A research team is studying strategies to reduce the energy demand of an industrial fluorination step summarized (in simplified form) by the endothermic reaction
CF4(g)→C(s,graphite)+2F2(g)ΔHrxn≈+480kJ⋅mol−1
The chemists are considering structural modifications to CF4 that would lower the positive enthalpy change without altering the overall stoichiometry (one carbon-centered molecule producing elemental carbon and diatomic fluorine). Average bond enthalpies relevant to the discussion are listed below.
C–F: 485kJ⋅mol−1
C–Cl: 328kJ⋅mol−1
C–H: 413kJ⋅mol−1
Assume that any C–Cl or C–H bonds introduced into the starting molecule would be broken and that the products would still be C(s) and only F2(g), Cl2(g), and/or H2(g) as appropriate.
(a) Explain, in terms of bonds broken and formed, why replacing one or more C–F bonds with C–Cl bonds in the reactant molecule is expected to change the magnitude of ΔH for the overall process.
(b) Describe qualitatively how substituting a single C–F bond with a C–H bond would affect the enthalpy change compared with the original CF4 reaction. Justify your answer using the data provided.
(c) If the goal is to bring the enthalpy change to within +350kJ⋅mol−1 while making only one type of single-bond substitution, argue which substitution (C–Cl or C–H) is more effective and explain your reasoning.