CHAPTER 12 LIQUIDS SOLIDS AND INTERMOLECULAR FORCES Problems 1 3 6 9 32 35 42 45 46 59 64 67 68 71 82 91 93 94 100 102 12 2 PROPERTIES OF LIQUIDS AND SOLIDS Properties of Liquids 1 Liquids have an indefinite shape They take the shape of their container 2 Liquids have a fixed volume They cannot be compressed nor can they be expanded very much 3 Liquids are much denser compared to gases Liquids are about 1000 times denser than gases 4 Liquids usually flow readily so liquids that mix will eventually form a homogeneous mixture This is because molecules in a liquid are constantly in motion at the molecular level Properties of Solids 1 Solids have definite shape Particles are fixed in place and vibrate but do not move 2 Solids have a fixed volume Like liquids they cannot be compressed nor can they be expanded very much 3 Solids are either crystalline or noncrystalline amorphous A crystalline solid contains particles in a regular repeating pattern A noncrystalline or amorphous solid is disordered 4 Like liquids solids have much greater densities than gases 5 Solids do not mix by diffusion because particles in a solid do not move and mix CHEM 121 Tro Chapter 12 page 1 of 17 condensed states solid and liquid phases where particles are in contact with one another In solids and liquids molecules are attracted to one another These attractions are called intermolecular forces To undergo a change in physical state e g solid liquid or liquid gas particles in the solid or liquid must overcome the attractive forces with surrounding molecules Depending on the liquid or solid the attractive forces may be intermolecular forces or they may be ionic bonds covalent bonds or metallic bonds 12 6 TYPES OF INTERMOLECULAR FORCES IMF s DISPERSION DIPOLE DIPOLE AND HYDROGEN BONDING Dispersion or London Forces also called Induced Dipole Forces In nonpolar molecules shown as green the electrons shift and concentrate on one end temporary dipole red end blue end Temporary dipole in one molecule causes electrons to shift in an adjacent molecule another temporary dipole Their temporary dipoles cause them to be attracted to one another Attraction lasts only until the electrons shift again and the temporary dipoles go away All molecules experience London forces LDF Nonpolar molecules experience only LDF Polar molecules also experience LDF but they also usually experience other IMFs that may be stronger than LDF The bigger the molecule more electrons the greater its polarizability ability to distort electron clouds to get temporary dipole the stronger its London dispersion forces Thus the more electrons in an atom or a molecule The stronger its London forces Dipole Dipole Forces Attraction between polar molecules generally stronger than dispersion forces because attraction is due to permanent dipoles so they do not go away like induced dipoles CHEM 121 Tro Chapter 12 page 2 of 17 The Hydrogen Bond Especially strong type of dipole dipole force Exist between molecules with the following bonds H F H O H N because these are small atoms with large electronegativity differences very strong dipole in molecules Strongest type of intermolecular force Responsible for the relatively high melting and boiling point for water compared to molecules of similar size Hydrogen bonds are also responsible for the bending and twisting in proteins DNA and other important biological molecules Image from http blog targethealth com p 6846 Note Hydrogen bonds are the strongest type of intermolecular forces between different molecules BUT ionic and covalent bonds holding ions or atoms together in compounds are stronger than hydrogen bonds CHEM 121 Tro Chapter 12 page 3 of 17 Ion Dipole Forces Attraction between an ion and the oppositely charged end of a polar molecule e g between Na and the negative end O atom of a H2O molecule or between Cl and the positive end H atoms of a H2O molecule Note that when an ionic compound like NaCl dissolves in water the formation of ion dipole forces between the Na or Cl ions with water molecules results in the ionic bonds breaking How to determine type of intermolecular forces involved hydrogen bonding yes polar Is the molecule polar or nonpolar Are there H F H O or H N bonds no nonpolar dipole dipole forces dispersion London forces Ex 1 Indicate the type s of intermolecular forces for each molecule below then circle the molecule in each pair that experiences the stronger intermolecular forces a N2 b H2S or or CHEM 121 Tro Chapter 12 NO H2O c Cl2 d PH3 or or Br2 CH4 page 4 of 17 Ex 2 For each of the following i Identify the bond between atoms as ionic polar covalent or nonpolar covalent ii Identify the intermolecular forces as London dispersion forces dipole dipole forces and or hydrogen bonding Water H2O B A A Oxygen O2 B A B A B HCN H white C charcoal N blue A A CHEM 121 Tro Chapter 12 B B page 5 of 17 Ex 3 Indicate the type s of bonds or intermolecular forces described for each below A ionic bond B polar covalent bond C nonpolar covalent bond i D London dispersion forces E dipole dipole forces F hydrogen bond G metallic bond H ion dipole forces What holds the atoms together in a HF molecule ii What holds two H2S molecules together in H2S l iii What holds two Br2 molecules together in Br2 l iv What holds the atoms together in a chlorine molecule v What holds the CO2 molecules together in dry ice CO2 s vi What are the bonds or attractive forces in a sample of CuO vii What holds atoms together in magnesium viii What holds atoms together in water ix What bonds or forces are broken when NaCl dissolves in water x What bonds or forces are made when KBr dissolves in water CHEM 121 Tro Chapter 12 page 6 of 17 12 4 EVAPORATION AND CONDENSATION vaporization liquid gas From a molecular viewpoint a molecule escaping from the liquid to gaseous state As the liquid evaporates more molecules go into the gas phase vapor The gas above a liquid when the liquid and gaseous states are both present vaporization liquid heat vapor condensation vapor liquid heat Liquid Gas Equilibrium vaporization vapor liquid heat condensati on When the molecules in the liquid have enough energy they escape to the gas phase In a closed system when enough vapor exists above the liquid some gaseous molecules condense back to the liquid Ultimately the rate of vaporization the rate of condensation The system has reached a state of dynamic equilibrium in which the forward process occurs at the same rate as the reverse process In an open system molecules