Chem:Chapter 9
Bond Energies and Bond Lengths 82. Ethanol is a possible fuel. Use average bond energies to calculate ∆Hrxn for the combustion of ethanol. CH3CH2OH( g) + 3 O2( g) ¡ 2 CO2( g) + 3 H2O( g)
-1245 kJ
Challenge Problems 120. The standard heat of formation of CaBr2 is -675 kJ/mol. The first ionization energy of Ca is 590 kJ/mol, and its second ionization energy is 1145 kJ/mol. The heat of sublimation of Ca3Ca(s) -> Ca( g)4 is 178kJ/mol. The bond enSergy of Br2 is 193 kJ/mol, the heat of vaporization of Br2(l) is 31 kJ/mol, and the electron affinity of Br is -325 kJ>mol. Calculate the lattice energy of CaBr2.
-2162 kJ mol-1
Simple Covalent Lewis Structures, Electronegativity, and Bond Polarity 56. Determine if a bond between each pair of atoms would be pure covalent, polar covalent, or ionic. a. C and N b. N and S c. K and F d. N and N
a) polar covalent b) polar covalent c) ionic d) pure covalent
Ionic Lewis Symbols and Lattice Energy 48. Use the Born-Haber cycle and data from Appendix IIB and Table 9.3 to calculate the lattice energy of CaO. (∆Hsub for calcium is 178 kJ/mol; IE1 and IE2 for calcium are 590 kJ/mol and 1145 kJ/mol, respectively; EA1 and EA2 for O are -141 kJ/mol and 744 kJ/mol, respectively.)
-3400 kJ mol-1
Review 24. What is the basic procedure for writing a covalent Lewis structure?
Determine the total number of valence electrons in the molecule or polyatomic ion. • Count the total number of valence electrons again! • Draw the skeletal structure. • Assign electrons in pairs around the terminal atoms to give each an octet. • Assign remaining electrons to the central atom as lone pairs. • If the central atom is still lacking an octet, move lone pairs from terminal atoms to form multiple bonds.
Review 19. What is electronegativity? What are the periodic trends in electronegativity?
Electronegativity is the tendency of an atom to draw the electrons in a covalent bond toward itself. Electronegativity increases across a period and decreases down a group.
Review 32. Explain the difference between endothermic reactions and exothermic reactions with respect to the bond energies of the bonds broken and formed
In an endothermic reaction stronger bonds are broken in the reactants and weaker bonds are formed in the products; in an exothermic reaction weaker bonds are broken in the reactants and stronger bonds are formed in the products.
Ionic Lewis Symbols and Lattice Energy 44. Rubidium iodide has a lattice energy of -617 kJ/mol, while potassium bromide has a lattice energy of -671 kJ/mol. Why is the lattice energy of potassium bromide more exothermic than the lattice energy of rubidium iodide?
K+ and Br- are both smaller ions than Rb+ and I-. Lattice energy becomes more negative (more exothermic) with decreasing interionic distance.
Review 12. How does lattice energy relate to ionic radii? To ion charge?
Lattice energy decreases with increasing ionic radius, and increases with increasing magnitude of ionic charge.
Covalent Lewis Structures, Resonance, and Formal Charge 66. Use formal charges to identify the better Lewis structure (On Doc)
The better Lewis structure has zero formal charge on all atoms:
Review 3. What are the three basic types of chemical bonds? What happens to electrons in the bonding atoms in each type?
ionic bonds—electrons are transferred covalent bonds—electrons are shared between bonded atoms metallic bonds—electrons are pooled to form an "electron sea"
