Bond Order and Bond Strength 1 (Are all bonds created equal?)

Transcription

1 hem 50 Activity on bond order and bond strength v. /07 Bond Order and Bond Strength (Are all bonds created equal?) Table. Bond orders and Lewis structures for selected molecules Molecule Lewis structure Bond Bond Order Bond Energy (kj/mol) 436 l l l l l 43 O O O O O O O 804 O O O N N N N N This activity is modified from hemistry: A Guided Inquiry (3/e) by R.S. Moog and J.J. Farrell, Wiley, 006. Page of 9

2 hem 50 Activity on bond order and bond strength v. /07 Q. There are three types of covalent bonds that can occur in molecules: single, double, and triple. Use Table to give an example of each type. Q. ow can you determine the bond order for any type of covalent bond (single, double, and triple)? Q3. What is the relationship between the bond order and the number of electrons shared by two adjacent atoms? Q4. The bond energy (kj/mol) is defined as the energy required to break the bond that holds two adjacent atoms together (expressed as kj per mole of these bonds). Based on the definition of bond energy and Table, rank the three types of bonds in Table in order of increasing strength. Page of 9

3 hem 50 Activity on bond order and bond strength v. /07 Table. More bond orders and Lewis structures. Molecule Lewis structure Bond Bond Order Bond Energy (kj/mol) F F 570 l l l 43 Br Br Br 366 I I I 98 l l l l l 43 Br Br Br Br Br 93 I I I I I 5 The bond length is defined as the distance between the nuclei of two bonding atoms. These distances are very small and often have units of Angströms (symbol Å, 0-0 m) Page 3 of 9

4 hem 50 Activity on bond order and bond strength v. /07 Q5. In the hydrogen halide series F, l, Br, and I: a) What is the bond order for each X bond (where X=halogen)? b) What trend is observed in the bond energy? c) Rank F, l, Br, and I in order of increasing size. onsidering their relative sizes, what trend would you predict in the X bond length? Q6. In the halogen series l, Br, and I : a) What is the bond order for each X X bond (where X=halogen)? b) What trend is observed in the bond energy? c) onsidering the relative sizes of these halogens, what trend would you predict in the X X bond length? Q7. Which of the following statements appears to be true, based on your understanding of Q5 and Q6? Then provide a brief explanation. a) The longer the bond, the stronger the bond. b) The shorter the bond, the stronger the bond. Q8. Based on both Tables and, a) What is the range of bond energies for all single bonds? b) What is the range of bond energies for all double bonds? c) What is the range of bond energies for all triple bonds? Page 4 of 9

5 hem 50 Activity on bond order and bond strength v. /07 Table 3. alculated bond orders and bond lengths for selected molecules. Name of molecule Lewis structure of molecule Bond order (predicted by Lewis structures) - Bond order (alculated) Bond length (alculated) (Å, 0-0 m) ethane ethene ethyne benzene* *Note: Benzene is drawn in its line-bond notation. If you have never learned this form of notation and no one in your group has either, then raise your hand and the instructor will be happy to help you. Q9. Why is the Lewis bond order for ethene given as in Table 3? Q0. ow do the calculated bond orders for ethane, ethene, and ethyne compare to those predicted by the Lewis structures? alculated values are given from hemistry: A Guided Inquiry (3/e) by R.S. Moog and J.J. Farrell, Wiley, 006. The authors used MOPA software. The calculations are generally in good agreement with experimental values. Page 5 of 9

6 hem 50 Activity on bond order and bond strength v. /07 Q. Based on the data in Table 3 for ethane, ethene, and ethyne, what is the relationship between bond order and bond length? Q. Predict the bond length for a molecule with a bond order of.5. Q3. ow do the calculated bond orders for benzene compare to the bond orders predicted by Lewis structures? Q4. Experimentally, we find that all six bonds have the same bond strength (or bond energy), 509 kj/mol. Is this fact more consistent with the bond orders predicted by the Lewis structure or with the calculated bond orders? Q5. What feature(s) of the Lewis structure of benzene, Table 3, is inconsistent with the calculated bond orders in benzene? Page 6 of 9

7 hem 50 Activity on bond order and bond strength v. /07 Figure. Resonance structures of benzene The structure from Table 3 is on the left. An alternative structure is provided on the right. Q6. What is the difference between these two Lewis structures? Q7. Are either of these structures consistent with the calculated bond order for benzene? Explain. Neither structure alone provides a good description of the true bonding that occurs in benzene. They are like snapshots of a molecule. These snapshots are called resonance structures and they are indicated by the double headed arrow. Any one snapshot of an object does not give as much information as an average of snapshots. Likewise, the best description of a molecule comes from the average of its resonance structures. This average is called the resonance hybrid. Thus, the resonance hybrid of benzene is one in which the bonds lengths are between that of a single bond and a double bond. Although either of the resonance structures shown above are adequate and commonly used, this structure has also been proposed to reflect the similarity in all the bonds in benzene: Resonance hybrid of benzene (an average of its resonance structures) It is not always easy to draw the resonance hybrid for any compound, so drawing one resonance structure is often adequate and the hybrid is implied. Page 7 of 9

8 hem 50 Activity on bond order and bond strength v. /07 Exercises This activity covers material in your textbook Moore, Stanitski, Jurs (Sections 8.6, 8.9). Read these sections to reinforce the concepts you have just learned in this activity. Recommended Questions for Review and Thought: hapter 8 #34-40 (even), 44, 64, 86. a) Which N bond is stronger? b) Which N bond is longer? c) What is the bond order of N in each molecule? N or N. Describe the relationship between bond order, bond energy, and the number of electrons shared in a bond. 3. onsider a molecule of the type X where X= F, l, Br, or I a) Based on atomic radii, which do you expect to have the longest bond length, F, l, Br, or I? Which do you expect to have the shortest bond length? Explain your reasoning. b) Which do you expect to have the strongest bond, F, l, Br, or I? Which do you expect to have the weakest bond? Explain your reasoning. 4. For each of the following, which has the stronger bond between the two atoms in bold? Give a brief explanation. (int: Write the Lewis structures for each molecule). a) 3 and 3 Si b) P and O c) S and O d) S and Se e) P and N f) F and N Page 8 of 9

9 hem 50 Activity on bond order and bond strength v. /07 5. Rank N, P, and As in order from weakest to strongest bond. 6. Indicate whether each of the following statements is true or false and explain your reasoning. a) The bonds in N 3 are shorter than the bonds in NF 3. b) The bonds in l 4 are stronger than those in Br 4. c) The carbon nitrogen bond in 3 N is easier to break than the carbon nitrogen bond in N. d) The carbon oxygen bond in O is the same as the carbon oxygen bond in 3 O. 7. The molecular formula of cyclobutadiene is 44 and one of the resonance structures is given below on the left: a) Give the other resonance structure for cclobutadiene. Draw it on the right. b) Based on both resonance structures, predict the bond order in cyclobutadiene. c) Predict the bond length in cyclobutadiene. 8. Put the following molecules in order of increasing N N bond length: N, NN, NN. 9. The typical bond length for N (bond order = ) is.47 Å. The typical bond length for O (bond order=) is expected to be: a).43 Å; b).47 Å; c).5 Å; d).05 Å; e) impossible to estimate from the information given. 0. Which molecule or ion has the strongest bond between the two atoms: O -, S -, F, l, or I? Explain. Page 9 of 9