Chapter 6.2

Two Types of Forces with Compounds Intramolecular Forces the forces of attraction that hold the atoms together in a molecule. Ex. ionic bonds, covalent bonds, metallic bonds Intermolecular Forces the forces of attraction that hold molecules together. Intermolecular Forces in Pure Substances: 1) London Dispersion Forces 2) Dipole-Dipole Forces 3) Hydrogen Bonding Others (mixed substances): 4) Ion-Dipole Forces 5) Induced Intermolecular Forces

The intermolecular forces (IMF) pull molecules closer together - this can determine the state of a substance Gas weak IMF molecules far apart low boiling pts Liquid relatively strong IMF molecules close together higher boiling pts Solid strong IMF molecules very close together even higher boiling pts Substance melt or boil when enough heat is added to weaken or break the IMF that hold that compound together. Once these forces are broken - the compound will melt or boil. Solid, Liquid Gas Animation

London Dispersion Forces (Dispersion Forces, Van der Waals forces) The shared pairs of electrons in covalent bonds are constantly vibrating. These vibrations can cause a momentary, uneven distribution of charge which can allow any molecule to become partially polar for an instant. At that instant, this molecule is capable of inducing a dipole in a nearby molecule. London Dispersion Force is the temporary attraction force between the two momentarily polar molecules. the weakest intermolecular force of attraction the only force of attraction with non-polar molecules present in polar molecules as well.

Two Factors Affect LDF 1. Number of Electrons more electrons makes stronger dispersion forces  the higher the boiling pt. F 2 versus Cl 2 - non-polar - LD - 18 electrons  Cl 2 would have stronger dispersion forces and would have a higher boiling point F2F2 Cl 2 - non-polar - LD - 34 electrons

Size of the Molecule the shape with more surface area has stronger dispersion forces  the higher boiling point CH 4 versus C 3 H 8 - non-polar - LD forces smaller molecule - weaker LD Larger molecule - stronger LD  C 3 H 8 has higher boiling pt. - non-polar - LD forces

Dipole-Dipole Forces Polar molecules will orientate themselves so that oppositely charged ends of the molecule will be near each other. Dipole-Dipole Forces is the attraction between the negatively charged end of one polar molecule and the positively charged end of another polar molecule.

Dipole-Dipole Forces: Are usually main forces of attraction in polar molecules Are medium strength of all the intermolecular forces Do not occur in non-polar molecules Magnitude of Dipole-Dipole Forces the strength of these attractions increase with increasing polarity. Ex. H-Cl versus H-Br HCl HBr  HCl is more polar, so it would have stronger dipole- dipole forces and would have a higher boiling point. PolarForces - LD and DDBond Polarity = 0.96 Polar Forces - LD and DDBond Polarity = 0.76

Hydrogen Bonding To form hydrogen bonding - TWO molecules must be involved!! Hydrogen bonding occurs when the hydrogen in one molecule that is directly bonded to an O, N, F is attracted to the O, N, F in another molecule that is directly bonded to a hydrogen. This is the attraction between hydrogen atoms on one molecule and the lone pairs of either nitrogen, oxygen or fluorine atoms on a different molecule

Hydrogen Bonds: are the strongest intermolecular force of attraction. are 5% as strong as a covalent bond. occur many times within numerous molecules making them even stronger.

Network Covalent Solids happens with carbon and silicon atoms covalently bond continuously in 3-D arrays. SiO 2 and C 40, C 82, C 70, C 74

Determining which compound has a higher boiling point or melting point. decide if the molecule if polar or non-polar state the forces involved between the molecules state the strongest force and why state which compound would have a higher boiling point.

Ranking of Intermolecular and Intramolecular Forces 1. Network Covalent Compounds 2. Metallic (in general) 3. Ionic Compounds 4. Covalent compounds - LD, DD, HB - LD, DD - check magnitude of polarity - LD - check # of electrons and surface area

6.3 Structure Determines Properties Melting Points and Boiling Points Solids melt when the average kinetic energy of the particles is large enough to weaken/break the force of attraction with neighbouring particles. In order to evaporate, or boil, a substance must break all FOA with all neighbouring particles. When almost no forces of attraction exist with another molecule - the substance is in the gaseous phase.

In order for substances to melt: Ionic Compounds energy must be added to break the ionic bonds. There is a large attractive force between the + and the - ions, therefore a large amount of energy must be added. an extremely large melting point o C to 3000 o C an extremely large boiling point o C to 5000 o C Metallic Compounds enough energy must be added to break the metallic bonds. There is a large attractive force between the + cations and the - free electrons - therefore a large amount of energy must be added. an relatively large melting point o C o C an relatively large boiling point o C o C

Covalent Compounds DO NOT BREAK COVALENT BONDS !!! enough energy must be added to break the intermolecular forces. (LD, DD, HB) How many forces that a compound has and how strong they are will determine the melting and boiling points.... overall they are relatively low. melting point...From -200 o C to about 100 o C boiling point...From -250 o C to about 100 o C

Mechanical Properties of Compounds Metallic compounds are malleable and ductile. Metals are held together using positive cations and free electrons (free- electron model). When stressed, the free electrons allow the substance to shift without breaking it. See figure 6.25 page 219

Ionic Compounds are brittle. Ionic compounds are held together by ionic bonds. When stressed, the substance will break along the plane where the molecules are bonded. See figure 6.27 page 219

Covalent Compounds are soft or a bit brittle. Non-Polar Covalent Compounds little attraction between neighbouring particles.. London Dispersion Forces. when stressed - the molecules will slip off each other, therefore these are soft compounds. Paraffin wax – C 20 H 42 Polar Covalent Compounds more forces of attraction - not all that strong because they are partial charges. when stressed, the molecules will break at opposite charge line, therefore these compounds are somewhat hard, a bit brittle.

Thermal and Electrical Conductivity conducting electricity and heat involves negative and/or positive charges. Metals Ionic Compounds - in liquid state has ions  good conductors - when dissolved in water – has ions  good conductors Covalent Compounds - negative charges  good conductors - no charges  poor conductors