Final answer:
The second step of an EAS reaction is exothermic, meaning it releases heat and has a negative change in enthalpy (ΔH). This corresponds to the formation of new bonds, which naturally releases energy.
Step-by-step explanation:
The second step of an electrophilic aromatic substitution (EAS) reaction is exothermic. In general, exothermic processes are associated with the release of heat and energy into the surroundings, which corresponds to a negative change in enthalpy (ΔH).
In an EAS reaction, the first step involves the generation of an activated complex, which requires breaking bonds and is consequently endothermic (ΔH positive). However, the second step, where the new bonds are formed, is exothermic as it involves the stabilization of the activated complex by releasing energy (ΔH negative). This follows the thermodynamic principle that forming bonds releases energy and breaking bonds consumes energy.
For instance, when considering the formation of a solution, the overall process can be either exothermic or endothermic, depending on the magnitude of the enthalpy changes in each step. If the energy released during solute-solvent interactions (ΔH3) is greater than the energy consumed in separating solute and solvent particles (ΔH1 + ΔH2), the process is exothermic with a negative overall enthalpy change (ΔHsoln < 0). Conversely, if the released energy is less, the process is endothermic (ΔHsoln > 0).