PROBLEM \(\PageIndex{1}\) Using the bond energies in Table 7.3.1 , determine the approximate enthalpy change for each of the following reactions: a. \(\ce{H2}(g)+\ce{Br2}(g)⟶\ce{2HBr}(g)\) b. \(\ce{CH4}(g)+\ce{I2}(g)⟶\ce{CH3I}(g)+\ce{HI}(g)\) c. \(\ce{C2H4}(g)+\ce{3O2}(g)⟶\ce{2CO2}(g)+\ce{2H2O}(g)\) Answer a −114 kJ Answer b 30 kJ Answer c −1055 kJ PROBLEM \(\PageIndex{2}\) Using the bond energies in Table 7.3.1 , determine the approximate enthalpy change for each of the following reactions: a. \(\mathrm{H_2C=CH_2}(g)+\ce{H2}(g)⟶\ce{H3CCH3}(g)\) b. \(\ce{2C2H6}(g)+\ce{7O2}(g)⟶\ce{4CO2}(g)+\ce{6H2O}(g)\) Answer a -128 kJ Answer b -5175 kJ Click here to see a video of the solution PROBLEM \(\PageIndex{3}\) How does the bond energy of HCl differ from the standard enthalpy of formation of HCl( g )? Answer The enthalpy of formation is -431.6 kJ, while the bond energy of H-Cl is -432 kJ. They are practically the same. PROBLEM \(\PageIndex{4}\) Using the standard enthalpy of formation data in Appendix G , show how the standard enthalpy of formation of HCl( g ) can be used to determine the bond energy. Answer \(\ce{HCl}(g)⟶\dfrac{1}{2}\ce{H2}(g)+\dfrac{1}{2}\ce{Cl2}(g)\hspace{20px}ΔH^\circ_1=−ΔH^\circ_{\ce f[\ce{HCl}(g)]}\\ \dfrac{1}{2}\ce{H2}(g)⟶\ce{H}(g)\hspace{105px}ΔH^\circ_2=ΔH^\circ_{\ce f[\ce H(g)]}\\ \underline{\dfrac{1}{2}\ce{Cl2}(g)⟶\ce{Cl}(g)\hspace{99px}ΔH^\circ_3=ΔH^\circ_{\ce f[\ce{Cl}(g)]}}\\ \ce{HCl}(g)⟶\ce{H}(g)+\ce{Cl}(g)\hspace{58px}ΔH^\circ_{298}=ΔH^\circ_1+ΔH^\circ_2+ΔH^\circ_3\) \(\begin{align} D_\ce{HCl}=ΔH^\circ_{298}&=ΔH^\circ_{\ce f[\ce{HCl}(g)]}+ΔH^\circ_{\ce f[\ce H(g)]}+ΔH^\circ_{\ce f[\ce{Cl}(g)]}\\ &=\mathrm{−(−92.307\:kJ)+217.97\:kJ+121.3\:kJ}\\ &=\mathrm{431.6\:kJ} \end{align}\) PROBLEM \(\PageIndex{5}\) Using the standard enthalpy of formation data in Appendix G , determine which bond is stronger: the S–F bond in SF 4 ( g ) or in SF 6 ( g )? Answer The S–F bond in SF 4 is stronger. PROBLEM \(\PageIndex{6}\) Complete the following Lewis structure by adding bonds (not atoms), and then indicate the longest bond: Answer The C–C single bonds are longest. PROBLEM \(\PageIndex{7}\) Use principles of atomic structure to answer each of the following: 1 a. The radius of the Ca atom is 197 pm; the radius of the Ca 2+ ion is 99 pm. Account for the difference. b. The lattice energy of CaO( s ) is –3460 kJ/mol; the lattice energy of K 2 O is –2240 kJ/mol. Account for the difference. c. Given these ionization values, explain the difference between Ca and K with regard to their first and second ionization energies. Element First Ionization Energy (kJ/mol) Second Ionization Energy (kJ/mol) K 419 3050 Ca 590 1140 d. The first ionization energy of Mg is 738 kJ/mol and that of Al is 578 kJ/mol. Account for this difference. Answer a When two electrons are removed from the valence shell, the Ca radius loses the outermost energy level and reverts to the lower n = 3 level, which is much smaller in radius. Answer b The +2 charge on calcium pulls the oxygen much closer compared with K, thereby increasing the lattice energy relative to a less charged ion. Answer c Removal of the 4 s electron in Ca requires more energy than removal of the 4 s electron in K because of the stronger attraction of the nucleus and the extra energy required to break the pairing of the electrons. The second ionization energy for K requires that an electron be removed from a lower energy level, where the attraction is much stronger from the nucleus for the electron. In addition, energy is required to unpair two electrons in a full orbital. For Ca, the second ionization potential requires removing only a lone electron in the exposed outer energy level. Answer d In Al, the removed electron is relatively unprotected and unpaired in a p orbital. The higher energy for Mg mainly reflects the unpairing of the 2 s electron. Contributors Paul Flowers (University of North Carolina - Pembroke), Klaus Theopold (University of Delaware) and Richard Langley (Stephen F. Austin State University) with contributing authors. Textbook content produced by OpenStax College is licensed under a Creative Commons Attribution License 4.0 license. Download for free at http://cnx.org/contents/85abf193-2bd...a7ac8df6@9.110 ). Adelaide Clark, Oregon Institute of Technology Feedback Think one of the answers above is wrong? Let us know here .