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CHEM 1E03 (53)
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McMaster University

Physical properties of the phases of matter Gases Liquids Solids • little or no interaction significant interaction • strong interaction • between molecules between molecules between molecules • low density • higher density • highest density • variable volume • fixed volume • fixed volume (compressible) • variable shape • fixed shape • variable shape • moderate viscosity • high viscosity • low viscosity (resistance to flow) Types of Bonding ♣ 164 Types of solids Ionic • electrons localized on ions • ions held in a rigid, brittle lattice by electrostatic forces Metallic • delocalized valence electrons • metal atom cores held in a deformable (ductile) lattice by sea of delocalized valence electrons Network covalent (crystalline or amorphous) • electrons localized in covalent bonds • extended (infinite) network of (polar or non-polar) covalent (directional) bonds, eg. graphite, diamond, sand (SiO )2, glass Molecular e.g. H O, I , C H O , CO , C , etc. 2 2 6 12 6 2 60 • low melting points • discrete molecular units • held together by intermolecular forces ... 165 Intermolecular forces What forces hold molecules together to form molecular solids, or molecular liquids? These forces are also responsible for non-ideal behavior in gases - non-ideal gas behavior is observed at sufficiently low temperature (approaching the boiling point of the liquid) or high pressure (approaching the vapor pressure of the liquid). Dipole moment interactions • Recall that molecules with polar bonds and non- symmetric shapes have permanent dipole moments • The negative end, or region, of one molecule will be attracted to the positive end, or region, of another • This leads to ordered structure in solids and some ordering in liquids • Larger dipole moments ⇒ stronger forces ♣ 166 Hydrogen bonding • An extreme form of dipole-dipole interaction is called hydrogen-bonding, or H-bonding. • When H is bonded to a very electronegative atom (F, O or N) there is a very large dipole associated with the X-H bond (egs. H 2O, HF, NH ,3CH C3 OH2 CH NH )3 2 • The positively charged H atom will be strongly attracted to negatively charged atoms on other molecules. • This attraction is "directional" like a covalent bond • H-bonding is responsible for the double helix of DNA & the ability of the cell to "read" DNA & produce the proteins encoded in the DNA H-bonding is responsible for the α-helix of proteins ♣ ♣ DNA double helix 167 Dipoles may interact with ions • stronger than dipole-dipole interactions • eg. ions dissolved in water: hydration energy increases with decreasing size and increasing charge of ions - relates to solubility H2O Hydration energy - eg. Li (g) → Li (aq) ∆H hydration Cation Ion Radius/pm Enthalpy of Hydration/(kJ/mol) + Li 90 -515 + Na 116 -405 + K 152 -321 + Rb 166 -296 + Cs 181 -263 • Another eg. ... soap dissolves grease (oil) in water because soap molecules are ionic at one end ~~~~~~~~COO − • the non-polar part of the soap molecule interacts well the the grease - How do non-polar molecules interact? First consider ... 168 Dipoles (or ions) may interact with non-polar species ... • by inducing a dipole moment • weaker then dipole-dipole interaction • e.g. O2 vs. C2 dissolved in H2O • polarizability characterizes the relative size of the induced dipole resulting from proximity of a permanent dipole (or net charge) • polarizability of atom increases with size of atom • larger molecules have larger polarizability Gas Molar Mass/ (g/mol) Solubility/ (g /100g H2O) H 2.01 0.000160 2 N 28.0 0.000190 2 O 2 32.0 0.000434 Cl 2 70.9 0.729 What about the interaction of non-polar molecules with non-polar molecules? Even in absence of permanent dipoles, random fluctuations of electron density gives rise to short-lived "spontaneous" dipoles in non-polar molecules, which then induce dipoles in their neighbors ⇒ Dispersion forces • important in I (s) and in large hydrocarbons, since 2 polarizability increases with size - spontaneous dipoles also increase with size • More electrons, more loosely held, are more easily displaced from their average distribution. • Note that dispersion forces exist in every system even when other forces also exist. 169 ♣ Larger nonpolar atoms & molecules have larger dispersion energy & consequently higher boiling points Problems 1. Describe/list the intermolecular forces that must be overcome in converting each of the following from a liquid to a gas: CH oiodide,O 2 methyl 3 ethanol, CH 3H O2 sulfur dioxide, S2 ammonia, NH 3 trChlCrlmethane, 3 tetrachloromethane, CCl4cadroxnide, 2 Which forces are strongest? Which liquids have the highest boiling points? 2. Rank the following in order of increasing strength of intermolecular forces in the pure substances: Ne, NH , CH , CCl 4 3 4 b.p.s /°C -246 -33 -161 +77 Which of these are gaseous at STP? 170 3. Which member of each pair has the higher boiling point? O 2 N 2 -138, -195 °C HF & HI +19.7, -35.6 °C HCl & HI -84.5, -35.6 °C ethanol, CH 3H O2 & +78.3, -24.8 °C dimethyl ether, CH 3CH 3 Other Properties of Liquids Surface Tension: • energy required to increase surface area (per unit area) • measures resistance to breaking of liquid surface, or spreading of liquid drop - as a film • there is a net inward attraction of molecules at the surface • drops "try" to minimize their surface area (i.e. we get spherical drops) ♣ Molecules at the surface are not stabilized (by intermolecular forces) to the extent of those in the bulk. 171 Capillary action: • liquids can "creep" up a solid surface - e.g. chromatography • the area of the liquid-solid interface increases at the expense of increased liquid-gas interface area (i.e. at the expense of surface tension) because of a favorable interaction between the liquid & solid Here, we distinguish ... • cohesive forces (with self) and adhesive forces (with others) • relative strength of these forces determines the shape of a meniscus curve: • adhesive stronger ⇒ concave - e.g. water (& glass) • cohesive stronger ⇒ convex - e.g. mercury (& glass) ♣ Viscosity: • resistance to flow • increases with increased intermolecular forces and is much increased for long chain molecules which tangle when the liquid flows – try it at home: starch in water 172 Structure of solids • Crystalline solids have a regularly repeating pattern of atoms • We define the "unit cell" - the smallest repeating unit of atoms • The crystal lattice consists of repetitions of the unit cell – the symmetry of the crystal lattice is determined by the symmetry of the unit cell • There are seven different unit cell shapes – we (mainly) consider the cubic unit cells (see, for example, http://chemwww.cwru.edu/Chime/solids/cubic.htm– i.e. you are not responsible for Fig. 11.15 on pg. 430 Simple cubic cell – scc # of nearest neighbors = Coordination # = 6 • All atoms are equivalent • A simple cubic cell has 8×(1/8) = 1 atom per unit cell – each of the 8 atoms in the unit cell is shared by 8
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