Bond enthalpies Knowledge-Based Practice

Questions

1. 1.

   Which of the following chemical equations represents the process whose enthalpy change corresponds to the first bond dissociation enthalpy of water?

   • A) $\text{H}_2\text{O(l)} \rightarrow 2\text{H(g)} + \text{O(g)}$
   • B) $\text{H}_2\text{O(g)} \rightarrow \text{H}_2\text{(g)} + \frac{1}{2}\text{O}_2\text{(g)}$
   • C) $\text{H}_2\text{O(g)} \rightarrow \text{H}^+\text{(g)} + \text{OH}^-\text{(g)}$
   • D) $\text{H}_2\text{O(g)} \rightarrow \text{OH(g)} + \text{H(g)}$
2. 2.

   Which of the following best explains why enthalpy changes of reaction calculated using mean bond enthalpies often deviate from values determined experimentally under standard conditions?

   • A) They assume that all covalent bonds undergo heterolytic fission instead of homolytic cleavage during a chemical reaction.
   • B) They account only for the energy absorbed when breaking bonds and omit the energy released when forming new bonds.
   • C) They are averaged across a wide variety of molecular environments and strictly apply to species in the gaseous state.
   • D) They vary unpredictably depending on whether the overall reaction mechanism follows an exothermic or endothermic pathway.
3. 3.

   Using the average bond enthalpies of 945 kJ/mol for $\text{N}\equiv\text{N}$, 436 kJ/mol for $\text{H}-\text{H}$, and 391 kJ/mol for $\text{N}-\text{H}$, what is the enthalpy change for the reaction $\text{N}_2\text{(g)} + 3\text{H}_2\text{(g)} \rightarrow 2\text{NH}_3\text{(g)}$?

   • A) +93 kJ/mol
   • B) +1080 kJ/mol
   • C) -93 kJ/mol
   • D) -965 kJ/mol
4. 4.

   Given that the enthalpy change for the reaction $\text{H}_2\text{(g)} + \text{Cl}_2\text{(g)} \rightarrow 2\text{HCl(g)}$ is $-184\text{ kJ/mol}$, and the bond enthalpies of $\text{H}-\text{H}$ and $\text{Cl}-\text{Cl}$ are $436\text{ kJ/mol}$ and $242\text{ kJ/mol}$ respectively, what is the bond enthalpy of the $\text{H}-\text{Cl}$ bond?

   • A) 431 kJ/mol
   • B) 862 kJ/mol
   • C) 247 kJ/mol
   • D) 339 kJ/mol
5. 5.

   Given the average bond enthalpies of $\text{C}=\text{C}$ (614 kJ/mol), $\text{C}-\text{C}$ (347 kJ/mol), $\text{H}-\text{H}$ (436 kJ/mol), and $\text{C}-\text{H}$ (413 kJ/mol), what is the enthalpy change ($\Delta H$) for the reaction $\text{C}_2\text{H}_4\text{(g)} + \text{H}_2\text{(g)} \rightarrow \text{C}_2\text{H}_6\text{(g)}$?

   • A) +123 kJ/mol
   • B) -123 kJ/mol
   • C) -267 kJ/mol
   • D) +267 kJ/mol

Answers and step-by-step solutions appear after you finish the test.