(a) Using average bond enthalpies, predict which of the following reactions will be most exothermic: (i) \(\mathrm{C}(g)+2 \mathrm{~F}_{2}(g) \longrightarrow \mathrm{CF}_{4}(g)\) (ii) \(\mathrm{CO}(g)+3 \mathrm{~F}_{2} \longrightarrow \mathrm{CF}_{4}(g)+\mathrm{OF}_{2}(g)\) (iii) \(\mathrm{CO}_{2}(g)+4 \mathrm{~F}_{2} \longrightarrow \mathrm{CF}_{4}(g)+2 \mathrm{OF}_{2}(g)\) (b) Explain the trend, if any, that exists between reaction exothermicity and the extent to which the carbon atom is bonded to oxygen.

First, we need to write down the bond enthalpies needed for each reaction: Average Bond Enthalpies (in kJ/mol): - C-F: \(488\) - C=O: \(805\) - C-O: \(360\) - F-F: \(159\) - O-F: \(190\)

Now, we will use the given bond enthalpies to calculate the enthalpy change for each reaction: For Reaction (i): \[\Delta H_{i} = \text{Bonds broken} - \text{Bonds formed} \newline \Delta H_{i} = (\text{4 C-F bonds formed}) - (\text{0 bonds broken + 4 F-F bonds broken}) \newline \Delta H_{i} = (4 \times 488) - (4 \times 159) = \text{1316 kJ/mol}\] For Reaction (ii): \[\Delta H_{ii} = (\text{1 C=O bond broken + 3 F-F bonds broken}) - (\text{4 C-F bonds formed + 2 O-F bonds formed}) \newline \Delta H_{ii} = (805 + 3 \times 159) - (4 \times 488 + 2 \times 190) = -\text{556 kJ/mol}\] For Reaction (iii): \[\Delta H_{iii} = (\text{2 C-O bonds broken + 1 O=O bond broken + 4 F-F bonds broken}) - (4 \times \text{C-F bonds formed + 4 O-F bonds formed}) \newline \Delta H_{iii} = (2 \times 360 + 1 \times 498 + 4 \times 159) - (4 \times 488 + 4 \times 190) = -\text{676 kJ/mol}\]

Comparing the reaction enthalpies, the most exothermic reaction is the one with the lowest (most negative) enthalpy change: Reaction (iii) is the most exothermic.

The reaction exothermicity decreases as the number of bonds between carbon and oxygen increases. This is because breaking the strong C=O and C-O bonds consumes more energy, while forming new bonds only releases energy. The more carbon is bonded to oxygen, the higher the energy needed to break these bonds, leading to lower overall exothermic reactions.