Thermodynamic and Kinetic Control

Delta G tells you whether a reaction is downhill overall, comparing the energy of reactants to products. It says nothing about the path between them. That path has its own obstacle: the activation energy, the energy needed to break old bonds and distort molecules into the transition state before new bonds can form. A reaction can have a very negative delta G and still face a towering activation barrier, which makes it kinetically trapped. The reaction is thermodynamically favored but frozen in practice, sometimes for a million years, sometimes forever on a human timescale.

Picture an energy profile: reactants sit high, products sit much lower, so the overall drop is large and favorable. But between them the curve spikes upward into a tall hump before it drops down to the product. That hump is the activation energy. The system has to climb it before it can fall into the lower, more stable valley. A big hump means almost no molecules at room temperature have enough energy to get over it, so the reaction crawls or simply does not proceed, regardless of how favorable the end state is.

This is why thermodynamics and kinetics are separate stories about the same reaction. Delta G, built from enthalpy and entropy, predicts the destination. Activation energy and rate constants predict how fast, or whether, you arrive. A catalyst changes the kinetic story only: it lowers the activation barrier by offering a different pathway, so a previously frozen but favorable reaction can suddenly proceed, while delta G and the final equilibrium stay exactly the same.