces (intermolecular)

1. V-nb

2. -a(n/V)2

Van der Waals Equation of State2

V

na

nbV

nRTP

States of MatterThere is no general law for the behaviour of condensed states

(i) Molecules are tightly packed

(ii) Intermolecular forces cannot be ignoredINTERMOLECULAR ATTRACTIONS

Intermolecular forces cause compounds to exist in solid, liquid, or gas and affect properties such as the mp and bp of compounds as well as the solubility of one substance in another (later).

Solid: not compressible nor does it flow (shape and volume).

Liquid: not compressible but can flow (volume).

Gas: compressible and can flow.

Solids-Ionic and MetallicElectrostatic forces - these forces occur between charged species and are responsible for the extremely high melting and boiling

points of ionic compounds and metals.

Metal ions interact with the sea of electrons that surround them. This attraction is strong (bp of calcium 1484C).

Substances which bear full charges, anions and cations, are attracted very strongly (bp NaF 1695C)

Bond strengths for these solids are normally >1000kJ/mol

Dipole-dipole interactionsPolar covalent molecules contain regions that are rich and poor in electrons.

The electron-rich regions of one molecule can attract the electron-poor region of an adjacent molecule.

Strengths of ~5-25kJ/mol

Hydrogen BondsHydrogen bonds are abnormally strong dipole-dipole attractions that involve molecules with -OH, -NH, or FH groups.

H atoms are very small (r= 37 pm, they're smaller than any other atom but helium).

When a bonded highly electronegative atom (oxygen, nitrogen, or fluorine) pulls electrons away from the hydrogen atom, the positive charge that results is tightly concentrated. The hydrogen is intensely attracted to small, electron-rich O, N, and F atoms on other molecules.

Strength for water=19kJ/mol

Dipole-induced dipoleA polar molecule can also induce a temporary dipole in a non-polar molecule. The electron cloud around a non-polar molecule responds almost instantaneously to the presence of a dipole, so this "dipole-induced dipole" force isn't as orientation-dependent as the dipole-dipole interaction.

Strength ~2-10kJ/mol

Ion-dipole

Important in solvation of salts to yield solutions.

Strength~40-600kJ/mol

BP for alkanes

0 2 4 6 8 10 12 14 16 18-200

-100

0

100

200

300

Bo

ilin

g P

oin

t (C

)

Number of Carbons in Alkane

Alkanes: non-polar. Increasing alkane chain length gives higher bp.

There must be some other intermolecular force!

Instantaneous dipole-induced dipoleRandom instantaneous imbalances in the distribution of electrons in non-polar molecules creates instantaneous dipoles that can induce a dipole on a neighbouring non-polar molecule. The net force is attractive and sometimes called the London force (in honour of their discoverer).

London forces always contribute to intermolecular attractions. Usually weaker than other forces. Explain intermolecular forces between non-polar molecules or noble gas atoms. Molecules with large, diffuse electron clouds can have London forces that are as strong as other forces.(hexadecane is a solid at room temperature!).

Properties that are dependent on intermolecular forces shape and volume

surface tension

Tendency of liquids to reduce their exposed surface to the smallest possible area.

The molecules within the liquid are attracted equally from all sides, but those near the surface experience unequal attractions and thus are drawn toward the centre of the liquid.

Because of surface tension, various small insects are able to skate across the surface of a pond, objects of greater density than water can be made to float. A steel needle placed carefully on water, for example, can be supported by the surface tension.

Wetting:

The spread of liquid across a surface to form a thin film.

Water on glass: glass is a solid silicon oxide network.

On clean glass H-bonding between water and the surface overcomes the energy needed to expand the surface area.

On greasy glass the water will try to minimise contact with the non-polar surface. Beads form.

To prevent this add a a surface-active agent called a surfactant that has a non-polar “tail” and a polar “head” to dissolve the grease. The surfactant reduces the surface tension of the water.

Viscocity

Resistance to change of form of a liquid.

Proportional to the strength of intermolecular forces and to the ability of the molecule to entangle (polymers)

Evapouration and sublimation

Molecules with sufficient speed (kinetic energy) can escape the attraction of neighbours and evapourate (or sublimate).

Equilibrium: lg, sg

Vapour PressureIn a closed container, the pressure exerted by evapourated molecules at equilibrium is called the vapour pressure (vp).

The factors that affect the vapour pressure are

(i) Temperature (see Maxwell’s distribution on previous slide)

(ii) Chemical composition (intermolecular forces!)

The boiling point of a liquid can now be defined as the temperature at which the vp of the liquid is equal to the prevailing atmospheric pressure.

At 1atm pressure: Normal Boiling Point (100C for water)

At 1bar pressure: Standard Boiling Point (99.6C for water; 1bar=0.987atm)