Chemistry 1212 w Dr Suggs Chapter 12 12 1 States of Matter and Intermolecular Forces Kinetic Molecular theory of gas assumes that gas molecules or atoms are widely separated and these particles can be considered to be independent of one another In ideal conditions the properties of gas fall under the ideal gas law PV nRT o P pressure of gas o V volume of gas o n amount of gas moles o R gas constant o T gas temperature In real gasses intermolecular forces IMF are at work If these forces are strong enough they cause the gas to condense to a liquid and eventually a solid There is a large increase in volume when converting liquid to a gas but no dramatic change in volume occurs when converting a liquid to a solid o It can therefore be assumed that the molecules in liquid are about as closely packed as those in solids How do IMF s influence chemistry 1 They are directly related to melting point boiling point 2 They are important in determining the solubility of solids gasses and liquids in different solvents 3 They help determine the structure of molecules IMF s between molecules are electrostatic 1 Dipole dipole forces between molecules with permanent dipoles molecule with permanent partial charges 2 Dipole induced dipole forces polar molecules and nonpolar ones 3 Induced dipole induced dipole forces nonpolar molecules also called London forces 12 2 Interactions between Ions and Molecules with a Permanent Dipole Ion dipole forces attraction between a positive or negative ion and a polar molecule Ion ion forces are the most strong Coulomb s Law a way to evaluate ion dipole attractions states that the force of attraction between two charged objects depends on the product of their charges divided by the square of the distance between them o The closer the molecules are the greater the attraction o The higher the ion charge the stronger the attraction o The greater the magnitude of the dipole the stronger the attraction Example of the interaction between an ion and a polar molecule o Hydration of ions enthalpy of solvation enthalpy of hydration o Has a substantial enthalpy change but it cannot be measured directly o The enthalpy of hydration depends on the charge of the ion and 1 d d distance between center of ion and oppositely charged pole of dipole 12 3 Interactions between Molecules with a Dipole Dipole Dipole Forces Dipole Dipole Interaction when one polar molecule interacts with another dipole molecule either different of the same kind the positive end of one molecule will be attracted to the negative end of the other molecule o These attractions influence the evaporation of a liquid and the condensation of gas each requires an energy change Evaporation requires the input of energy enthalpy of vaporization this change has a positive enthalpy so this process is endothermic The enthalpy change for condensation is the opposite exothermic so it has a negative enthalpy The greater the force of attraction between the molecules the greater the energy that must be applied to separate them o So we would expect polar compounds a higher value of enthalpy of vaporization than nonpolar compounds with similar molar masses Boiling Point o As the temperature of a liquid is raised its molecules gain kinetic energy o Once enough kinetic energy is attained at the liquid s boiling point the molecules have sufficient kinetic energy to break the IMF s between themselves and neighboring molecules o For molecules of similar molar mass the greater the polarity of the molecule the higher the boiling point Solubility o Like dissolves like meaning polar molecules are likely to dissolve in polar solvents and nonpolar molecules are likely to dissolve in nonpolar solvents o Water is a polar solvent Hydrogen Bonding In general the boiling point of hydrogen compounds i e CH4 SiH4 etc increase with increasing molar mass There are a few exceptions to this rule o Hydrogen compounds containing Nitrogen N Fluorine F or Oxygen O have much greater electro negativities and therefore a much higher boiling point that would be assumed from their molar masses o The high electronegativity of these compounds is due to the large difference in electronegativity between H 2 2 and N 3 0 O 3 5 or F 4 0 In these cases the more electronegative atom N O or F takes on a strong negative charge and the hydrogen atom acquires a strong positive charge A hydrogen bond is the unusually strong attraction between an electronegative atom with a lone pair and the hydrogen atom of the N H O H or F H bond The hydrogen in this molecule is then available to make a bridge between the N O or F in its molecule and the N O or F of another molecule o Examples N H N F H N N H O F H O N H F F H F O H N O H O O H F The dotted line after the hydrogen represents the hydrogen bond Hydrogen Bonding and the Unusual Properties of Water Water s unique properties are due to its strong hydrogen bonding each water molecule can participate in four hydrogen bonds Ice is an example where water molecules bond together through hydrogen bonds and this creates a tetrahedral structure as a result is made up of tons of little cages of water molecules with lots of free space available The free space in the ice is the reason that it 10 less dense than water which explains why ice floats on water When ice melts the hydrogen bonds break down causing the increased density of water Water s density reaches a maximum at about 4 degrees Celsius and then declines in density with increasing temperature like other liquids This is why lakes bodies of water freeze from the surface down o Once the whole body of water reaches 4 degrees Celsius all at same density some water begins to freeze and it floats to the top due to its greater density Hydrogen bonding is also the reason for waters high boiling point High specific heat o It takes a greater input of energy heat to break the hydrogen bonds o This is the reason that bodies of water affect our weather as temperature drops outside the water gives off heat in the atmosphere 12 4 Intermolecular Forces Involving Nonpolar Molecules Dipole Induced Dipole Forces Polar molecules such as water can create a dipole in molecules that do not have a permanent dipole nonpolar o Example O2 s electrons are evenly distributed around the two oxygen atoms and the molecule is therefore nonpolar When a polar molecule such as water approaches the O2 the negative end O causes the O2 molecule to distort and become polar as the protons are attracted to the