Understanding Energy Diagrams

An energy diagram is a graphical representation showing the energy changes that occur during a chemical reaction.
The vertical axis of an energy diagram represents the potential energy, whereas the horizontal axis typically represents the progression of the reaction from reactants to products.
The starting point of the plot represents the energy of the reactants.
The endpoint of the plot shows the energy of the products.

Parts of an Energy Diagram

Every energy diagram features a peak, which represents the maximum energy point in the reaction. This peak is called the transition state or the activated complex.
The energy difference between the reactants and the transition state is termed the activation energy (Ea). It’s the minimum energy required to start a reaction.
The enthalpy change (∆H) for a reaction is represented by the energy difference between the reactants and the products. It gives the net energy absorbed or released by the reaction.
For an exothermic reaction, ∆H is negative and the products have lower energy than the reactants. For an endothermic reaction, ∆H is positive and the products have higher energy than the reactants.

Progress of a reaction can be traced by following the energy plot from the reactants towards the products.
The height of the peak on the plot conveys the difficulty or ease with which the reaction proceeds. A high peak indicates a large activation energy and a slower reaction, while a low peak indicates a small activation energy and a faster reaction.
Reactions involving a decrease in energy (exothermic reactions) feature an energy plot that slopes downward, while those involving an increase in energy (endothermic reactions) feature an energy plot that slopes upward.

Energy diagrams are vital tools for visualising the energy transactions in chemical reactions.
They provide insight into the activation energy and the enthalpy change, which are crucial for predicting the feasibility and identifying the nature (endothermic or exothermic) of a reaction.
By illustrating the energy changes through the progression of the reaction, energy diagrams enable easier comprehension of complex chemical reactions.