5.1.2 Reaction Profiles

Chemical reactions require the collision of reacting particles with both sufficient energy and proper orientation. The minimum energy required for a reaction to occur is called the activation energyMinimum amount of energy available for reactants to enable a chemical reaction to occur.. Without reaching this energy threshold, the reaction does not proceed.

Reaction Profiles

To visualise and analyse the energy changes that occur during a chemical reaction, scientists use reaction profiles. A reaction profile is a graph that represents the relative energies of the reactants and products, as well as the activation energy and the overall energy change of the reaction.

The vertical axis of a reaction profile represents the energy level, while the horizontal axis represents the progress of the reaction from reactants to products. The reactants are situated on the left side of the graph, and the products are on the right side. The relative energy of the species is indicated by the height of their respective positions on the graph.

The reaction profile shows the energy changes that occur during the reaction. The difference in energy between the reactants and products represents the overall energy change, which can be exothermicA reaction that releases heat, where the products have less energy than the reactants. (releasing energy) or endothermicA reaction that ‘absorbs’ heat, where the products have greater energy than the reactants. (absorbing energy).

Exothermic Reaction

For an exothermic reaction, the reactants start with a higher energy level compared to the products. The reaction profile will show a downward slope from left to right, indicating that energy is released to the surroundings. The overall energy change is negative, and the products have a lower energy level than the reactants.

Endothermic Reaction

In contrast, an endothermic reaction involves the absorption of energy from the surroundings. The energy of the products is higher than that of the reactants. As a result, the reaction profile will have an upward slope from left to right, indicating the increase in energy. The overall energy change is positive.

On the reaction profile, the activation energy is represented by the peak or highest point. It is the minimum amount of energy required for reactant particles to overcome the energy barrier and proceed to the transition stateRefers to any structure which corresponds to a bond being broken/formed. Mathematically this corresponds to a maxima on the energy profile of the molecule. TS are generally unstable and short lived. Reactions have to proceed through TS to occur.. The activation energy is essential for a reaction to occur.

Conclusion

By examining the reaction profile, scientists can gain insights into the energy changes that occur during a chemical reaction. They can analyse the stability of reactants and products, determine the activation energy required for the reaction, and assess the overall energy change. Understanding reaction profiles helps in studying the factors that affect reaction rates, such as temperature, concentration, and catalysts. It also provides a visual representation of the energy considerations involved in chemical reactions, which aids in explaining reaction mechanisms and predicting the feasibility of reactions.