WS 5:Bond Order and Bond strength1

In covalent bonding, atoms fill their outer valence shells by sharing electrons with neighboring atoms. Each atom in a covalent bond “counts” the shared electrons as belonging entirely to itself. The bond order is the number of electron pairs being shared by a pair of bonded atoms. The covalent bond in H2, HCl, or Cl2 is a single bond. This bond consists of a single pair of electrons. A single bond has a bond order of 1. Single bonds are the most common type of bond, but many molecules (and polyatomic ions) contain multiple bonds. Multiple bonds most frequently involve C, O, N, and sometimes P and S. There are two types of multiple bonds: double and triple. A double bond consists of two bonding electron pairs, four electrons shared between two atoms, so the bond order is 2. Ethylene (C2H4) is a simple hydrocarbon that contains a carbon-carbon double bond. A triple bond consists of three bonding pairs; two atoms share six electrons so the bond order is 3. In the N2 molecule, the atoms are held together by a triple bond. Six shared electrons and two unshared electrons give each nitrogen an octet. Bond numbers can be fractions as well as whole numbers. (Think resonance!) The strength of a covalent bond depends on the magnitude of the mutual attraction between bonded nuclei and shared electrons. The more electron density that is located between the atoms, the stronger the bond. The bond energy bond enthalpy or bond strength is the energy required to overcome this attraction and is defined as the standard enthalpy change for breaking the bond in mole of gaseous molecules. Bond breaking is always positive. A—B(g) ® A(g) + B(g) ∆H°Bond breaking =BE(A—B); +BE(A—B) >0. The same amount of energy that is absorbed to break the bond is the energy released when the bond forms. A(g) + B(g) ® A—B(g) ∆H°bond forming =–BE(A—B); BE(A—B) <0. Bond energies depend on characteristics of the bonded atoms, their electron configurations, nuclear charges, and atomic radii; each bond type has its own energy. Stronger bonds are larger in energy (have a deeper energy well), weaker bonds are lower in energy (have shallower energy wells). The energy of a given type of bond varies slightly from molecule-to-molecule, and even with in the same molecule. The bond energies given below are averages (Table 2)

1 Background reading for this handout can be found in sections 8.8 & 9.7 of Brown and LeMay.

WS 5-bond order & strength.docx Page 1 of 6 Alscher Table 1: Electronegativities H 2.1 Li Be B C N O F 1.0 1.5 2.0 2.5 3.0 3.5 4.0 Na Mg Electronegativities of various elements Al Si P S Cl 0.9 1.2 1.5 1.8 2.1 2.5 3.0 K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br 0.8 1.0 1.3 1.5 1.6 1.6 1.5 1.8 1.9 1.9 1.9 1.6 1.6 1.8 2.0 2.4 2.8 Rb Sr Y Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn Sb Te I 0.8 1.0 1.2 1.4 1.6 1.8 1.9 2.2 2.2 2.2 1.9 1.7 1.7 1.8 1.9 2.1 2.5 Cs Ba La Hf Ta W Re Os Ir Pt Au Hg Tl Pb Bi Po At 0.7 0.9 1.0 1.3 1.5 1.7 1.9 2.2 2.2 2.2 2.4 1.9 1.8 1.9 1.9 2.0 2.2 Table 2: Average bond enthalpies (kJ/mol) Single bonds C—H 413 N—H 391 O—H 463 F—F 155 C—C 348 N—N 163 O—O 146 C—N 293 N—O 201 O—F 190 Cl—F 253 C—O 358 N—F 272 O—Cl 203 Cl—Cl 242 C—F 485 N—Cl 200 O—I 234 C—Cl 328 N—Br 243 Br—F 237 C—Br 276 N—P 209 S—H 339 Br—Cl 218 C—I 240 N—I 159 S—F 327 Br—Br 193 C—S 259 S—Cl 253 C—P 264 H—H 436 S—Br 218 I—Cl 208 H—F 567 S—S 266 I—Br 175 H—Cl 431 I—I 151 H—Br 366 H—I 299 Multiple bonds C=C 614 N=N 418 O2 495 CºC 839 NºN 941 C=N 615 N=O 607 S=O 523 CºN 891 S=S 418 C=O 745 C=O 799 for carbon dioxide CºO 1072 C=O 745

A covalent bond has a characteristic bond length, the distance between nuclei of two bonded atoms. Here too, we look at the average bond lengths for the given bond in different substances. A close relationship exists among bond order, bond length, and bond energy. Two nuclei are more strongly attracted to two shared electron pairs than to one. For a given pair of atoms, a higher bond order results in shorter bond lengths and higher bond energy. One way of thinking about bond enthalpy and bond length is to examine a potential energy diagram for the atoms in question. The potential energy diagram for molecular hydrogen is shown below. The potential energy of two hydrogen atoms is plotted against the distance between the nuclei, with a depiction of the atomic systems above. As we look at the plot from right to left, first we see the

WS 5-bond order & strength.docx Page 2 of 6 Alscher atoms are too far apart to attract each other. As the atoms approach each other, the nuclei become attracted to each other. Soon the attractions between the nuclei of one atom and the electron of the other atom and the repulsion of electrons and repulsions of the two nuclei are balanced. We see the molecule is in a deep energy well. Here, the energy is at a minimum. The bond energy is the energy released when bonds are formed and absorbed when bonds are broken. When the nuclei get too close together, the repulsions increase the systems energy.

WS 5-bond order & strength.docx Page 3 of 6 Alscher

Energy

absorbed when bond breaks, the bond enthalpy

Potential energy (kJ/mol)

74pm Inter-nuclear Separation

Bond orders and bond energies for selected molecules. molecule Bond Bond order Bond energy (kJ/mol) HF H—F 1 565 HCl H—Cl 1 427 HI H—I 1 363 Cl2 Cl—Cl 1 295 Br2 Br—Br 1 193 I2 I—I 1 151 CH3CH3 C—C 1 376 CH2CH2 C=C 2 720 CO2 C=O 2 799 H2CO C=O 2 782 N2 NºN 3 945 HCCH CºC 3 962

Questions to think about:

WS 5-bond order & strength.docx Page 4 of 6 Alscher 1. Consider the bond order for each H—X bond [H—F, H—Cl, H—Br, H—I]. What is the bond order for each H—X? What trend is observed in bond energy?

2. Considering the relative size of F, Cl, Br, and I, what trend would you predict in H—X bond length?

3. Consider the relative size of Cl, Br, and I, what trend would you predict in the X—X bond length? How does this compare for the trend of the bond energies?

4. What statement can you make that summarizes the trends of bond lengths and strengths?

5. Using a grammatically correct English sentence, describe the relation ships between bond order, bond energy, and the number of electrons shared in a bond.

6. Consider the molecule CH3X, where X = F, Cl, Br, I. Based on atomic radii, which do you expect to have the longest bond length, C—F, C—Cl, C—Br, C—I? Explain your reasoning. Which of these will have the strongest bond? Explain your reasoning.

7. Which do you think has a longer bond length: the N—H bond in ammonia or the N—F bond in nitrogen tri-fluoride? Explain your reasoning.

8. Which do you think has a stronger bond : the C—Cl bond in carbon tetra-chloride or the C—Br bond in carbon tetra-bromide? Explain your reasoning.

9. Which do you think easier to break: C—N bond in CH3NH2 or C—N bond in hydrogen cyanide? Explain your reasoning.

WS 5-bond order & strength.docx Page 5 of 6 Alscher

10. Do problem 8.54, on page 325 in the book.

11. Using the structure of nitroglycerin (in book or on internet), calculate the ∆H for the reaction of 1 mole of nitroglycerin decomposing to produce nitrogen gas, molecular oxygen, gaseous water, and carbon dioxide.

WS 5-bond order & strength.docx Page 6 of 6 Alscher