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Energy Diagrams

Reaction Energy Diagrams

It's convenient to describe chemical reactions with energy diagrams.  In a Reaction Energy diagram, the vertical axis represents the energy of the reactants, intermediates, and products.  Higher energy indicates less stable and lower energy indicates more stable structures. The horizontal axis is the "reaction coordinate".  It usually traces from left to right the progress of the reaction from reactants to final products.  Reaction Energy Diagrams are sometimes referred to as potential energy surfaces (PES).  

Exothermic Reactions

Reactions that release heat are termed exothermic.   In an exothermic reaction, the resulting products have more or more stable bonds than the reactants.  The ΔH of reaction for an exothermic reaction is less than zero (ΔHrxn < 0).  Recall that ΔHrxn = Hproducts-Hreactants.  Thus if the products are lower in energy than the reactants the ΔHrxn will be less than zero.  The ΔHrxn is a thermodynamic quantity and tells us nothing about how fast a reaction proceeds.

The rate of a reaction is governed by its Activation Energy or Activation Barrier (Ea).  The activation energy (Ea) is the difference in energy between the reactants and the transition state complex.  To go from reactants to products you must go over the activation barrier.


  

  

Endothermic Reactions

Reactions that absorb heat are termed endothermic.  In an endothermic reaction, the products have less or less stable bonds.  The ΔH of reaction for an exothermic reaction is greater than zero (ΔHrxn > 0).

 

Note chemists also occasionally use the terms endergonic and exergonic.  An exergonic reaction has ΔG < 0 and occurs spontaneously, while an endergonic reaction has ΔG > 0  Recall that ΔG is the Gibbs free energy and is defined as follows ΔG = ΔH - TΔS.

Multi-Step Reactions

The reaction energy diagrams shown above are for single-step reactions.  In a single reaction step there is only one barrier (hump) in the diagram.  Likewise in a two-step reactions, there are two humps or activation barriers.  In multi-step reactions, the species between both steps is called an intermediate.  In the diagram below the first step is the slowest step since it has the largest barrier.  This is called the rate-determining step (RDS) or rate-limiting step.

 

 

Helpful Steps to Create an Energy Diagram

1) Label the axes. The x-axis is labeled as the reaction coordinate, and the y-axis is labeled as energy.

2) Draw a line at the beginning of the graph for the reactants and a line at the end for the products. The products will be higher or lower depending on if the reaction is endothermic or exothermic.

3) Add a hump that connects the reactant and product lines. Remember to add multiple humps if the reaction has multiple transition steps.

4) Label the activation energy, transition step, and enthalpy!

 

Tips for Drawing Transition States

The transition state is a structure between the reactants and products