A reaction coordinate is a graph that can be used to observe the progress of a reaction as reactants are turned into products. These graphs can be used to predict whether a reaction will occur based on the activation energy for the reaction. If the reactants have a large potential energy, the activation energy will be small, meaning the reaction is likely to proceed. If the reactants have a small potential energy, the activation energy will be large, meaning the reaction will not proceed without an external energy source. So, reactions that require energy such as those with a high activation energy will have a positive delta H because they require energy to proceed, while reactions that release energy and have small activation energies will have a negative delta H.
The potential energy of the particles is dependent upon their positions and complexity, along with their activation energy. In order for the reaction to occur, the particles must collide in the correct orientation with the correct speed. In addition, concentration, temperature, and presence of a catalyst all affect reaction rates and whether a reaction will occur. As explained before, if these factors are favorable, the activation energy will be low, and the reaction will occur. By observing the activation energy on a reaction coordinate graph, the rate of a reaction can be predicted. If the reaction has a high activation energy it will not happen at all, if there is little to no activation energy, the reaction will proceed quickly, and if the amount of activation energy is somewhere between the two, the reaction will proceed slowly.
The approximate activation energy for the forward and reverse reactions is dependent on the difference in potential energy between the products and reactants.