liquids_solids_15dec2017_1st.notebook Liquids and solids Chapters 11 and 12

Transcription

1 liquids_solids_15dec2017_1st.notebook December 15, 2017 Liquids and solids Chapters 11 and 12

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4 Intermolecular forces Intermolecular: forces between molecules Intramolecular: within molecules (i.e. covalent) State changes: molecules intact; phase changes due to changes between the molecules (intermolecular forces only!!!)

5 Types of Intermolecular Forces Dipole Dipole Forces Act between polar molecules Polar: have a net dipole moment About 1% as strong as covalent or ionic Become weaker as the distance between the dipoles increases SOLIDS Molecules are more ordered line up so there are mostly attractive interactions LIQUIDS Molecules have more disorder some repulsive forces

6 For molecules of similar mass/size strength of intermolecular forces increases with increasing polarity Boiling point also increases

7 Types of Intermolecular forces HYDROGEN BONDING Strong dipole-dipole force H bonded to N, O, F Strong because: The bond between H and the highly electronegative atom is very polar The hydrogen is so small that the dipoles can get very close to each other Still much weaker than covalent or the electrostatic force between ions Boiling point is higher than expected b/c the molecules are held more tightly together To form a hydrogen bond what must the non-hydrogen atom (N, O or F) involved in the bond possess?

8 1. In which of these substances is hydrogen bonding likely to play an important role in determining physical properties: methane (CH 4 ), hydrazine (H 2 NNH 2 ), methyl fluoride (CH 3 F), hydrogen sulfide (H 2 S)? 2. In which of these substances is significant hydrogen bonding possible: methylene chloride (CH 2 Cl 2 ), phosphine (PH 3 ), hydrogen peroxide (HOOH), acetone (CH 3 COCH 3 )?

9 Types of Intermolecular Forces LONDON DISPERSION FORCES Exist between all types of molecules Only kind of intermolecular force for noble gas atoms and nonpolar molecules Due to a momentary nonsymmetrical electron distribution Forms a temporary dipole called an instantaneous dipole The temporary dipole can then induce a dipole in a neighboring atom Weak and short lived Polarizability the ease with which the e- charge distribution is distorted

10 Notice that the halogens have a much higher boiling point than the noble gases in the same period. Why? Also notice that the boiling point increase down a group. Why? Dispersion forces increase in strength with increasing molecular mass More electrons = more polarizability Can be more important than dipole dipole forces for large atoms

11 TYPES OF INTERMOLECULAR FORCES ION-DIPOLE FORCES Between an ion and a polar molecule Increases with increasing ionic charge or as the magnitude of the dipole moment increases (more polar molecule) Important for solutions of ionic substances in polar liquids NaCl in water

12 Comparing intermolecular forces MOLECULES OF SIMILAR MASS/SHAPE Dispersion forces about equal Differences in intermolecular forces due to difference in the strength of dipole-dipole interactions Molecule with hydrogen bonding have the strongest IMF MOLECULES WITH BIG DIFFERENCE IN MASS Depends mainly on dispersion forces Generally higher in the substance with the higher molecular mass 1. List the substances BaCl 2, H 2, CO, HF, and Ne in order of increasing boiling point. 2. (a) Identify the intermolecular attractions present in the following substances, and (b) select the substance with the highest boiling point: CH 3 CH 3, CH 3 OH, and CH 3 CH 2 OH.