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Energy profiles

An energy profile shows how the chemical energy changes throughout the course of a reaction.

The reaction coordinate charts the progress of the reaction from start to finish. It is sometimes labelled as the 'reaction pathway'.

This coordinate does not represent a physical variable, but it is useful for studying chemical energy changes.

Before the reaction, the chemical energy is equal to the energy of the reactants.

As the reaction begins, the energy increases because the bonds of the reactant molecules are broken. This frees the individual atoms to bond with different atoms and form new products.

At the peak of the energy profile curve, the bonds have been completely broken.

In an energy profile, the chemical energy must reach at least the energy needed to break all the bonds apart.

This required energy is identified as the peak on the energy profile. Energy decreases after this point because new bonds start to form, releasing energy from the system.

The difference between the peak energy and the energy of the reactants is called the activation energy ($$E_A$$). The activation energy has to be overcome for the reaction to occur.

After the peak, the energy decreases until all the new bonds are formed. When all products are formed, the reaction ends.

At this point, the energy remains constant and is equal to the energy of the products.

Energy profile of the exothermic combustion of glucose. The lower activation energy peak represents the alternative pathway observed when a catalyst is used.

Every reaction is classified as either endothermic or exothermic by examining the overall chemical energy change on an energy profile.

The energy of products is higher than the energy of reactants in an endothermic reaction. In an exothermic reaction, however, the energy of products is lower than the energy of reactants.

$$\Delta H$$ is labelled on as the vertical difference between the reactant and product energies.

In a forward reaction, the reactants react to form the products. In a reverse reaction, the products react to reform the reactants.

A reverse reaction has the same energy profile pattern, but it proceeds in the opposite direction.

If a reaction is endothermic, then the reverse reaction is exothermic. The magnitude of enthalpy change is the same for the forward and reverse reaction, but the sign is different.

$$\Delta H$$ of the formation of water is $$-286\text{ kJ/mol}$$. Breaking down water into hydrogen and oxygen has a $$\Delta H$$ of $$+286\text{ kJ/mol}$$.