Chapter 11 Intermolecular Forces

A phase is a homogeneous part of the system in contact with other parts of the system but separated from them by a well-defined boundary. 2 Phases Solid phase - ice Liquid phase - water 11.1

11.2 Intermolecular forces are attractive forces between molecules. Intramolecular forces hold atoms together in a molecule. Intermolecular vs Intramolecular 41 kJ to vaporize 1 mole of water (inter) 930 kJ to break all O-H bonds in 1 mole of water (intra) Generally, intermolecular forces are much weaker than intramolecular forces. “Measure” of intermolecular force boiling point melting point  H vap  H fus  H sub

Intermolecular forces are feeble; but without them, life as we know it would be impossible. Water would not condense from vapor into solid or liquid forms if its molecules didn't attract each other. Intermolecular forces are responsible for many properties of molecular compounds, including crystal structures (e. g. the shapes of snowflakes), melting points, boiling points, heats of fusion and vaporization, surface tension, and densities. Intermolecular forces pin gigantic molecules like enzymes, proteins, and DNA into the shapes required for biological activity.

Intermolecular Forces 1. London Forces (Dispersion Forces) 2.Dipole-Dipole Interactions 3. Ion-Dipole Interactions (Salt dissolving in solution) 4. Hydrogen Bonding

Dispersion Forces Occur between every compound and arise from the net attractive forces amount molecules which is produced from induced charge imbalances The magnitude of the Dispersion Forces is dependent upon how easily it is to distort the electron cloud. The larger the molecule the greater it’s Dispersion Forces are. Figure 10-8 Olmsted Williams Figure 10-9 Olmsted Williams

The boiling point of alkanes increase with the length of the carbon chain. Long-chain alkanes have larger dispersion forces because of the increased polarizability of their larger electron cloud.

How molecular shape affects the strength of the dispersion forces The shapes of the molecules also matter. Long thin molecules can develop bigger temporary dipoles due to electron movement than short fat ones containing the same numbers of electrons. Long thin molecules can also lie closer together - these attractions are at their most effective if the molecules are really close. For example, the hydrocarbon molecules butane and 2-methylpropane both have a molecular formula C 4 H 10, but the atoms are arranged differently. In butane the carbon atoms are arranged in a single chain, but 2-methylpropane is a shorter chain with a branch. Butane has a higher boiling point because the dispersion forces are greater. The molecules are longer (and so set up bigger temporary dipoles) and can lie closer together than the shorter, fatter 2- methylpropane molecules.

Polarizability 11.2 the ease with which the electron distribution in the atom or molecule can be distorted. Polarizability increases with: greater number of electrons more diffuse electron cloud Dispersion forces usually increase with molar mass.

Is the Molecule Polar? We have already talked about diatomic molecules. The more Electronegative atom will pull the electron density of the bond Closer to itself giving it a partial negative charge leaving the other Atom with a partially positive charge. Thus giving the molecule A dipole moment. But what about molecules made up of more than two molecules?

Molecules with 3 Atoms HCN Even though the C-O bond is polar, the bonds cancel each other out because the molecule is linear the dipole moments are equal and in opposite directions.Therefore CO 2 is non-polar. The dipole moment between H-C points in the direction of C. The dipole moment points between C-N points in the direction of the N. Therefore the dipole vectors are additive and HCN is polar CO 2 SO 2 is a polar molecule because the S-O dipole Moments don’t cancel each other out due to the angle SO 2

CCl 4 is non-polar CHCl 3 is polar Molecules with 4 Atoms

How to Determine if a Molecule Is Polar 1.Draw Lewis Structure 2.If all of the regions of electron density are bound to the same thing (CCl 4 ; CO 2 ) than the molecule is non-polar 3.If the regions of electron density are not bound to the same thing than the molecule is polar (HCN; SO 2 )

10.2 Which of the following molecules are polar (have a dipole moment)?H 2 O, CO 2, SO 2, and CH 4 O H H dipole moment polar molecule S O O CO O no dipole moment nonpolar molecule dipole moment polar molecule C H H HH no dipole moment nonpolar molecule

Dipole-Dipole Forces Attractive forces between polar molecules Orientation of Polar Molecules in a Solid 11.2

Figure Dipole Forces occur between molecules containing a dipole moment. The positive end of the dipole moment on one mole is attracted to the Negative end of the dipole moment on a nearby molecule. Consider 2-methyl propane (left) and acetone (right) Both compounds are about Equal in size and shape therby Having similar dispersion forces, But Acetone contains an Oxygen (red) and causes the Molecule to have a dipole Moment allowing it to have Dipole forces and thus a Higher boiling point Olmsted Williams

Ion-Dipole Forces Attractive forces between an ion and a polar molecule 11.2 Ion-Dipole Interaction The larger the charge the stronger the force

Fig A molecular picture showing the ion-dipole Interaction that helps a solid ionic crystal dissolve in water. The arrows indicate ion-dipole interactions. Olmsted Williams

S O O What type(s) of intermolecular forces exist between each of the following molecules? HBr HBr is a polar molecule: dipole-dipole forces. There are also dispersion forces between HBr molecules. CH 4 CH 4 is nonpolar: dispersion forces. SO 2 SO 2 is a polar molecule: dipole-dipole forces. There are also dispersion forces between SO 2 molecules. 11.2

Hydrogen Bond 11.2 The hydrogen bond is a special dipole-dipole interaction between they hydrogen atom in a polar N-H, O-H, or F-H bond and an electronegative O, N, or F atom. A H … B A H … A or A & B are N, O, or F

Crystals of benzoic acid contain pairs of molecules held together head to head by hydrogen bonds. These pairs then stack in planes which are Held together by dispersion forces. Fig 10-16B Pg 444 Courtesy Stephen Frisch

Intermolecular Forces 1. London Forces (Dispersion Forces) 2.Dipole-Dipole Interactions 3. Ion-Dipole Interactions (Salt dissolving in solution) 4. Hydrogen Bonding These forces affect how molecules will interact with each other and As a general rule as the strength of the force increases the boiling Point of the compound increases

Surface tension is the amount of energy required to stretch or increase the surface of a liquid by a unit area. Strong intermolecular forces High surface tension 11.3 Liquids and Surface Tension

Properties of Liquids Cohesion is the intermolecular attraction between like molecules 11.3 Adhesion is an attraction between unlike molecules Adhesion Cohesion

Evaporation Greatest Order Least Order 11.8 Condensation T 2 > T 1

The equilibrium vapor pressure is the vapor pressure measured when a dynamic equilibrium exists between condensation and evaporation H 2 O (l) H 2 O (g) Rate of condensation Rate of evaporation = Dynamic Equilibrium 11.8 A substance with a high Vapor pressure is considered To be volitile therefore, the lower The boiling point the higher the Vapor pressure and the weaker The intermolecular forces

The boiling point is the temperature at which the (equilibrium) vapor pressure of a liquid is equal to the external pressure. The normal boiling point is the temperature at which a liquid boils when the external pressure is 1 atm. 11.8

The critical temperature (T c ) is the temperature above which the gas cannot be made to liquefy, no matter how great the applied pressure. The critical pressure (P c ) is the minimum pressure that must be applied to bring about liquefaction at the critical temperature. 11.8

Melting 11.8 Freezing H 2 O (s) H 2 O (l) The melting point of a solid or the freezing point of a liquid is the temperature at which the solid and liquid phases coexist in equilibrium

11.8

Sublimation 11.8 Deposition H 2 O (s) H 2 O (g) Molar heat of sublimation (  H sub ) is the energy required to sublime 1 mole of a solid.  H sub =  H fus +  H vap ( Hess’s Law)

A phase diagram summarizes the conditions at which a substance exists as a solid, liquid, or gas. Phase Diagram of H 2 O 11.9