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Chapter 7: Q65E (page 400)

Question: Using the bond energies in Table \({\rm{7}}{\rm{.2}}\), determine the approximate enthalpy change for each of the following reactions:

(a) \({{\rm{H}}_{\rm{2}}}{\rm{(g) + B}}{{\rm{r}}_{\rm{2}}}{\rm{(g)}} \to {\rm{2HBr(g)}}\)

(b) \({\rm{C}}{{\rm{H}}_{\rm{4}}}{\rm{(g) + }}{{\rm{I}}_{\rm{2}}}{\rm{(g)}} \to {\rm{C}}{{\rm{H}}_{\rm{3}}}{\rm{I(g) + HI(g)}}\)

(c) \({{\rm{C}}_{\rm{2}}}{{\rm{H}}_4}{\rm{(g) + 3}}{{\rm{O}}_{\rm{2}}}{\rm{(g)}} \to {\rm{2C}}{{\rm{O}}_{\rm{2}}}{\rm{(g) + 2}}{{\rm{H}}_{\rm{2}}}{\rm{O(g)}}\)

Short Answer

Expert verified

(a) For the reaction \({{\rm{H}}_{\rm{2}}}{\rm{(g) + B}}{{\rm{r}}_{\rm{2}}}{\rm{(g)}} \to {\rm{2HBr(g)}}\), the value for enthalpy change is \({\rm{\Delta H}}_{{\rm{298}}}^{\rm{^\circ }}{\rm{ = - 144\;kJ}}\).

(b) For the reaction\({\rm{C}}{{\rm{H}}_{\rm{4}}}{\rm{(g) + }}{{\rm{I}}_{\rm{2}}}{\rm{(g)}} \to {\rm{C}}{{\rm{H}}_{\rm{3}}}{\rm{I(g) + HI(g)}}\), the value for enthalpy change is\({\rm{\Delta H}}_{{\rm{298}}}^{\rm{^\circ }}{\rm{ = 30\;kJ}}\).

(c) For the reaction \({{\rm{C}}_{\rm{2}}}{{\rm{H}}_4}{\rm{(g) + 3}}{{\rm{O}}_{\rm{2}}}{\rm{(g)}} \to {\rm{2C}}{{\rm{O}}_{\rm{2}}}{\rm{(g) + 2}}{{\rm{H}}_{\rm{2}}}{\rm{O(g)}}\), the value for enthalpy change is \({\rm{\Delta H}}_{{\rm{298}}}^{\rm{^\circ }}{\rm{ = - 1055\;kJ}}\).

Step by step solution

01

Concept Introduction

Bond energy, also known as mean bond enthalpy or average bond enthalpy in chemistry, is a measure of the bond strength in a chemical bond. The average value of the gas-phase bond-dissociation energy for all bonds of the same type within the same chemical species is defined by IUPAC as bond energy.

02

Enthalpy Change for first reaction

(a)

Bond Energy for\({\rm{H - H}}\)is:\({\rm{436 kJ/mol}}\)

Bond Energy for\({\rm{Br - Br}}\)is:\({\rm{190 kJ/mol}}\)

Bond Energy for\({\rm{H - Br}}\)is:\({\rm{370 kJ/mol}}\)

Calculate the enthalpy change according to the reaction 鈥�

\(\begin{array}{l}{\rm{\Delta H}}_{{\rm{298}}}^{\rm{^\circ }}{\rm{ = }}\sum {{{\rm{D}}_{{\rm{bonds broken }}}}} {\rm{ - }}\sum {{{\rm{D}}_{{\rm{bonds formed }}}}} \\{\rm{\Delta }}{{\rm{H}}^{\rm{^\circ }}}_{{\rm{298}}}{\rm{ = }}{{\rm{D}}_{{\rm{H - H}}}}{\rm{ + }}{{\rm{D}}_{{\rm{Br - Br}}}}{\rm{ - 2}}{{\rm{D}}_{{\rm{H - Br}}}}\\{\rm{\Delta }}{{\rm{H}}^{\rm{^\circ }}}_{{\rm{298}}}{\rm{ = 436 + 190 - 2(370) = - 144\;kJ}}\end{array}\)

Therefore, the enthalpy change is \({\rm{\Delta H}}_{{\rm{298}}}^{\rm{^\circ }}{\rm{ = - 144\;kJ}}\).

03

Enthalpy Change for second reaction

(b)

Bond Energy for\({\rm{C - H}}\)is:\({\rm{415 kJ/mol}}\)

Bond Energy for\({\rm{I - I}}\)is:\({\rm{150 kJ/mol}}\)

Bond Energy for\({\rm{C - I}}\)is:\({\rm{240 kJ/mol}}\)

Bond Energy for\({\rm{H - I}}\)is:\({\rm{295 kJ/mol}}\)

Calculate the enthalpy change according to the reaction 鈥�

\(\begin{array}{l}{\rm{\Delta }}{{\rm{H}}^{\rm{^\circ }}}_{{\rm{298}}}{\rm{ = }}\sum {{D_{{\rm{bonds broken }}}}} {\rm{ - }}\sum {{D_{{\rm{bonds formed }}}}} \\{\rm{\Delta }}{{\rm{H}}^{\rm{^\circ }}}_{{\rm{298}}}{\rm{ = 4}}{{\rm{D}}_{{\rm{C - H}}}}{\rm{ + }}{{\rm{D}}_{{\rm{I - I}}}}{\rm{ - 3}}{{\rm{D}}_{{\rm{C - H}}}}{\rm{ - }}{{\rm{D}}_{{\rm{C - I}}}}{\rm{ - }}{{\rm{D}}_{{\rm{H - I}}}}\\{\rm{\Delta }}{{\rm{H}}^{\rm{^\circ }}}_{{\rm{298}}}{\rm{ = 4(415) + 150 - 3(415) - 240 - 295 = 30\;kJ}}\end{array}\)

Therefore, the enthalpy change is \({\rm{\Delta H}}_{{\rm{298}}}^{\rm{^\circ }}{\rm{ = 30\;kJ}}\).

04

Enthalpy Change for third reaction

(c)

Bond Energy for\({\rm{C - H}}\)is:\({\rm{415 kJ/mol}}\)

Bond Energy for\({\rm{C = C}}\)is:\({\rm{611 kJ/mol}}\)

Bond Energy for\({\rm{O = O}}\)is:\({\rm{498 kJ/mol}}\)

Bond Energy for\({\rm{C = O}}\)is:\({\rm{741 kJ/mol}}\)

Bond Energy for\({\rm{O - H}}\)is:\({\rm{464 kJ/mol}}\)

Calculate the enthalpy change according to the reaction 鈥�

\(\begin{array}{l}{\rm{\Delta H}}_{{\rm{298}}}^{\rm{^\circ }}{\rm{ = }}\sum {{{\rm{D}}_{{\rm{bonds broken }}}}} {\rm{ - }}\sum {{{\rm{D}}_{{\rm{bonds formed }}}}} \\{\rm{\Delta }}{{\rm{H}}^{\rm{^\circ }}}_{298}{\rm{ = }}{{\rm{D}}_{{\rm{C = C}}}}{\rm{ + 4}}{{\rm{D}}_{{\rm{C - H}}}}{\rm{ + 3}}{{\rm{D}}_{{\rm{O = O}}}}{\rm{ - 4}}{{\rm{D}}_{{\rm{C = O}}}}{\rm{ - 4}}{{\rm{D}}_{{\rm{O - H}}}}\\{\rm{\Delta H}}_{{\rm{298}}}^{\rm{^\circ }}{\rm{ = 611 + 4(415) + 3(498) - 4(741) - 4(464) = - 1055\;kJ}}\end{array}\)

Therefore, the enthalpy change is \({\rm{\Delta H}}_{{\rm{298}}}^{\rm{^\circ }}{\rm{ = - 1055\;kJ}}\).

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Most popular questions from this chapter

Identify the more polar bond in each of the following pairs of bonds: (a) \({\rm{HF or HCl}}\) (b) \({\rm{NO or CO}}\) (c) \({\rm{SH or OH}}\) (d) \({\rm{PCl or HCl}}\) (e) \({\rm{CH or NH}}\) (f) \({\rm{SO or PO}}\) (g) \({\rm{CN or NN}}\) .

Use the Molecule Shape simulator (http://openstaxcollege.org/l/16MolecShape) to build a molecule. Starting with the central atom, click on the double bond to add one double bond. Then add one single bond and one lone pair. Rotate the molecule to observe the complete geometry. Name the electron group geometry and molecular structure and predict the bond angle. Then click the check boxes at the bottom and right of the simulator to check your answers.

Calculate the formal charge of chlorine in the molecules \({\rm{C}}{{\rm{l}}_{\rm{2}}}\), \({\rm{BeC}}{{\rm{l}}_{\rm{2}}}\), and \({\rm{Cl}}{{\rm{F}}_{\rm{5}}}\).

From their positions in the periodic table, arrange the atoms in each of the following series in order of increasing electronegativity: (a)\({\rm{C, F, H, N, O}}\)(b)\({\rm{Br, Cl, F, H, I }}\)(c)\({\rm{F, H, O, P, S }}\)(d)\({\rm{Al, H, Na, O, P}}\)(e)\({\rm{Ba, H, N, O, As}}\).

The reaction of a metal, \({\rm{M}}\), with a halogen, \({{\rm{X}}_{\rm{2}}}\), proceeds by an exothermic reaction as indicated by this equation: \({\rm{M(s) + }}{{\rm{X}}_{\rm{2}}}{\rm{(g)}} \to {\rm{M}}{{\rm{X}}_{\rm{2}}}{\rm{(s)}}\). For each of the following, indicate which option will make the reaction more exothermic. Explain your answers.

(a) a large radius vs. a small radius for \({{\rm{M}}^{{\rm{ + 2}}}}\)

(b) a high ionization energy vs. a low ionization energy for \({\rm{M}}\)

(c) an increasing bond energy for the halogen

(d) a decreasing electron affinity for the halogen

(e) an increasing size of the anion formed by the halogen
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