Back
Problem 58b
Mothballs are composed of naphthalene, C10H8, a molecule that consists of two six-membered rings of carbon fused along an edge, as shown in this incomplete Lewis structure:
(b) Do you expect the C—C bond lengths in the molecule to be similar to those of C—C single bonds, C ═ C double bonds, or intermediate between C—C single and C ═ C double bonds?
Problem 58c
Mothballs are composed of naphthalene, C10H8, a molecule that consists of two six-membered rings of carbon fused along an edge, as shown in this incomplete Lewis structure:
(c) Not all of the C—C bond lengths in naphthalene are equivalent. Based on your resonance structures, how many C—C bonds in the molecule do you expect to be shorter than the others?
Problem 59a
(a) Which of these compounds is an exception to the octet rule: carbon dioxide, water, ammonia, phosphorus trifluoride, or arsenic pentafluoride?
Problem 59b
(b) Which of these compounds or ions is an exception to the octet rule: borohydride (BH4-), borazine (B3N3H6, which is analogous to benzene with alternating B and N in the ring), or boron trichloride?
- Draw the dominant Lewis structures for these chlorine–oxygen molecules/ions: ClO, ClO-, ClO2-, ClO3-, ClO4-. Which of these do not obey the octet rule?
Problem 61
- For Group 13–17 elements in the third row of the periodic table and beyond, the octet rule is often not obeyed. A friend of yours says this is because these heavier elements are more likely to make double or triple bonds. Another friend of yours says that this is because the heavier elements are larger and can make bonds to more than four atoms at a time. Which friend is more correct?
Problem 62
- Draw the Lewis structures for each of the following ions or molecules. Identify those in which the octet rule is not obeyed; state which atom in each compound does not follow the octet rule; and state, for those atoms, how many electrons surround them: (a) HCl, (b) ICl5, (c) NO, (d) CF2Cl2, (e) I3-.
Problem 63
Problem 64
Draw the Lewis structures for each of the following molecules or ions. Identify instances where the octet rule is not obeyed; state which atom in each compound does not follow the octet rule; and state how many electrons surround these atoms: (a) PF6-, (b) BeCl2, (c) NH3, (d) XeF2O (the Xe is the central atom), (e) SO42- .
Problem 65a
In the vapor phase, BeCl2 exists as a discrete molecule. (a) Draw the Lewis structure of this molecule, using only single bonds. Does this Lewis structure satisfy the octet rule?
Problem 65b
In the vapor phase, BeCl2 exists as a discrete molecule. (b) What other resonance structures are possible that satisfy the octet rule?
Problem 65c
In the vapor phase, BeCl2 exists as a discrete molecule. (c) On the basis of the formal charges, which Lewis structure is expected to be dominant for BeCl2?
Problem 66
(a) Describe the molecule xenon trioxide, XeO3, using four possible Lewis structures, one each with zero, one, two, or three Xe¬O double bonds. (b) Do any of these resonance structures satisfy the octet rule for every atom in the molecule? (c) Do any of the four Lewis structures have multiple resonance structures? If so, how many resonance structures do you find? (d) Which of the Lewis structures in part (a) yields the most favorable formal charges for the molecule?
Problem 67a
There are many Lewis structures you could draw for sulfuric acid, H2SO4 (each H is bonded to an O). (a) What Lewis structure(s) would you draw to satisfy the octet rule?
Problem 67b
There are many Lewis structures you could draw for sulfuric acid, H2SO4 (each H is bonded to an O). (b) What Lewis structure(s) would you draw to minimize formal charge?
Problem 68a
Some chemists believe that satisfaction of the octet rule should be the top criterion for choosing the dominant Lewis structure of a molecule or ion. Other chemists believe that achieving the best formal charges should be the top criterion. Consider the dihydrogen phosphate ion, H2PO4-, in which the H atoms are bonded to O atoms. (a) What is the predicted dominant Lewis structure if satisfying the octet rule is the top criterion?
Problem 68b
Some chemists believe that satisfaction of the octet rule should be the top criterion for choosing the dominant Lewis structure of a molecule or ion. Other chemists believe that achieving the best formal charges should be the top criterion. Consider the dihydrogen phosphate ion, H2PO4-, in which the H atoms are bonded to O atoms. (b) What is the predicted dominant Lewis structure if achieving the best formal charges is the top criterion?
Problem 71a,c
State whether each of these statements is true or false. (a) The longer the bond, the stronger the bond. (c) A typical double bond length is in the 500–1000 pm range.
Problem 71e
State whether each of these statements is true or false. (e) The longer the bond, the more energy is stored chemical bonds.
- State whether each of these statements is true or false. (a) An oxygen–oxygen double bond is shorter than an oxygen–oxygen single bond. (b) The C—C bond in ethene is longer than the C—C bond in polyethene.
Problem 72
- We can define average bond enthalpies and bond lengths for ionic bonds, just like we have for covalent bonds. Which ionic bond is predicted to be stronger, Na—Cl or Ca—O?
Problem 73
- We can define average bond enthalpies and bond lengths for ionic bonds, just like we have for covalent bonds. Which ionic bond is predicted to have the smaller bond enthalpy, Li—F or Cs—F?
Problem 74
- Predict the length of the carbon–carbon bond you would expect if two H2C molecules reacted with each other by a combination reaction.
Problem 75
- Draw the Lewis structure for NO+ . Is the nitrogen– oxygen bond in NO+ longer, shorter, or the same length as the nitrogen–oxygen bond in NO? Explain. Draw the Lewis structure for NO+ .
Problem 76
- A new compound has a C—O bond length of 120 pm. Is this bond likely to be a single, double, or triple C—O bond?
Problem 77
- A new compound is made that has a C—N bond length of 118 pm. Is this bond likely to be a single, double, or triple C—N bond?
Problem 78
Problem 79a
Consider the lattice energies of the following Group 2A compounds: BeH2, 3205 kJ/mol; MgH2, 2791 kJ/mol; CaH2, 2410 kJ/mol; SrH2, 2250 kJ/mol; BaH2, 2121 kJ/mol. (a) What is the oxidation number of H in these compounds?
Problem 79c
Consider the lattice energies of the following Group 2A compounds: BeH2, 3205 kJ/mol; MgH2, 2791 kJ/mol; CaH2, 2410 kJ/mol; SrH2, 2250 kJ/mol; BaH2, 2121 kJ/mol. (c) Consider BeH2. Does it require 3205 kJ of energy to break one mole of the solid into its ions, or does breaking up one mole of solid into its ions release 3205 kJ of energy?
Problem 79d
Consider the lattice energies of the following Group 2A compounds: BeH2, 3205 kJ/mol; MgH2, 2791 kJ/mol; CaH2, 2410 kJ/mol; SrH2, 2250 kJ/mol; BaH2, 2121 kJ/mol. (d) The lattice energy of ZnH2 is 2870 kJ/mol. Considering the trend in lattice enthalpies in the Group 2 compounds, predict which Group 2 element is most similar in ionic radius to the Zn2+ ion.
- A metal oxide has a lattice energy in the range of 3300 kJ/mol. Is the charge on the metal ion M likely to be 1+, 2+, or 3+? Explain.
Problem 81
Problem 82a
The ionic compound CaO crystallizes with the same structure as sodium chloride (Figure 8.3). (a) In this structure, how many O2- are in contact with each Ca2+ ion (Hint: Remember the pattern of ions shown in Figure 8.3 repeats over and over again in all three directions.)