1 KINETIC THEORY OF MATTER molecules in matter are always in motion speed of molecules is proportional to the temperature THE STATES OF MATTER 1 Gas a ideal gas molecules move freely molecules have no interactions with each other no attraction or repulsion like ping pong balls b real gas molecules move freely molecules have small attractions for each other like sticky ping pong balls attractions decrease pressure of gas molecules have a volume inclusion of the molecular volume increases the pressure of gas more crowding 2 Liquid molecules move but movement is heavily influenced by other molecules molecules must move through a crowd of molecule ideal gas can never condense to be a liquid 3 Solid molecules or ions vibrate about fixed position in crystal lattice motion is not great enough to overcome attractions molecules or ions stay in a specific arrangement INTERMOLECULAR FORCES Overriding Principles 1 Opposite charges attract 2 The greater the charge the greater the attraction These principles are a restatement of Coulomb s Law 2 CHARGE DISTRIBUTIONS OF MATTER 1 Ion 2 Dipole A polar molecule has a dipole The value of a dipole depends on a polarity of bonds b molecular geometry H O H H C N 3 Induced Dipole A molecule can have its electron cloud polarized by neighboring charge Consider F2 F F F F Fluorine molecule has no dipole Consider F2 next to sodium ion Na Na F F Negative electron cloud adjusts to move closer to positive ion Fluorine molecule now has a dipole Sodium ion induces dipole in fluorine Thus fluorine may have an induced dipole Some atoms in molecules have electron clouds that adjust easier than others Na Na F F I I Sodium ion induces larger dipole in iodine molecule than in fluorine 3 The ability of an atom or molecule to adjust its electron cloud in response to an outside charge is called its polarizability The less tightly bound to the nucleus the electrons are the more polarizable they are As a periodic trend polarizability increases as row increases Each electron in an atom molecule or ion contributes to the overall polarizability Thus the more electrons a molecule has the greater the strength of its induced dipole Example Arrange the following atoms in order of increasing polarizability Ar He Kr Ne and Xe Answer He Ne Ar Kr Xe Xe very polarizable He barely polarizable TYPES OF INTERMOLECULAR FORCES 1 Ion Dipole Example salt water O H H O H O H O H H H O H H H O H H H H O H H O strongest type of intermolecular force as the charge increases the strength of the force increases Ca2 H2O forces are stronger than Na H2O forces 4 2 Dipole Dipole Example Formaldehyde H O C H H H C dipole dipole forces O H H C O Molecules with greater dipole moments have greater attractions to each other Intermolecular forces such dipole dipole forces can occur between different molecules H C C N H H HH C O 3 Hydrogen bonding A hydrogen bond is a very strong dipole dipole bond Molecules with N O F covalently bonded to H can form hydrogen bonds N O F atoms have high electronegativity Electrons in sigma bond are greatly polarized O H O H Hydrogen is bare naked A hydrogen bond is formed when polarized H is attracted to nonbonding pair of electrons 5 Example H2O H O H H H hydrogen bond O Hydrogen bond is much stronger than dipole dipole bond Hydrogen bonding gives water many important and distinctive properties Hydrogen bonding holds together DNA helix and proteins 4 Induction forces Dipole Induced Dipole Example HCl in I2 I I I I H Cl I I I I Dipole can induced by other dipoles Induction forces are weaker than dipole dipole forces 5 Dispersion forces Induced dipole induced dipole also known as London dispersion forces Charge distribution of molecules fluctuates over time i e charge distribution is unequal in nonpolar molecules at times i e nonpolar molecules become polar for a short time though average dipole is zero 6 Spontaneous fluctuation of electron cloud Consider a single bromine molecule Br Br Br Br Br Br Br Br dipole appears then disappears then appears reversed then disappears etc Fluctuating dipole induces dipole in another molecule so that they fluctuate together Consider two bromine molecules Br Br Br Br Br Br Br Br Strength of dispersion forces depends on strength of induced dipole Molecules with greater polarizability have greater dispersion forces Illustration Br Br Br stronger bond Br Cl Cl Cl Cl weaker bond Bond between bromine molecules is stronger than bond between chlorine molecules degree of contact with other molecules is also important 7 Aside abbreviated carbon chain structure C C C C Example Pentane C5H12 has stronger dispersion forces than butane C4H10 pentane butane Bonding between butane molecules is weaker than bonding between pentane molecules pentane 2 methylbutane Bonding between 2 methylbutane molecules is weaker than bonding between pentane molecules Butane 2 methylbutane Pentane Boiling Point 0 5 C 27 8 C 36 1 C Increasing Intermolecular Forces 8 Summary of Intermolecular Forces Molecule A Molecule B ionic bonding ion ion ion dipole dip o le hydrogen bonding dipole dipole dipole in du ctio n induced dipole dis persion induced dipole Principles of Intermolecular Force Strength 1 Opposite charges attract 2 The greater the charge the greater attraction 3 Intermolecular forces are additive 4 Nonpolar molecules have only dispersion forces 5 Polar molecules have dipole dipole induction forces and dispersion forces and possibly hydrogen bonding 6 Induced dipoles are often stronger than permanent dipoles Dispersion forces can be a significant reason why polar molecules are attracted to each other Example Fluoromethane CH3F 18 e has stronger total forces between molecules than the force between fluorine gas molecules F2 18 e Example Iodine I2 nonpolar 106 e has stronger forces in its molecules than does hydrogen sulfide polar 18 e Why Because the induced dipole of iodine is much greater than induced dipole and permanent dipole of hydrogen sulfide since iodine has so many more electrons 9 INTERMOLECULAR FORCES AND PHASE TRANSITIONS Solid condensation Gas Liquid evaporation Phase changes occur when intermolecular bonds are affected by an increase or decrease in kinetic energy Temperature during a phase transition is always constant All energy input goes into phase transition not temperature increase Heating Curve of Water 250 Temperature C 200 150 100 50 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 63 60 57 54 51 48 45 42 39 36 33 30 27 24 21 18 15 90 12 60 0 30 0 Tim e sec