Catalyzed Reactions
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ΔG
Uncatalyzed reaction
Energy diagram
Transition state
Activation energy, reactants
Activation energy, products
Catalyzed reaction
Energy diagram
Transition state
Activation energy
Desolvation energy and
entropy change
Binding energy
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G reaction progress Reactants Products

Here, the energy changes during a chemical reaction are depicted:

A + B –> C + D

This reaction will proceed as indicated, because the energy level of the products is lower than that of the substrates ( i.e., ΔGo is negative ).

X ΔGo is defined as the difference between the free energy of the products of a reaction and the free energy of the reactants.

A negative value indicates a spontaneously proceeding reaction. It does not indicate a reaction that immediately starts. This is dependent on the probability of the reactants to gain the necessary activation energy as shown in the following parts of this tutorial.

During most reactions, the energy change does not follow a linear path from substrates to products. Instead, reactants first have to gain energy in order to reach a higher energy level.

X The energy that has to be collected to reach this state is the activation energy. In this transition state of the reaction, reactants have already undergone part of the changes on their way to products, but the reaction is not yet complete.

The height of the activation energy governs the rate of the reaction, according to the equation:

Eyring ☰

There is an equal probability that the transition state decays into products or falls back to reactants. But note that reaching the transition state needs more energy when starting at the energy level of the products. This explains why this reaction will reach it's equilibrium at the side of the products: the transition state is approached more often from the reactant's level.

Enzymes stabilize the transition state by additional interactions. The released binding energy lowers the energy level and thereby accelerates the reaction.

X Binding of substrates to the enzyme is reflected not only in the height of the activation energy changing, energy changes also occur at the beginning and the end of the reaction cycle:

Upon approaching the active site of the enzyme, substrates loose their water shell and get immobilized, resulting in both, increase of the Enthalpy (less interactions with water) and decrease of the Entropy (less freedom of motion). In a second step, substrates get bound to the enzyme and the corresponding binding energy is released.

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