CHM 361

CHM 361Chapter 1: Intermolecular Forces In Covalent CompoundsVan der Waals Hydrogen BondingDipole-Dipole InteractionIon-Dipole Interaction Instantaneous Dipole-Dipole Interaction

Outlines INTRODUCTIONIntermolecular forces ?? Forces that holding molecules together

play important roles in determining the properties of a substances (boiling and melting points, vapor pressures, and viscosities.)The attractions between molecules are not nearly as strong as the intramolecular attractions that hold compounds together These intermolecular forces as a group are referred to as van der Waals forces.1. Named after the Dutch physicist Johannes van der Waals

2.He had studied the effects of inter-molecular forces on the behavior of real gas

3. Several types of intermolecular forces (Van der Waals ) such as:

VAN DER WAALS

Hydrogen bondingDipole-dipole interactionIon-dipole interactionDispersion forces1)Known as :London Dispersion Forces2)These forces are present in all molecules, whether they are polar or nonpolar.

3)In nonpolar molecules, this is the only intermolecular forces that operates

4)Dispersion forces result from temporary dipoles induced in atoms or molecules

What is temporary/instantaneous dipole and induced dipole?London Dispersion Forces1) Movement of atoms/molecules, together with their electrons, may result in the electrons gathering at one side of the molecule

2)This will result in temporary/instantaneous dipole for the molecule

3) If this molecule is close enough with another molecule, it will induce a dipole with its neighbour 4)Due to random movement, a different temporary dipole is created the next moment, which will induce the dipole from its neighbours

5) the arrangement keep on changing as the molecules are movingThe likelihood of induced dipole depends on the polarisability of the atom or molecule

Polarisability the ease with which the distribution of electron cloud can be distorted

The larger the electron cloud (or the higher number of electrons), the easier it is to be polarisedPolarisability increased with increase of molar mass

London Dispersion ForcesAs you can see in this table, the melting point increase with the increase of molar mass of the moleculesThe increase of melting point shows increase in intermolecular forces in this case, London dispersion forces

London Dispersion Forces

Dipole-dipole interaction Dipole-dipole forces attractive forces between polar molecules

Dipole-dipole forces exist between molecules that have dipole moment

The larger the dipole moment, the greater the force

Polar molecules align in a way that maximises the attractive interaction

Dipole-dipole interaction

The more polar the molecule, the higher is its boiling pointIf two molecules are of comparable size and shape, dipole-dipole interactions will likely be the dominating force.

If one molecule is much larger than another, dispersion forces will likely determine its physical propertiesWhich Have a Greater Effect:Dipole-Dipole Interactions or Dispersion Forces?The dipole-dipole interactions experienced when H is bonded to N, O, or F are unusually strong.2. We call these interactions hydrogen bondsThe hydrogen must also be chemically bonded with either one of the three element Note that N, O and F all have at least one lone pair to interact with H atom in other molecules

Hydrogen Bonding

AHBAHAorA & B are N, O, or FHydrogen Bonding

Hydrogen bonding arises in part from the high electronegativity of nitrogen, oxygen, and fluorine.Also, when hydrogen is bonded to one of those very electronegative elements, the hydrogen nucleus is exposed.Hydrogen BondingThe strength of hydrogen bond depends on the interaction of the lone pair electrons with hydrogenExample: The boiling point of HF is lower than H2O

Although F is more elecronegative than O, but H2O can form 4 intermolecular hydrogen bonds. HF can only form 2 hydrogen bonds, as shown in this figureIon-dipole forces are attraction between an ion and a polar moleculeThe strength of this interaction depends on the charge of the ion, the dipole moment of the molecule & the size of bothA cation interacts more with dipoles than anion with the same magnitudeThe strength of these forces are what make it possible for ionic substances to dissolve in polar solvents

Ion-dipole forces

Hydration is an example of common ion-dipole interactionThe ion-dipole interaction of 2 different ions, Na+ and Mg2+ is shown here

Mg2+ ion, with higher charge and smaller, interacts strongly with water

Ion-dipole forces In aqueous solutions, metal ions are usually surrounded by water molecules in octahedral arrangement

Ion-dipole forces Summary In general, the strength of intermolecular forces increase in this order:dispersion < dipole-dipole < hydrogen bond < ion-dipole

Dispersion forces operates on all molecules and depend only on molecular mass Dipole-dipole operates on polar molecules, but compete with dispersionThe contribution of dipole-dipole forces in polar molecules is usually smaller than dispersion forcesHowever, if hydrogen bond is present, it makes a significant contribution to intermolecular interaction between molecules

SummaryHow to compare strength of intermolecular interaction?? similar in weights & shapes, differences is due to different polarity of molecules the higher polarity will have greater intermolecular forcesmolecules differ widely in weights, dispersion forces will determine the strengths of intermolecular forces the higher molecular weight will have greater intermolecular forces

SummaryFor each pair of substances, identify the key intermolecular forces(s) in each substance, and select the substance with the higher boiling point:

a) CH3NH2 or CH3F b) CH3OH or CH3CH2OHc) MgCl2 or PCl3Sample problem 1) we examine the formulas and picture the structure difference to identify the key difference2)Are ions present? Polar or nonpolar? Is N, O, F bonded to H?3)do the molecules have different masses or shape?

Remember that :Bonding forces are stronger than intermolecular forcesHydrogen bonding is a strong type of dipole-dipole force.Dispersion are always present, but they are decisive when the differences is molar mass or molecular shape

Plan

CH3NH2 and CH3F both consist of polar molecules of about the same molar mass, CH3NH2 has N-H bonds, so it can form H bonds. But, CH3F contain C-F bond but no H-F bond, so dipole-dipole forces occur but not H bonds. Therefore CH3NH2 has the higher boiling point.

(b) CH3OH and CH3CH2OH molecules both have O-H bond, so the can form H bond, CH3CH2OH have additional CH2- group and the larger molecular mass, which correlates with stronger dispersion forces, therefore have the higher boiling points

(c) MgCl2 =ionic bonding PCl3 = dipole-dipole moment,the forces of MgCl2 > PCl3. so it has a higher boiling point.

Solution