Average bond enthalpies are generally defined for gas-phase molecules. Many substances are liquids in their standard state. coo (Section 5.7) By using appropriate thermochemical data from Appendix C, calculate average bond enthalpies in the liquid state for the following bonds, and compare these values to the gas-phase values given in Table 8.4: (a) \(\mathrm{Br}-\mathrm{Br}\), from \(\mathrm{Br}_{2}(l) ;\) (b) \(\mathrm{C}-\mathrm{Cl},\) from \(\mathrm{CCl}_{4}(l) ;\) (c) \(\mathrm{O}-\mathrm{O},\) from \(\mathrm{H}_{2} \mathrm{O}_{2}(I)\) (assume that the \(\mathrm{O}-\mathrm{H}\) bond enthalpy is the same as in the gas phase). (d) What can you conclude about the process of breaking bonds in the liquid as compared to the gas phase? Explain the difference in the \(\Delta H\) values between the two phases.

Step by step solution

01

a. Br-Br bond in Br₂(l)

We need to determine the Enthalpy of vaporization from liquid state to gas state for the Br₂(l) molecule to calculate the average bond enthalpy in the liquid state. From Appendix C, we find that the enthalpy of vaporization for Br₂(l) is 29.96 kJ/mol. Now, the bond enthalpy for the Br-Br bond in Br₂(g) can be found from Table 8.4, which is 193 kJ/mol. In order to find the average bond enthalpy for the Br-Br bond in the liquid state, we need to subtract the enthalpy of vaporization from the bond enthalpy: Br-Br(l) bond enthalpy = 193 kJ/mol (gas-phase) - 29.96 kJ/mol (enthalpy of vaporization) Br-Br(l) bond enthalpy = 163.04 kJ/mol

02

b. C-Cl bond in CCl₄(l)

Similarly, for CCl₄(l), we find the enthalpy of vaporization from Appendix C which is 30.6 kJ/mol. The bond enthalpy for the C-Cl bond in CCl₄(g) can be found from Table 8.4, which is 328 kJ/mol. Now, we subtract the enthalpy of vaporization to calculate the bond enthalpy for the C-Cl bond in the liquid state: C-Cl(l) bond enthalpy = 328 kJ/mol (gas-phase) - 30.6 kJ/mol (enthalpy of vaporization) C-Cl(l) bond enthalpy = 297.4 kJ/mol

03

c. O-O bond in H₂O₂(l)

For H₂O₂(l), we find the enthalpy of vaporization from Appendix C which is 51.7 kJ/mol. The bond enthalpy for the O-O bond in H₂O₂(g) can be found from Table 8.4, which is 145 kJ/mol. Again, we subtract the enthalpy of vaporization to find the bond enthalpy for the O-O bond in the liquid state: O-O(l) bond enthalpy = 145 kJ/mol (gas-phase) - 51.7 kJ/mol (enthalpy of vaporization) O-O(l) bond enthalpy = 93.3 kJ/mol We assume that the O-H bond enthalpy is the same as in the gas phase.

04

d. Comparison and Conclusion

In summary, we obtained the following bond enthalpies for the liquid state: Br-Br(l) bond enthalpy = 163.04 kJ/mol C-Cl(l) bond enthalpy = 297.4 kJ/mol O-O(l) bond enthalpy = 93.3 kJ/mol Comparing these values with the gas-phase bond enthalpies, we notice that the bond enthalpies are lower in the liquid state. This means that less energy is required to break these bonds in the liquid state as compared to the gas phase. The difference in the ΔH values between the two phases can be attributed to the fact that in the liquid phase, the attractive forces between the molecules are stronger than in the gas phase due to the close proximity of the molecules. This results in a decrease in the enthalpy required to break the bonds in the liquid phase compared to the gas phase.

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