Question

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estimate the reaction energy for the following reaction (all bonds are single bonds except as noted):
h₂ + f₂ → 2hf

Explanation:

Step1: Recall bond energy values

We need the bond energy values for \(H - H\), \(F - F\), and \(H - F\). From standard bond energy tables:

• Bond energy of \(H - H\) (\(E_{H - H}\)) is \(436\space kJ/mol\)
• Bond energy of \(F - F\) (\(E_{F - F}\)) is \(158\space kJ/mol\)
• Bond energy of \(H - F\) (\(E_{H - F}\)) is \(568\space kJ/mol\)

Step2: Calculate energy required to break bonds

In the reactants, we have 1 mole of \(H_2\) (1 \(H - H\) bond) and 1 mole of \(F_2\) (1 \(F - F\) bond). The energy required to break bonds (\(\Delta H_{break}\)) is the sum of the bond energies of the bonds broken.
\(\Delta H_{break}=E_{H - H}+E_{F - F}\)
\(\Delta H_{break}=436 + 158=594\space kJ/mol\)

Step3: Calculate energy released when forming bonds

In the products, we have 2 moles of \(HF\) (2 \(H - F\) bonds). The energy released when forming bonds (\(\Delta H_{form}\)) is the sum of the bond energies of the bonds formed (but with a negative sign since energy is released).
\(\Delta H_{form}=- 2\times E_{H - F}\)
\(\Delta H_{form}=-2\times568=- 1136\space kJ/mol\)

Step4: Calculate reaction energy (\(\Delta H_{rxn}\))

The reaction energy is the sum of the energy required to break bonds and the energy released when forming bonds.
\(\Delta H_{rxn}=\Delta H_{break}+\Delta H_{form}\)
\(\Delta H_{rxn}=594-1136=- 542\space kJ/mol\)

Answer:

The reaction energy for the reaction \(H_2 + F_2
ightarrow2HF\) is \(\boldsymbol{- 542\space kJ/mol}\) (the negative sign indicates that the reaction is exothermic).