CHEM 139: Zumdahl Chapter 14 page 1 of 16 CHAPTER 14: LIQUIDS AND SOLIDS Active Learning Problems: 1-19, 21-22 End-of-Chapter Problems: 1-3, 6-11, 19, 21-27, 31-34, 39-41, 45, 47, 63-65, 69, 71, 75-78, 81, 83, 85-90 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. phase (=physical state): solid, liquid, or gas Solids have the lowest kinetic energy (KE)—i.e. do not move very much – Highest attraction between particles → particles are stuck in specific sites = very confined Liquids have slightly higher KE—i.e. particles moving more than in solid – Particles are still attracted and maintain contact but can move past one another → particles are less confined Gases have greatest KE—i.e. particles move quickly and randomly – Attractive forces almost (if not) completely overcome, so particles can fly freely within container → particles are far away from each other = unrestrictedCHEM 139: Zumdahl Chapter 14 page 2 of 16 condensed states: solid and liquid phases – In solids and liquids, molecules are in close contact and attracted to one another. → These attractions are called intermolecular forces. Example: What is the biggest difference at the molecular level between particles in a gas and particles in a liquid? 14.2 ENERGY REQUIREMENTS FOR THE CHANGES OF STATE – To undergo a change in physical state (e.g. solid → liquid or liquid → gas), particles on the solid or liquid must overcome the intermolecular forces with surrounding molecules. – Thus, for a liquid or solid to become a gas, the thermal energy (associated with motion) of particles in the liquid/solid must be great enough to overcome the attractive forces with surrounding particles in the liquid/solid. Those attractive forces are ionic bonds, covalent bonds, metallic bonds, or intermolecular forces.CHEM 139: Zumdahl Chapter 14 page 3 of 16 14.3 INTERMOLECULAR FORCES (IMF’s) Dispersion (or London) Forces (also called Induced-Dipole Forces) – In nonpolar molecules (shown as green) the electrons can shift and concentrate on one end → temporary dipole (red = − end; blue = + end) – The partial positive charge caused by the temporary dipole in one molecule causes the electrons in an adjacent molecule to concentrate around the atom closest to the first molecule. → A temporary dipole results in the second molecule. → The temporary dipoles in both molecules cause them to be attracted to one another. – But that attraction lasts only until the electrons shift again, and the temporary dipoles go away. – Every molecule experiences London forces. – They are the ONLY type of IMFs between nonpolar molecules – Polar molecules also experience London forces they usually have other types of IMFs that are stronger than London forces. The strength of London forces is determined by the number of electrons. – The bigger the molecule → more electrons → the greater its polarizability (ability to distort electron clouds to get a temporary dipole) → the stronger its London/dispersion forces – Thus, the more electrons in an atom or molecule → The stronger the London/Dispersion forces Dipole-Dipole Forces: Attraction between polar molecules – generally stronger than dispersion forces because attraction is due to permanent dipoles. – The permanent dipoles mean the molecules are always attracted to nearby molecules. → Dipole-dipole forces are stronger than London forces for molecules of similar size (or number of electrons).CHEM 139: Zumdahl Chapter 14 page 4 of 16 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 139: Zumdahl Chapter 14 page 5 of 16 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: 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 or NO c. Cl2 or Br2 b. H2S or H2O d. PH3 or CH4 Is the molecule polar