Question

A) The reaction between hydrogen and oxygen to form water is highly exothermic. Which statement is true of the energies of the bonds that break and form during the reaction?

The energy needed to break the required bonds is about the same as the energy released when the new bonds form.

The energy needed to break the required bonds is less than the energy released when the new bonds form.

The energy needed to break the required bonds is greater than the energy released when the new bonds form

B)Use bond energies to determine ΔHrxn for the reaction between ethanol and hydrogen chloride.

CH3CH2OH(g)+HCl(g)→CH3CH2Cl(g)+H2O(g)


-12 kJ
1549 kJ
12 kJ
-1549 kJ

Answer 1

a) B. The energy needed to break the required bonds is less than the energy released when the new bonds form.

b)

$$ \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH}(\mathrm{g})+\mathrm{HCl}(\mathrm{g}) \longrightarrow \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{Cl}(\mathrm{g})+\mathrm{H}_{2} \mathrm{O} $$

\(\Delta \mathrm{H}_{\mathrm{mn}}=\Sigma\) (number of bond broken) \(-\Sigma\) (number of bond forms) \(\begin{aligned} &=[5(\mathrm{C}-\mathrm{H})+(\mathrm{C}-\mathrm{O})+(\mathrm{O}-\mathrm{H})+(\mathrm{H}-\mathrm{C})]-[5(\mathrm{C}-\mathrm{H})+(\mathrm{C}-\mathrm{Cl})+2(\mathrm{O}-\mathrm{H})] \\ &=[(\mathrm{C}-\mathrm{O})+(\mathrm{O}-\mathrm{H})+(\mathrm{H}-\mathrm{Cl})]-[(\mathrm{C}-\mathrm{Cl})+2(\mathrm{O}-\mathrm{H})] \\ &=[431+360+464]-[339+2(464)] \\ &=1255-1267 \mathrm{~kJ} \\ \Delta \mathrm{H}_{\mathrm{an}} &=-12 \mathrm{~kJ} \end{aligned}\)