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TAMU CHEM 101 - Intermolecular forces, Dipole Molecules and Properties of Liquids
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CHEM 101 1st Edition Lecture 25Outline of Last Lecture I. Molecular Orbital Theory (cont.)II. Gas PropertiesIII. Boyle’s LawIV. Charles’ LawV. Avogadro’s HypothesisOutline of Current Lecture I. Diffusion and EffusionII. Intermolecular ForcesIII. Dipole Molecules and Ionic CompoundsIV. Properties of LiquidsV. Phase DiagramsCurrent Lecture- Diffusion and Effusiono Effusion is the movement of molecules through a small hole into an empty container that is under vacuumo Effusion is governed by graham’s Law: o The rate of effusion of a gas is proportional to its rms speeds:rate is (proportional) to vrms (proportional) to sqrt(1/M)o Real gases (as opposed to ideal gases) have a particle volume and undergo intermolecular forceso At higher pressure and/or lower pressure conditions the influence of particle volume and intermolecular forces is not negligible- Non-ideal Behavior of Gaseso This non-ideal behavior is corrected by the van der Waals equation:(P +( n^(2)a/V^(2))) x (V – nb) = nRTwhere P is the measured PV is the Measured V = V(ideal)nb is the correction for volumeV^(2) is the correction for intermolecular forcesChapter 11: Intermolecular Forces and Liquids- Intermolecular Forceso Attractive and repulsive intermolecular forces are summarized as so-called van der Waals forces. We classify forces between  Molecules with permanent dipoles (dipole-dipole forces) Polar and nonpolar molecules (dipole-induced dipole forces) Nonpolar molecules (induced dipole-induced dipole forces of London forces)- Dipole-dipole forceso Nonsymmetrical molecules such as H2O or HCl have a dipole moment. Therefore,they undergo attractive interactions, which are weaker than a chemical bondo The electrostatic inter- or intramolecular interaction of the lone pair(s) of an electronegative atom X and (a) hydrogen atom(s) of (a) X – H bond(s) are called hydrogen bond(s).o Hydrogen bonds are responsible for the unusual properties of water (and HF, NH3)- Dipole Molecules and Ionic Compoundso Ionic compounds such as NaCl are soluble in polar liquids (also called solvents) like water, since they are solvated by the H2O molecules The fronts of the dipoles are facing towards positively charged moleculeso We remember that electrostatic forces in ionic compounds are described by Coulomb’s law. o It generally applies for charged particleso As ion charge increases, force of attraction increaseso As distance increases, force of attraction decreaseso Elements with the greater charge and smaller distance between the nuclei of two ions has the higher melting pointo Enthalpy of solation; in water: enthalpy of hydration: Na^(+) (g) + water  Na^(+)(aq) Δhydration H° = -405 kJ/mol- Dipole-Induced Dipole Forceso A polar molecule can induce a dipole in a nonpolar molecule by distorting the electrono The degree to which the electron cloud of an atom or molecule can be distorted is measured by its polarizabilityo The larger the molecule, the more easily it can be polarized- Induced Dipole-Induced Dipole Forceso Even nonpolar atoms or molecules can be polarized by momentary induced dipoles (London dispersion forces): Two nonpolar atoms or molecules (depicted as having an electron cloud that has a time-averaged spherical shape) Momentary attractions and repulsions between nuclei and electrons in neighboring molecules lead to induce dipoles Correlation of the electron motions between the two atoms or molecules (which are now polar) leads to a lower energy and stabilizes the system.- Summary of Intermolecular Forces:o Interacting particles. No Are ions involved? YesAre polar molecules Are polar molecules involved? involved? NoLondon Forces (induced dipoles)- Properties of Liquidso Out of the three states of matter, liquids are the most difficult to describe precisely: (Ideal) gases can be considered as independent particles Solids undergo significant intermolecular forces. They are usually in an orderly arrangement; they have a long-range order Liquids undergo significant intermolecular forces; they have a short range, but no pronounced long-range order Molecules of a liquid on the surface are only affected by neighbors ar or below the surface layer; they experience a net inward force of attraction This leads to surface tension- the energy required to break the surface of the liquid- Water molecules on the surface aren’t completely surrounded by other water molecules- Water molecules under the surface are completely surrounded by other water moleculeso Capillary action causes a concave meniscus for a water column: Concave meniscus: attractive forces between water and glass H2O in glass tube: cohesive forces between water moleculeso Movement of water up a piece of paper Is a result of H-bonds between water and the OH groups of the cellulose in the papero In order for a liquid to vaporize, sufficient energy much be available to overcomethe intermolecular forceso The process of vaporization is therefore endothermico Standard molar enthalpy of vaporization (Δvap H°):1 mol liquid (vaporization, energy absorbed by liquid)  1 mol vaporo Standard molar enthalpy of condensing (Δcon H°)1 mol liquid (condensation, energy dispensed by liquid)  1 mol condensation- Vapor Pressureo In an open container, a liquid such as water will evaporate completelyo If that container is sealed, a certain portion of water vapor will recondense; a dynamic equilibrium is achieved: liquid  vaporo The higher the temperature, the higher the vapor pressureo If the natural logarithm of the vapor pressure (ln P) is plotted against the reciprocal of the temperature value 1/T, a linear relationship results (Clausius-Clapeyron equation)ln P = - ((Δvap H°)/RT) + Cln P2 – lnP1 = [((-Δvap H°)/RT2) + C] – [((-Δvap H°)/RT1) + C]- The Thermite ReactionFe2O3(s) + Al(s)  2 Fe(l) + Al2O3(s)o ΔrH= -851.5 kJ/mol; a highly exothermic reactiono The achieved temperature is about 2200 °C or moreo This reaction was patented (H. Goldschmidt, 1895) and applied (for example)


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TAMU CHEM 101 - Intermolecular forces, Dipole Molecules and Properties of Liquids

Type: Lecture Note
Pages: 4
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