Final answer:
To ascertain whether reactions A and B have a positive enthalpy change (ΔH), assess each reaction's enthalpy given the convention that positive ΔH indicates endothermic processes, and negative ΔH indicates exothermic processes. Use Hess's law that the sum of individual reaction enthalpies equates to the overall reaction enthalpy to determine the correct answer.
Step-by-step explanation:
When comparing the halogenation of differently substituted carbons and assessing the heat of reaction (ΔHo), it's critical to understand the concept of enthalpy (ΔH) and how it relates to chemical reactions. The statement provided indicates that one must determine whether reactions A and B have positive heat of reactions. Since positive enthalpy change (ΔH > 0) indicates that a reaction is endothermic, and negative (ΔH < 0) indicates exothermic, the correct answer must align with the given enthalpy changes of the reactions.
Based on the principle that breaking bonds requires energy (endothermic) and forming bonds releases energy (exothermic), and considering that summation of enthalpies from reactions A and B (AH₁ and AH₂ respectively) equals the enthalpy of reaction C (AH₃), and that Hess's law states that the total enthalpy change for a chemical reaction is the sum of the enthalpy changes for individual steps in the reaction (AHₒ + AHₓ = AHC), comparing given values of AH for reactions A and B will lead to the conclusion whether one or both have a positive enthalpy change.