Activation energy is the minimum energy for a chemical reaction to occur, that is, it is one of the determining factors for the occurrence of a reaction, together with the favorable contact and collision between the reactant molecules.
Favorable collision model between the reactants of a reaction
When there is contact and collision between the reactant molecules, an intermediate compound is formed (before the product is formed), called an activated complex. This complex is a grouping of all the atoms of the reactants.
Representation of the activated complex
However, the time to form the activated complex indicates whether one chemical reaction will be faster or slower than the other. Thus, the favorable collision, combined with the activation energy, is decisive for the speed of the reaction.
It should be noted that the activation energy does not cease until the activated complex is formed.
The most common way to analyze the activation energy and the activated complex is using a graph, which presents, as a pattern, energy or enthalpy (in KJ or Kcal) on the y-axis, the reaction path (from the reactant to the products) on the x-axis, and a curve, as we can see in the follow:
Model of a standard graph used in chemical kinetics
In this graph template, the activated complex is given by the highest point of the curve (point a of the next graph), and the activation energy is the entire path of the reactants to the activated complex (red arrow on graphic):
Checking the complex and activation energy on the graph
To determine the value of the activation energy, then, just subtract the energy of the reactants from the energy of the activated complex, as in the mathematical expression below:
Do not stop now... There's more after the advertising ;)
Eat = Hcomplex activated - Hreagents
Suppose, for example, that a chemical reaction is taking place between reagents AB and CD, for the formation of products AD and CB, according to the following graph:
AB + CD → AD + CB
AD and CB formation reaction graph
Analyzing this chart, we have to:
The energy required for the formation of the activated complex is 30 Kcal, as it is the energy related to the highest point on the curve;
THE activation energy of this reaction would be 10 Kcal only, because it starts from the reactants that have an energy of 20 Kcal and ends up in the activated complex that has an energy of 30 Kcal, as shown below:
Eat = Hcomplex activated - Hreagents
Eat = 30 - 20
Eat = 10 Kcal
One way to speed up a chemical reaction, from what we've seen, is to reduce the activation energy, because the smaller it is, the faster the activated complex will be formed and, consequently, the faster will be the obtaining of the products.
To reduce the activation energy and thus speed up the reaction, an alternative is to use a catalyst, which is a chemical substance that must be added to the reaction medium (reaction), which participates in the formation of the activated complex, but does not form any substance in the product, that is, it is collected at the end and can be reused.
Graphically, the presence of the catalyst is noted by the increment of a second curve, as we can see below:
Graph showing the curve with a catalyst
Thus, the curve with the catalyst (in red) has a new activation energy (blue arrow) and a new value (x) of energy for the activated complex, always lower in relation to the curve of the reaction carried out without the presence of the catalyst.
By Me. Diogo Lopes Dias
Would you like to reference this text in a school or academic work? Look:
DAYS, Diogo Lopes. "What is activation energy?"; Brazil School. Available in: https://brasilescola.uol.com.br/o-que-e/quimica/o-que-e-energia-ativacao.htm. Accessed on June 28, 2021.
