Chemistry » Energy and Chemical Reactions » Mechanism Of Reaction And Catalysis

Activation Energy Continued

Mechanism of reaction and catalysis

Earlier it was mentioned that it is the collision of particles that causes reactions to occur and that only some of these collisions are successful. This is because the reactant particles have a wide range of kinetic energies, and only a small fraction of the particles will have enough energy (and the correct orientation) to actually break bonds so that a chemical reaction can take place. The minimum energy that is needed for a reaction to take place is called the activation energy. For more information on the energy of reactions, refer to previous lessons.

Activation Energy

Definition: Activation energy

The minimum energy required for a chemical reaction to proceed.

Even at a fixed temperature, the energy of the particles varies, meaning that only some of them will have enough energy to be part of the chemical reaction, depending on the activation energy for that reaction (see figure below). Increasing the reaction temperature has the effect of increasing the number of particles with enough energy to take part in the reaction, and increasing the reaction rate.

Activation Energy Continued

The distribution of particle kinetic energies at a fixed temperature.

Remember that a molecule must have energy greater than the activation energy, as well as the correct orientation, for the reaction to take place.

Increasing the temperature of a reaction mixture raises the average kinetic energy of the particles. As can be seen on the graph (see figure below), a higher proportion of the particles can now react making the reaction faster. With the increased movement of the molecules the chances of a molecule having the correct orientation is also increased.

Activation Energy Continued

The distribution of particle kinetic energies with an increase in temperature. There is an increased number of particles with sufficient energy due to the higher temperature.

An endothermic reaction can be represented by:

\(\text{Reactants} + \color{red}{\text{Energy}} \to \text{Products}\) i.e. a reaction that absorbs energy

This can be shown in an activation energy diagram (see figure below). These graphs are also sometimes called a reaction profile or a potential energy graph.

Activation Energy Continued

An activation energy diagram with reactant energy lower than product energy, i.e. endothermic 

An exothermic reaction can be represented by:

\(\text{Reactants}\to \text{Products} + \color{red}{\text{Energy}}\) i.e. a reaction that releases energy

This can be shown in an activation energy diagram (see figure below):

Activation Energy Continued

An activation energy diagram with reactant energy greater than product energy, i.e. exothermic 

You could revise previous lessons on activation energy.

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