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Summary Notes (Lesmahagow High School) |  |
It is clear from the potential energy diagram below that as we move from reactants to products the enthalpy change does not occur as a straight fall (the reaction shown is exothermic, so shows a drop in overall energy – it would be an increase for an endothermic reaction) from reactant to product. Clearly, on the diagram, the chemical potential energy of the reactants initially increases. This increase in energy is referred to as the activation energy (EA), and it can be calculated from the potential energy diagram (on the diagram, EA is shown for the forward reaction).
The activation energy is the minimum kinetic energy required by colliding molecules for a reaction to occur.
As a chemical reaction proceeds from products to reactants the reacting molecules go through a transition state or form an activated complex* where they are neither reactants nor products (this complex exists only briefly). This intermediate structure(s) forms while bonds are breaking and new bonds are forming. The activated complex has the highest chemical potential energy in the reaction pathway (always higher than either the products or reactants), because it is an unstable arrangement of the atoms or molecules reacting, so is found at the top of the reaction pathway. Consequently, the reactants must gain energy to reach this transition state. This energy gain is the activation energy (EA) and represents a barrier to the reaction, so can be referred to as the potential energy barrier.
The diagram below illustrates the activation energy required for the reverse reaction (EA’). Note the activation energy for the reverse reaction is greater, meaning that it is less likely to happen (the reverse reaction is endothermic).
* The activated complex on a reaction profile diagram is often indicated by a star(*).
The energy to provide reactants with the Activation Energy is described by the reactants kinetic energy. Temperature is a measure of this kinetic energy and so as we increase the temperature of a reaction we are increasing the kinetic energy of th reactants. Where a reaction has a low activation energy, then it is likely that reactants will have sufficient kinetic enrgy to react at room temperature, where
