Prof Gilbert LECTURE 26 CHEM 1211 11 17 10 Last time Chapter 9 Molecular Geometry Valence Bond Theory assumes that atomic orbitals mix hybridize among themselves to form orbitals with the shapes that give the observed electron and molecular geometries In hybridization lone pairs and bonding pairs are in hybrid orbitals the orbitals involved in bond formation are unhybridized atomic p orbitals This time Chapter 9 continued Polar molecules and dipole moments The extent of separation of charge along polar bonds as in H F or within any polar molecule is expressed by the molecule s dipole moment Q r where Q is the partial charge that is separated by a distance r inside the molecule The value of in debye D can be determined experimentally and r is based on the distance between charge centers and the geometry of the molecule To calculate Q a unit conversion is required 1 D 2 24 10 30 coulomb meters The value of Q when divided by the charge on an electron 1 602 10 19 C yields the ionic character of a bond or polar substance Chapter 10 Intermolecular Forces Learning Goals 1 Relate physical properties of substances melting points boiling points solubilities in different solvents to the interactions among their particles and between their particles and those of other substances 2 Predict the most stable state of a substance from its phase diagram 3 Relate the colligative properties of a solution to the concentration of solute in it Basic Concepts 1 Likes dissolve likes meaning polar solvents dissolve polar solutes but not nonpolar solutes this is why gasoline and vegetable oil don t dissolve in water 2 Ionic solutes separate into their free ions when they dissolve but not if the ions are very tightly held together The more highly charged they are and the smaller they are the less soluble they are See point 1 below 1 Ion ion interactions and Lattice Energy The ions in ionic compounds are held together by the electrostatic attraction positive and negative ions have for each other To use the above equation we need the value of d the distance between the centers of the and ions This is the same as the sum of their ionic radii For a Na ion and a Cl ion d 102 181 283 pm or 0 283 nm Inserting this value into the above equation Eel 2 31 10 19 J nm 1 1 283 nm 8 16 10 19 J The negative value Eel means the pair of ion pair is lower in energy and so more stable than the individual ions The value can be scaled up to a mole of NaCl if we multiply by Avogadro s number 8 16 10 19 J 6 022 1023 492 kJ mol This value is in the ball park of a parameter called the Lattice Energy U of NaCl which is defined as the enthalpy change when a mole of an ionic compound forms from its gas phase ions Na g Cl g NaCl s H 786 kJ mol U where So why is the actual lattice energy more negative than the energy of a mole of ion pairs Answer the differences in the k terms reflect the fact that in a crystal of NaCl each ion is surrounded by 6 ions of the opposite charge Inquiry how can we explain the differences in U values among the following ionic compounds Compound U kJ mol NaF NaCl NaBr MgO MgCl2 910 786 732 3795 2326 1 2 Compare the lattice energies of the three Na compounds Compare the lattice energies of NaF and MgO Physical properties that depend on U Why should they Melting point Sublimation point solid turns directly into a gas Solubility in water Inquiry Which of the following ionic solids has 1 the highest melting point 2 is the least soluble in water NaCl NH4NO3 K2SO4 CuSO4 TiO2 NOTE Ionic attraction is a much stronger force than other truly intermolecular forces not including covalent bonds Consider the properties of KCl vs Cl2 Similar molar masses but Determining the value of U experimentally Consider this exothermic reaction Na s Cl2 g NaCl s H 411 kJ mol Let s break it down into several steps and then apply Hess s law to calculate U Step Process 1 2 3 4 5 Vaporize of mole of Na s atoms Na g atoms Break a mole of Cl Cl bonds Cl g atoms Ionize a mole of Na s atoms Na g ions Ionize a mole of Cl g atoms Cl g ions Gas phase Na and Cl ions combine to form 1 mol NaCl s H kJ mol 109 120 495 IE1 349 EA1 U NOTE The enthalpy change in Step 4 is called the first electron affinity EA It is the energy released when one mole of gas phase atoms of an element acquires a mole of electrons To calculate the value of U we subtract the sum of the values of H for Steps 1 4 from the overall H U 411 109 120 495 349 786 kJ mol Why salt dissolves in water or How does the salt water system overcome lattice energy Answer strong ion dipole interactions They are not as strong as ion ion interactions ionic bonds but one ion may interact with as many as 6 molecules Plus there is a big gain in entropy Dipole Dipole interactions and H bonds The partial charges on polar molecules create these interactions If N O or F is bonded to H we have the extreme case of dipole dipole interactions among molecules called hydrogen bonds the dashed lines in the structures below H bonds are only about 1 10 the strength of real covalent bonds but hugely important in defining the shapes of large molecules such as proteins and DNA Among nonpolar molecules their can be temporary dipoles or induced dipoles One example is O2 dissolved in water Water distorts the electron cloud around the O atoms in O O Sometimes random motion of the valence electrons does the same thing The result is a kind of interaction called a London force or dispersion force The bigger the size of the cloud the stronger the interaction Boiling points and Vapor Pressures of Liquids The weaker the intermolecular interactions between molecules in the liquid state the easier it is for them to vaporize reflected in high vapor pressures and low boiling points Vapor pressure increases with increasing temperature When vapor pressure reaches 1 00 atm the substance boils Phase diagrams are graphical presentations such as these of CO2 and H2O of the dependence of the stabilities of the physical states of a substance on temperature and pressure The lines represent combinations of pressure and temperature at which physical states exist in equilibrium with each other All three exist at the triple point The critical point is located at the end of the liquid gas curve Note the differences between the diagram of CO2 and a portion of the phase diagram of water on the left Supercritical fluids exist at temperatures and pressures above the critical point Unusual properties they have the viscosities …