Final answer:
At point a in the reaction systems, an activated complex is formed as reactant molecules collide with sufficient energy, leading to an exothermic reaction where the enthalpy of the system decreases as products are formed.
Step-by-step explanation:
When examining the reaction systems shown at point a, the reactions depicted can typically be described by the equation 2A = B. At this point, it suggests that the collision of molecules A with sufficient energy has occurred, leading to the formation of an unstable, high-energy activated complex or transition state. As the system progresses, A molecules are converted into B molecules, shown by a decrease in A and an increase in B over time. The reaction proceeds in the direction of the products until it reaches equilibrium. If we are discussing the activation energy (Ea) and enthalpy change (∆H), we are highlighting that Ea represents the required energy to reach the transition state, while ∆H indicates that the reaction is exothermic (energy is released and ∆H < 0) as the enthalpy of the system decreases when products are formed from reactants.