Final answer:
The first step of an electrophilic aromatic substitution (EAS) is highly endothermic as it involves bond breaking which requires an input of energy, resulting in a positive change in enthalpy (ΔH).
Step-by-step explanation:
The student asks whether the first step of an electrophilic aromatic substitution (EAS) is highly endothermic or exothermic. The first step of EAS is characterized by the breaking of bonds in the reactants to form intermediates and is usually associated with the absorption of energy, which corresponds to an increase in enthalpy (ΔH positive). Therefore, it is indeed an endothermic process.
For example, when considering the formation of a solution in both exothermic and endothermic scenarios, the initial steps that involve the separation of solvent and solute particles are always endothermic (ΔH positive). It is only the later interaction between solute and solvent that may result in an exothermic release of energy if interactions are favorable. This aligns with the principle that breaking bonds requires energy input, while bond formation releases energy.