Chemical Bonding I Basic Concepts

CHAPTER 9 CHEMICAL BONDING I: BASIC CONCEPTS 9.15 9.16 9.17 9.18 We use Coulomb’s law to answer this question: E = k Qcation Qanion r (a) Doubling the radius of the cation would increase the distance, r, between the centers of the ions. A larger value of r results in a smaller energy, E, of the ionic bond. Is it possible to say how much smaller E will be? (b) Tripling the charge on the cation will result in tripling of the energy, E, of the ionic bond, since the energy of the bond is directly proportional to the charge on the cation, Qcation. (c) Doubling the charge on both the cation and anion will result in quadrupling the energy, E, of the ionic bond. (d) Decreasing the radius of both the cation and the anion to half of their original values is the same as halving the distance, r, between the centers of the ions. Halving the distance results in doubling the energy. (a) RbI, rubidium iodide (b) Cs2SO4, cesium sulfate (c) Sr3N2, strontium nitride (d) Al2S3, aluminum sulfide Lewis representations for the ionic reactions are as follows. + − + 2− (a) Na + F Na F (b) 2K + S 2K S (c) Ba + O Ba O (d) Al + N Al N 2− 2+ 3+ 3− The Lewis representations for the reactions are: 2+ (a) Sr + Se Sr (b) Ca + 2H Ca 2H (c) 3Li + N 3Li N (d) 2Al + 3S Se 2+ + 3+ 2− − 3− 2Al 3 S 2− CHAPTER 9: CHEMICAL BONDING I: BASIC CONCEPTS 9.19 241 (a) I and Cl should form a molecular compound; both elements are nonmetals. One possibility would be ICl, iodine chloride. (b) Mg and F will form an ionic compound; Mg is a metal while F is a nonmetal. The substance will be MgF2, magnesium fluoride. 9.20 (a) Covalent (BF3, boron trifluoride) 9.25 (1) Na(s) → Na(g) ΔH1o = 108 kJ/mol (2) 12 Cl2(g) → Cl(g) ΔH 2o = 121.4 kJ/mol + − ΔH 3o = 495.9 kJ/mol (3) Na(g) → Na (g) + e − − ΔH 4o = − 349 kJ/mol (4) Cl(g) + e → Cl (g) + − (5) Na (g) + Cl (g) → NaCl(s) Na(s) + 1 2 (b) ionic (KBr, potassium bromide) Cl2(g) → NaCl(s) ΔH 5o = ? o ΔH overall = − 411 kJ/mol o ΔH 5o = ΔH overall − ΔH1o − ΔH 2o − ΔH 3o − ΔH 4o = (−411) − (108) − (121.4) − (495.9) − (−349) = − 787 kJ/mol The lattice energy of NaCl is 787 kJ/mol. 9.26 (1) Ca(s) → Ca(g) ΔH1o = 121 kJ/mol (2) Cl2(g) → 2Cl(g) ΔH 2o = 242.8 kJ/mol (3) Ca(g) → Ca (g) + e + + − 2+ ΔH 3o ' = 589.5 kJ/mol − Ca (g) → Ca (g) + e − − ΔH 3o " = 1145 kJ/mol ΔH 4o = 2(−349 kJ/mol) = − 698 kJ/mol (4) 2[Cl(g) + e → Cl (g)] (5) Ca (g) + 2Cl (g) → CaCl2(s) ΔH 5o = ? Ca(s) + Cl2(g) → CaCl2(s) o ΔH overall = − 795 kJ/mol 2+ − Thus we write: o ΔH overall = ΔH1o + ΔH 2o + ΔH 3o ' + ΔH 3o " + ΔH 4o + ΔH 5o ΔH 5o = (−795 − 121 − 242.8 − 589.5 − 1145 + 698)kJ/mol = − 2195 kJ / mol The lattice energy is represented by the reverse of equation (5); therefore, the lattice energy is +2195 kJ/mol. 9.35 The degree of ionic character in a bond is a function of the difference in electronegativity between the two bonded atoms. Figure 9.5 lists electronegativity values of the elements. The bonds in order of increasing ionic character are: N−N (zero difference in electronegativity) < S−O (difference 1.0) = Cl−F (difference 1.0) < K−O (difference 2.7) < Li−F (difference 3.0). 242 CHAPTER 9: CHEMICAL BONDING I: BASIC CONCEPTS 9.36 Strategy: We can look up electronegativity values in Figure 9.5 of the text. The amount of ionic character is based on the electronegativity difference between the two atoms. The larger the electronegativity difference, the greater the ionic character. Solution: Let ΔEN = electronegativity difference. The bonds arranged in order of increasing ionic character are: C−H (ΔEN = 0.4) < Br−H (ΔEN = 0.7) < F−H (ΔEN = 1.9) < Li−Cl (ΔEN = 2.0) < Na−Cl (ΔEN = 2.1) < K−F (ΔEN = 3.2) 9.37 We calculate the electronegativity differences for each pair of atoms: DE: 3.8 − 3.3 = 0.5 DG: 3.8 − 1.3 = 2.5 EG: 3.3 − 1.3 = 2.0 DF: 3.8 − 2.8 = 1.0 The order of increasing covalent bond character is: DG < EG < DF < DE 9.38 The order of increasing ionic character is: Cl−Cl (zero difference in electronegativity) < Br−Cl (difference 0.2) < Si−C (difference 0.7) < Cs−F (difference 3.3). 9.39 9.40 9.43 (a) The two carbon atoms are the same. The bond is covalent. (b) The elelctronegativity difference between K and I is 2.5 − 0.8 = 1.7. The bond is polar covalent. (c) The electronegativity difference between N and B is 3.0 − 2.0 = 1.0. The bond is polar covalent. (d) The electronegativity difference between C and F is 4.0 − 2.5 = 1.5. The bond is polar covalent. (a) The two silicon atoms are the same. The bond is covalent. (b) The electronegativity difference between Cl and Si is 3.0 − 1.8 = 1.2. The bond is polar covalent. (c) The electronegativity difference between F and Ca is 4.0 − 1.0 = 3.0. The bond is ionic. (d) The electronegativity difference between N and H is 3.0 − 2.1 = 0.9. The bond is polar covalent. (a) Cl N Cl (b) O C S (c) H O O H H H H C C N H H H Cl H (d) H C O C O − (e) − C N (f) H H 9.44 (a) F O F (b) F N N F (c) H + H H Si Si H H H H CHAPTER 9: CHEMICAL BONDING I: BASIC CONCEPTS O H (d) − O H (e) H C C O − (f) H Cl 9.45 H H C N H H + 243 H The lewis dot structures are: (a) I P (b) H Cl (c) H (d) H P P H S H P P (e) H 9.46 N N H H H (f) H O Cl (g) O Br C Br O O Strategy: We follow the procedure for drawing Lewis structures outlined in Section 9.6 of the text. Solution: (a) Step 1: It is obvious that the skeletal structure is: O O 2 4 Step 2: The outer-shell electron configuration of O is 2s 2p . Also, we must add the negative charges to the number of valence electrons, Thus, there are (2 × 6) + 2 = 14 valence electrons Step 3: We draw a single covalent bond between each O, and then attempt to complete the octets for the O atoms. − O O − Because this structure satisfies the octet rule for both oxygen atoms, step 4 outlined in the text is not required. − Check: As a final check, we verify that there are 14 valence electrons in the Lewis structure of O2 . Follow the same procedure as part (a) for parts (b), (c), and (d). The appropriate Lewis structures are: H − (b) C − C (c) N + O (d) H + N H 9.47 (a) Too many electrons. The correct structure is H C N H 244 CHAPTER 9: CHEMICAL BONDING I: BASIC CONCEPTS (b) (c) (d) Hydrogen atoms do not form double bonds. The correct structure is H C O Sn C H Too few electrons. O Too many electrons. The correct structure is F F B F (e) Fluorine has more than an octet. The correct structure is O H (f) F Oxygen does not have an octet. The correct structure is O H (g) C F Too few electrons. The correct structure is F N F F 9.48 − (a) Neither oxygen atom has a complete octet. The left-most hydrogen atom is forming two bonds (4 e ). Hydrogen can only be surrounded by at most two electrons. (b) The correct structure is: H O H C C O H H Do the two structures have the same number of electrons? Is the octet rule satisfied for all atoms other than hydrogen, which should have a duet of electrons? CHAPTER 9: CHEMICAL BONDING I: BASIC CONCEPTS 9.51 The resonance structures are: O H - O H C C O O O H - O H + C C O - O H - + C N H - - + - N H 9.52 245 N H O O - Strategy: We follow the procedure for drawing Lewis structures outlined in Section 9.6 of the text. After we complete the Lewis structure, we draw the resonance structures. Solution: Following the procedure in Section 9.6 of the text, we come up with the following Lewis − structure for ClO3 . O − O + Cl − O We can draw two more equivalent Lewis structures with the double bond between Cl and a different oxygen atom. The resonance structures with formal charges are as follows: O O 9.53 + Cl N O − − O + O Cl O − − − + O Cl O + N N − − H + N N H N + + N N The structures of the most important resonance forms are: H H 9.55 O The structures of the most important resonance forms are: H 9.54 − C H + N − N − H C + N N − Three reasonable resonance structures for OCN are: O C − N − O C N + O C N 2− 2− N 246 CHAPTER 9: CHEMICAL BONDING I: BASIC CONCEPTS 9.56 Three reasonable resonance structures with the formal charges indicated are − 9.61 N + N O + N The resonance structures are + − I Al N O − − Al I I I + N N + + O I I I 9.62 2− − Al I I + Strategy: We follow the procedure outlined in Section 9.6 of the text for drawing Lewis structures. We assign formal charges as discussed in Section 9.7 of the text. Solution: Drawing the structure with single bonds between Be and each of the Cl atoms, the octet rule for Be is not satisfied. The Lewis structure is: Cl Be Cl An octet of electrons on Be can only be formed by making two double bonds as shown below: 2− + Cl Be Cl + This places a high negative formal charge on Be and positive formal charges on the Cl atoms. This structure distributes the formal charges counter to the electronegativities of the elements. It is not a plausible Lewis structure. 9.63 For simplicity, the three, nonbonding pairs of electrons around the fluorine atoms are omitted. F F (a) F Xe F (b) F Xe (c) F F O O F F Xe The octet rule is exceeded in each case. Xe F F (d) F F F (e) F Xe O F F F CHAPTER 9: CHEMICAL BONDING I: BASIC CONCEPTS 9.64 2 247 3 The outer electron configuration of antimony is 5s 5p . The Lewis structure is shown below. All five valence electrons are shared in the five covalent bonds. The octet rule is not obeyed. (The electrons on the chlorine atoms have been omitted for clarity.) Cl Cl Cl Sb Cl Cl Can Sb have an expanded octet? 9.65 For simplicity, the three, nonbonding pairs of electrons around the fluorine are omitted. F Se F F F F Se F F F F F The octet rule is not satisfied for Se in both compounds (why not?). 9.66 The reaction can be represented as: Cl Cl Al Cl + Cl − Cl Cl − Al Cl Cl The new bond formed is called a coordinate covalent bond. 9.69 The enthalpy change for the equation showing ammonia dissociating into a nitrogen atom and three hydrogen atoms is equal to three times the average bond enthalpy of the N−H bond (Why three?). NH3(g) → N(g) + 3H(g) ΔH° = 3ΔH°(N−H) The equation is the sum of the three equations given in the problem, and by Hess's law (Section 6.6 of the text) the enthalpy change is just the sum of the enthalpies of the individual steps. NH3(g) → NH2(g) + H(g) NH2(g) → NH(g) + H(g) NH(g) → N(g) + H(g) ΔH° = 435 kJ/mol ΔH° = 381 kJ/mol ΔH° = 360 kJ/mol NH3(g) → N(g) + 3H(g) ΔH° = 1176 kJ/mol ΔH °(N − H) = 9.70 1176 kJ/mol = 392 kJ/mol 3 Strategy: Keep in mind that bond breaking is an energy absorbing (endothermic) process and bond making is an energy releasing (exothermic) process. Therefore, the overall energy change is the difference between these two opposing processes, as described in Equation (9.3) of the text. 248 CHAPTER 9: CHEMICAL BONDING I: BASIC CONCEPTS Solution: There are two oxygen-to-oxygen bonds in ozone. We will represent these bonds as O−O. However, these bonds might not be true oxygen-to-oxygen single bonds. Using Equation (9.3) of the text, we write: ΔH° = ∑BE(reactants) − ∑BE(products) ΔH° = BE(O=O) − 2BE(O−O) In the problem, we are given ΔH° for the reaction, and we can look up the O=O bond enthalpy in Table 9.4 of the text. Solving for the average bond enthalpy in ozone, −2BE(O−O) = ΔH° − BE(O=O) BE (O − O) = BE (O=O) − ΔH o 498.7 kJ/mol + 107.2 kJ/mol = = 303.0 kJ/mol 2 2 Considering the resonance structures for ozone, is it expected that the O−O bond enthalpy in ozone is between the single O−O bond enthalpy (142 kJ) and the double O=O bond enthalpy (498.7 kJ)? 9.71 When molecular fluorine dissociates, two fluorine atoms are produced. Since the enthalpy of formation of atomic fluorine is in units of kJ/mol, this number is half the bond enthalpy of the fluorine molecule. F2(g) → 2F(g) ΔH° = 156.9 kJ/mol ΔH ° = 2ΔH fo (F) − ΔH fo (F2 ) 156.9 kJ/mol = 2ΔH fo (F) − (1)(0) ΔH fo (F) = 9.72 (a) Bonds Broken C−H C−C O=O Bonds Formed C=O O−H 156.9 kJ/mol = 78.5 kJ/mol 2 Number Broken 12 2 7 Number Formed 8 12 Bond Enthalpy (kJ/mol) 414 347 498.7 Bond Enthalpy (kJ/mol) 799 460 Enthalpy Change (kJ) 4968 694 3491 Enthalpy Change (kJ) 6392 5520 ΔH° = total energy input − total energy released = (4968 + 694 + 3491) − (6392 + 5520) = −2759 kJ/mol (b) ΔH ° = 4ΔH fo (CO 2 ) + 6ΔH fo (H 2 O) − [2ΔH fo (C2 H 6 ) + 7ΔH fo (O2 )] ΔH° = (4)(−393.5 kJ/mol) + (6)(−241.8 kJ/mol) − [(2)(−84.7 kJ/mol) + (7)(0)] = −2855 kJ/mol The answers for part (a) and (b) are different, because average bond enthalpies are used for part (a). 9.73 CH4, CO, and SiCl4 are covalent compounds. KF and BaCl2 are ionic compounds. CHAPTER 9: CHEMICAL BONDING I: BASIC CONCEPTS 9.74 249 Typically, ionic compounds are composed of a metal cation and a nonmetal anion. RbCl and KO2 are ionic compounds. Typically, covalent compounds are composed of two nonmetals. PF5, BrF3, and CI4 are covalent compounds. 9.75 9.76 (a) electron affinity of fluorine (b) bond enthalpy of molecular fluorine (c) ionization energy of sodium (d) standard enthalpy of formation of sodium fluoride Recall that you can classify bonds as ionic or covalent based on electronegativity difference. The melting points (°C) are shown in parentheses following the formulas. Ionic: NaF (993) MgF2 (1261) AlF3 (1291) Covalent: SiF4 (−90.2) PF5 (−83) SF6 (−121) ClF3 (−83) Is there any correlation between ionic character and melting point? 9.77 By Hess's law, the overall enthalpy (energy) change in a reaction is equal to the sum of the enthalpy (energy) changes for the individual steps. The reactions shown in the problem are just the sums of the ionization energy of the alkali metal and the electron affinity of the halogen. (a) Taking data from the referenced figures we have: + − Li(g) → Li (g) + e − − I(g) + e → I (g) ΔH° = 520 kJ/mol ΔH° = −295 kJ/mol + − Li(g) + I(g) → Li (g) + I (g) ΔH° = 225 kJ/mol Parts (b) and (c) are solved in an analogous manner. (b) (c) + − Na(g) + F(g) → Na (g) + F (g) + − K(g) + Cl(g) → K (g) + Cl (g) ΔH° = 163 kJ/mol ΔH° = 71 kJ/mol + − 9.78 KF is an ionic compound. It is a solid at room temperature made up of K and F ions. It has a high melting point, and it is a strong electrolyte. Benzene, C6H6, is a covalent compound that exists as discrete molecules. It is a liquid at room temperature. It has a low melting point, is insoluble in water, and is a nonelectrolyte. 9.79 The three pairs of nonbonding electrons around each fluorine have been omitted for simplicity. F F F Br F F F Cl F F F F F F I F F F The octet rule is not obeyed in any of the compounds. In order for the octet rule to be obeyed, what would the value of n in the compound ICln have to be? [Hint: see Problem 9.45 (a)] 250 CHAPTER 9: CHEMICAL BONDING I: BASIC CONCEPTS 9.80 The resonance structures are: − + N N N − + N N N 2− 2− N + N N Which is the most plausible structure based on a formal charge argument? 9.81 A resonance structure is: O + N C H 9.82 − (a) An example of an aluminum species that satisfies the octet rule is the anion AlCl4 . The Lewis dot structure is drawn in Problem 9.66. 3− An example of an aluminum species containing an expanded octet is anion AlF6 . (How many pairs of electrons surround the central atom?) An aluminum species that has an incomplete octet is the compound AlCl3. The dot structure is given in Problem 9.66. (b) (c) 9.83 Four resonance structures together with the formal charges are − O − O + P F O 9.84 − O O P O − O F O P O − − − F O O − P F O − CF2 would be very unstable because carbon does not have an octet. (How many electrons does it have?) + − LiO2 would not be stable because the lattice energy between Li and superoxide O2 would be too low to stabilize the solid. CsCl2 requires a Cs lattice energy. 2+ cation. The second ionization energy is too large to be compensated by the increase in PI5 appears to be a reasonable species (compared to PF5 in Example 9.10 of the text). However, the iodine atoms are too large to have five of them "fit" around a single P atom. 9.85 Reasonable resonance structures are: O (a) H O S O + O − O − H O S − + O − O O H O S − + O O O − H O S − 2+ O− O − CHAPTER 9: CHEMICAL BONDING I: BASIC CONCEPTS O (b) − O P O O − − O − O P O− O − − O O − + P O 251 O − − There are two more equivalent resonance structures to the first structure above. O O (c) H O S O − H O (d) − O S O O O − − O − S O O H O S − + O − − S+ O − There are two more equivalent resonance structures to the first structure. 9.86 (a) false (b) true (c) false (d) false For question (c), what is an example of a second-period species that violates the octet rule? 9.87 If the central atom were more electronegative, there would be a concentration of negative charges at the central atom. This would lead to instability. 9.88 The formation of CH4 from its elements is: → CH4(g) C(s) + 2H2(g) ⎯⎯ The reaction could take place in two steps: Step 1: C(s) + 2H2(g) ⎯⎯ → C(g) + 4H(g) o ΔH rxn = (716 + 872.8)kJ/mol = 1589 kJ/mol → CH4(g) Step 2: C(g) + 4H(g) ⎯⎯ o ΔH rxn ≈ − 4 × (bond energy of C − H bond) = −4 × 414 kJ/mol = −1656 kJ/mol Therefore, ΔH fo (CH 4 ) would be approximately the sum of the enthalpy changes for the two steps. See Section 6.6 of the text (Hess’s law). o o ΔH fo (CH 4 ) = ΔH rxn (1) + ΔH rxn (2) ΔH fo (CH 4 ) = (1589 − 1656)kJ/mol = − 67 kJ/mol The actual value of ΔH fo (CH 4 ) = − 74.85 kJ/mol. 9.89 (a) Bond broken: Bond made: C−H C−Cl o ΔH rxn = 414 − 338 = 76 kJ/mol ΔH° = 414 kJ/mol ΔH° = −338 kJ/mol 252 CHAPTER 9: CHEMICAL BONDING I: BASIC CONCEPTS (b) Bond broken: Bond made: ΔH° = 414 kJ/mol ΔH° = −431.9 kJ/mol C−H H−Cl o ΔH rxn = 414 − 431.9 = − 18 kJ/mol Based on energy considerations, reaction (b) will occur readily since it is exothermic. Reaction (a) is endothermic. 9.90 Only N2 has a triple bond. Therefore, it has the shortest bond length. 9.91 The rest of the molecule (in this problem, unidentified) would be attached at the end of the free bond. (a) O O O O C (b) H O C − − C O + 9.92 To be isoelectronic, molecules must have the same number and arrangement of valence electrons. NH4 and CH4 are isoelectronic (8 valence electrons), as are CO and N2 (10 valence electrons), as are B3N3H6 and C6H6 (30 valence electrons). Draw Lewis structures to convince yourself that the electron arrangements are the same in each isoelectronic pair. 9.93 The Lewis structures are: H paramagnetic (a) C + (d) H 9.94 - C diamagnetic N H paramagnetic (b) O (e) H O paramagnetic C The reaction can be represented as: H N− + H O + −O H H N H H H 9.95 (c) C H The Lewis structures are: (a) F F C C F F (b) H H H H C C C H H H C C H H (c) H C C C C H H H H H (d) H C H H C diamagnetic CHAPTER 9: CHEMICAL BONDING I: BASIC CONCEPTS 253 O (e) C O H − 9.96 The central iodine atom in I3 has ten electrons surrounding it: two bonding pairs and three lone pairs. The central iodine has an expanded octet. Elements in the second period such as fluorine cannot have an − expanded octet as would be required for F3 . 9.97 See Problem 9.71. The bond enthalpy for F2 is: F2(g) → F(g) + F(g) + ΔH° = 156.9 kJ/mol − The energy for the process F2(g) → F (g) + F (g) can be found by Hess's law. Thus, F2(g) → F(g) + F(g) + − F(g) → F (g) + e − − F(g) + e → F (g) + ΔH° = 156.9 kJ/mol ΔH° = 1680 kJ/mol [I1(F), see Table 8.2 of the text] ΔH° = −328 kJ/mol [EA1(F), see Table 8.3 of the text] − F2(g) → F (g) + F (g) ΔH° = 1509 kJ/mol It is much easier to dissociate F2 into two neutral F atoms than it is to dissociate it into a fluorine cation and anion. 9.98 The skeletal structure is: H H C N C O H The number of valence electron is: (1 × 3) + (2 × 4) + 5 + 6 = 22 valence electrons We can draw two resonance structures for methyl isocyanate. H H C H N C H 9.99 O H C + N C O − H A reasonable Lewis structure is: O Cl 9.100 O N + O - (a) This is a very good resonance form; there are no formal charges and each atom satisfies the octet rule. (b) This is a second choice after (a) because of the positive formal charge on the oxygen (high electronegativity). 254 9.101 CHAPTER 9: CHEMICAL BONDING I: BASIC CONCEPTS (c) This is a poor choice for several reasons. The formal charges are placed counter to the electronegativities of C and O, the oxygen atom does not have an octet, and there is no bond between that oxygen and carbon! (d) This is a mediocre choice because of the large formal charge and lack of an octet on carbon. For C4H10 and C5H12 there are a number of structural isomers. C2H6 H C5H12 H 9.102 H H C C H H H H H H H C C C C H H H H H H H H H C C C C C H H H H H H The nonbonding electron pairs around Cl and F are omitted for simplicity. F C F Cl Cl F Cl 9.104 H H Cl 9.103 C4H10 C H C F F F Cl Cl F H C C F F F The structures are (the nonbonding electron pairs on fluorine have been omitted for simplicity): H H H H H C C C C C H F H (a) H H H H H H C C C C H H H Using Equation (9.3) of the text, ΔH = ∑BE(reactants) − ∑BE(products) ΔH = [(436.4 + 151.0) − 2(298.3)] = −9.2 kJ/mol (b) Using Equation (6.18) of the text, ΔH ° = 2ΔH fo [HI( g )] − {ΔH fo [H 2 ( g )] + ΔH fo [I2 ( g )]} ΔH° = (2)(25.9 kJ/mol) − [(0) + (1)(61.0 kJ/mol)] = −9.2 kJ/mol H CHAPTER 9: CHEMICAL BONDING I: BASIC CONCEPTS 9.105 Note that the nonbonding electron pairs have been deleted from oxygen, nitrogen, sulfur, and chlorine for simplicity. (a) H H C O (b) H H H C C H H H H (d) H H H C N H (f) H O H H N C N H H O (c) C 2H 5 H 5C 2 Pb C 2H 5 (e) H H Cl C C H H H S (g) H H O H N C C H H C C H H O H Cl H Note: in part (c) above, ethyl = 9.106 255 The Lewis structures are: − (a) C + (b) N O C2H 5 = H + − H H C C H H (c) C O N (d) N 9.107 O 2− O O oxide 2− peroxide O O − superoxide 9.108 True. Each noble gas atom already has completely filled ns and np subshells. 9.109 The resonance structures are: (a) N C O − − N C O 2− N C + O N C 2H 5 256 CHAPTER 9: CHEMICAL BONDING I: BASIC CONCEPTS (b) − C + N O − 2− C + N 3− O C + O + N In both cases, the most likely structure is on the left and the least likely structure is on the right. 9.110 (a) − − − The bond enthalpy of F2 is the energy required to break up F2 into an F atom and an F ion. − − → F(g) + F (g) F2 (g) ⎯⎯ We can arrange the equations given in the problem so that they add up to the above equation. See Section 6.6 of the text (Hess’s law). − − F2 (g) ⎯⎯ → F2(g) + e ΔH° = 290 kJ/mol F2(g) ⎯⎯ → 2F(g) ΔH° = 156.9 kJ/mol − F(g) + e − ΔH° = −333 kJ/mol ⎯⎯ → F (g) − − F2 (g) ⎯⎯ → F(g) + F (g) − The bond enthalpy of F2 is the sum of the enthalpies of reaction. − BE(F2 ) = [290 + 156.9 + (−333 kJ)]kJ/mol = 114 kJ/mol (b) − The bond in F2 is weaker (114 kJ/mol) than the bond in F2 (156.9 kJ/mol), because the extra electron increases repulsion between the F atoms. 9.111 The description involving a griffin and a unicorn is more appropriate. Both mule and donkey are real animals whereas resonance structures are nonexistent. 9.112 In (a) there is a lone pair on the C atom and the negative formal charge is on the less electronegative C atom. 9.113 Except for nitric oxide, lone pairs of electrons have been left off for simplicity. H−H (hydrogen), O=O (oxygen), N≡N (nitrogen), F−F (fluorine), Cl−Cl (chlorine), Br−Br (bromine), I−I (iodine), C≡O (carbon monoxide), H−F (hydrogen fluoride), H−Cl (hydrogen chloride), H−Br (hydrogen bromide), H−I (hydrogen iodide), and N 9.114 (a) N O (nitric oxide). − O N + O The first structure is the most important. Both N and O have formal charges of zero. In the second structure, the more electronegative oxygen atom has a formal charge of +1. Having a positive formal charge on an highly electronegative atom is not favorable. In addition, both structures leave one atom with an incomplete octet. This cannot be avoided due to the odd number of electrons. (b) It is not possible to draw a structure with a triple bond between N and O. N O Any structure drawn with a triple bond will lead to an expanded octet. Elements in the second row of the period table cannot exceed the octet rule. CHAPTER 9: CHEMICAL BONDING I: BASIC CONCEPTS 9.115 The unpaired electron on each N atom will be shared to form a covalent bond: O2 N + NO 2 − 9.116 O + N O + N 2 O4 O N O − − O + N O O − + − O N + N O + O − N O O − O + N O O N C O O One resonance form showing formal charges is: O + N − O + O N O O − 9.118 Cl Cl Al Cl Cl Al Cl Cl The arrows indicate coordinate covalent bonds. 9.119 + O − The OCOO structure violates the octet rule (expanded octet). The structure shown below satisfies the octet 2− rule with 22 valence electrons. However, CO3 has 24 valence electrons. Adding two more electrons to the structure would cause at least one atom to exceed the octet rule. O 9.117 257 (a) O H (b) The O−H bond is quite strong (460 kJ/mol). To complete its octet, the OH radical has a strong tendency to form a bond with a H atom. (c) One C−H bond is being broken, and an O−H bond is being formed. ΔH = ΣBE(reactants) − ΣBE(products) ΔH = 414 − 460 = −46 kJ/mol (d) Energy of one O − H bond = λ = 460 × 103 J 1 mol × = 7.64 × 10−19 J/bond 1 mol 6.022 × 1023 bonds hc (6.63 × 10−34 J ⋅ s)(3.00 × 108 m / s) = ΔH 7.64 × 10−19 J λ = 2.60 × 10 −7 m = 260 nm 258 CHAPTER 9: CHEMICAL BONDING I: BASIC CONCEPTS 9.120 There are four C−H bonds in CH4, so the average bond enthalpy of a C−H bond is: 1656 kJ/mol = 414 kJ/mol 4 The Lewis structure of propane is: H H H H C C C H H H H There are eight C−H bonds and two C−C bonds. We write: 8(C−H) + 2(C−C) = 4006 kJ/mol 8(414 kJ/mol) + 2(C−C) = 4006 kJ/mol 2(C−C) = 694 kJ/mol 694 kJ/mol = 347 kJ/mol 2 So, the average bond enthalpy of a C−C bond is: 9.121 Three resonance structures with formal charges are: + − O S O O S + O O S O − According to the comments in Example 9.11 of the text, the second and third structures above are more important. 9.122 (a) (c) H H C C H Cl (b) H H H H H H C C C C C C H Cl H Cl H Cl In the formation of poly(vinyl chloride) form vinyl chloride, for every C=C double bond broken, 2 C−C single bonds are formed. No other bonds are broken or formed. The energy changes for 1 mole of vinyl chloride reacted are: total energy input (breaking C=C bonds) = 620 kJ total energy released (forming C−C bonds) = 2 × 347 kJ = 694 kJ ΔH° = 620 kJ − 694 kJ = −74 kJ The negative sign shows that this is an exothermic reaction. To find the total heat released when 3 1.0 × 10 kg of vinyl chloride react, we proceed as follows: heat released = (1.0 × 106 g C2 H3Cl) × 1 mol C2 H3Cl −74 kJ × = − 1.2 × 106 kJ 62.49 g C2 H3Cl 1 mol C2 H3Cl CHAPTER 9: CHEMICAL BONDING I: BASIC CONCEPTS 9.123 259 Work done = force × distance = (2.0 × 10 −9 = 4 × 10 −19 = 4 × 10 −19 N) × (2 × 10 −10 m) N⋅m J to break one bond Expressing the bond enthalpy in kJ/mol: 4 × 10−19 J 1 kJ 6.022 × 1023 bonds × × = 2 × 102 kJ/mol 1 bond 1000 J 1 mol 9.124 EN (O) = 1314 + 141 = 727.5 2 EN (F) = 1680 + 328 = 1004 2 EN (Cl) = 1251 + 349 = 800 2 Using Mulliken's definition, the electronegativity of chlorine is greater than that of oxygen, and fluorine is still the most electronegative element. We can convert to the Pauling scale by dividing each of the above by 230 kJ/mol. EN (O) = 727.5 = 3.16 230 EN (F) = 1004 = 4.37 230 EN (Cl) = 800 = 3.48 230 These values compare to the Pauling values for oxygen of 3.5, fluorine of 4.0, and chlorine of 3.0. 9.125 F F H C C F Cl F Br H F C C F H C C F Cl 9.127 C F F enflurane H O C F Cl F isoflurane 9.126 O Cl F halothane F H H F C C Cl F H O C H H methoxyflurane + − (1) You could determine the magnetic properties of the solid. An Mg O solid would be paramagnetic 2+ 2− while Mg O solid is diamagnetic. (2) You could determine the lattice energy of the solid. Mg O would have a lattice energy similar to + − 2+ 2− Na Cl . This lattice energy is much lower than the lattice energy of Mg O . + − The equations for the preparation of sulfuric acid starting with sulfur are: S(s) + O2(g) → SO2(g) 2SO2(g) + O2(g) → 2SO3(g) SO3(g) + H2O(l) → H2SO4(aq) 260 CHAPTER 9: CHEMICAL BONDING I: BASIC CONCEPTS The equations for the preparation of sulfuric acid starting with sulfur trioxide are: SO3(g) + H2SO4(aq) → H2S2O7(aq) H2S2O7(aq) + H2O(l) → 2H2SO4(aq) Formation of oleum Generation of sulfuric acid Based on the discussion in Example 9.11 of the text, there are two resonance structures for oleum. Formal charges other than zero are shown in the structures. O O 9.128 O S O S O O H O H − O 2+ − O S O O H − 2+ S O O H − We can arrange the equations for the lattice energy of KCl, ionization energy of K, and electron affinity of Cl, to end up with the desired equation. + − K (g) + Cl (g) → KCl(s) + − K(g) → K (g) + e − − Cl(g) + e → Cl (g) K(g) + Cl(g) → KCl(s) H 9.129 O H+ ΔH° = −699 kJ/mol (equation for lattice energy of KCl, reversed) ΔH° = 418.7 kJ/mol (ionization energy of K) ΔH° = −349 kJ/mol (electron affinity of Cl) ΔH° = (−699 + 418.7 + −349) kJ/mol = −629 kJ/mol H+ H+ H H (b) This is an application of Hess’s Law. + H H (a) 2H + H → H3 H2 → 2H + ΔH = −849 kJ/mol ΔH = 436.4 kJ/mol H + H2 → H3 + ΔH = −413 kJ/mol + + H + The energy released in forming H3 from H and H2 is almost as large as the formation of H2 from 2 H atoms. 9.130 From Table 9.4 of the text, we can find the bond enthalpies of C−N and C=N. The average can be calculated, and then the maximum wavelength associated with this enthalpy can be calculated. The average bond enthalpy for C−N and C=N is: (276 + 615) kJ/mol = 446 kJ/mol 2 We need to convert this to units of J/bond before the maximum wavelength to break the bond can be calculated. Because there is only 1 CN bond per molecule, there is Avogadro’s number of bonds in 1 mole of the amide group. 446 kJ 1 mol 1000 J × × = 7.41 × 10−19 J/bond 23 1 mol 1 kJ 6.022 × 10 bonds The maximum wavelength of light needed to break the bond is: λ max = hc (6.63 × 10−34 J ⋅ s)(3.00 × 108 m/s) = = 2.68 × 10−7 m = 268 nm −19 E 7.41 × 10 J CHAPTER 9: CHEMICAL BONDING I: BASIC CONCEPTS 9.131 N 9.132 (a) + N N + N N − − N + N N + N N N + N − N + N 261 N We divide the equation given in the problem by 4 to come up with the equation for the decomposition of 1 mole of nitroglycerin. C3H5N3O9(l) → 3CO2(g) + 52 H2O(g) + 32 N2(g) + 14 O2(g) We calculate ΔH° using Equation (6.18) and the enthalpy of formation values from Appendix 3 of the text. o ΔH rxn = ∑ nΔH fo (products) − ∑ mΔH fo (reactants) ( ) o ΔH rxn = (3)(−395.5 kJ/mol) + 52 (−241.8 kJ/mol) − (1)(−371.1 kJ/mol) = − 1413.9 kJ/mol Next, we calculate ΔH° using bond enthalpy values from Table 9.4 of the text. Bonds Broken Number Broken C−H C−C C−O N−O N=O 5 2 3 6 3 Bonds Formed Number Formed C=O O−H N≡N O=O 6 (5/2)(2) = 5 1.5 0.25 Bond Enthalpy (kJ/mol) 414 347 351 176 607 Bond Enthalpy (kJ/mol) 799 460 941.4 498.7 Enthalpy Change (kJ/mol) 2070 694 1053 1056 1821 Enthalpy Change (kJ/mol) 4794 2300 1412.1 124.7 From Equation (9.3) of the text: ΔH° = ∑BE(reactants) − ∑BE(products) ΔH° = (6694 − 8630.8) kJ/mol = −1937 kJ/mol The ΔH° values do not agree exactly because average bond enthalpies are used, and nitroglycerin is a liquid (strictly, the bond enthalpy values are for gases). (b) One mole of nitroglycerin generates, (3 + 2.5 + 1.5 + 0.25) = 7.25 moles of gas. One mole of an ideal gas occupies a volume of 22.41 L at STP. 7.25 mol × (c) 22.41 L = 162 L 1 mol We calculate the pressure exerted by 7.25 moles of gas occupying a volume of 162 L at a temperature of 3000 K. P = nRT (7.25 mol)(0.0821 L ⋅ atm / mol ⋅ K)(3000 K) = = 11.0 atm 162 L V