Question

Sketch an energy diagram for a two-step reaction in which both steps are exergonic and in which the second step has a higher-energy transition state than the first. Label the parts of the diagram corresponding to reactant, product, intermediate, overall \(\Delta G^{\mp}\), and overall \(\Delta G^{\circ}\).

Short answer

Draw an energy diagram with two exergonic steps, the second transition state higher than the first, label reactants, products, intermediates, \(\Delta G^{\mp}\), and \(\Delta G^{\circ}\).

Step-by-step solution

Identify the Reaction Components

In a two-step reaction process, you will have reactants, intermediates, and products. Begin by identifying each of these components. For this situation, we know the reaction is exergonic for both steps, indicating that the products of each step have lower energy than the starting materials, including the intermediate.

Illustrate the Reactants and Products

Draw the initial energy level on your diagram, representing the energy of the reactants. The final energy level should be lower since both steps are exergonic, representing the energy of the products.

Draw the Two Transition States

For two-step reactions, there are two transition states. Draw two peaks on the diagram, showing that both steps have activation energy thresholds. Ensure that the second peak is higher than the first to represent the higher-energy transition state in the second step.

Indicate the Intermediate

Place an energy level between the two peaks to represent the intermediate state. The intermediate should be at a lower energy than the starting reactants but higher than the final products due to the exergonic nature of the step.

Label the Energy Changes

Now label the diagram with \(\Delta G^{\circ}\), which is the overall energy change from reactants to products as both steps are exergonic, this should be negative. Also, label \(\Delta G^{\mp}\), which represents the activation energies needed for each step. Be sure to highlight the higher activation energy for the second step.

Key concepts

Exergonic Reaction

An exergonic reaction is a type of chemical reaction where energy is released to the surroundings. In these reactions, the energy of the products is lower than that of the reactants, which results in an overall release of energy.
This makes the reaction spontaneous, or at least favorable under standard conditions.
For a two-step exergonic reaction, each step will release energy.

• The energy of the reactants is higher than that of the products.
• An exergonic reaction is represented by a negative Gibbs free energy change ( abla G^{ ext{°}}).
• Exergonic reactions are commonly observed in biochemical processes, providing energy to drive other reactions or biological work.

Transition State

The transition state in a chemical reaction is a high-energy state that occurs during the transformation of reactants into products. It is often represented as a peak on an energy diagram.
In the context of a two-step reaction with two transition states, each peak represents the point at which bonds are in the process of breaking and forming.

• The transition state is not a stable state; it is fleeting and difficult to observe directly.
• For the given problem, the second step's transition state has a higher energy level, indicating a larger energy barrier for that step.
• Transition states help in understanding the kinetics of a reaction.

Activation Energy

Activation energy ( abla G^{ ext{†}}) is the minimum energy barrier that reactants must overcome to transform into products. It is represented as the height of the peak in the energy diagram.
Each step in a multi-step reaction may have its own activation energy.

• This energy is necessary to reach the transition state from the reactants.
• In the diagram provided, the second step requires higher activation energy, shown by the taller peak.
• While higher activation energy can slow down a reaction, catalysts can be used to lower this energy barrier.

Reaction Intermediate

In a multi-step reaction, an intermediate is formed during one step and consumed in a subsequent step. It lays between two transition states in energy diagrams.
The energy of an intermediate is higher than that of the final products, but lower than the initial reactants if the overall process is exergonic.

• The intermediate is usually less stable than the reactants or products.
• It appears as a plateau or a low point between two peaks in the energy diagram.
• Intermediates are crucial in understanding reaction mechanisms, providing insight into how reactants are converted to products.

Gibbs Free Energy Change

Gibbs free energy change ( abla G^{ ext{°}}) is a measure of the overall energy difference between reactants and products in a chemical reaction. It indicates whether a reaction is spontaneous.
If abla G_{ ext{°}} is negative, as in exergonic reactions, the process releases energy and tends to occur spontaneously.

• abla G^{ ext{°}} = Energy of products - Energy of reactants.
• In an energy diagram, abla G^{ ext{°}} can be visualized as the difference between the initial reactants' energy level and the final products' energy level.
• This change is critical for understanding the thermodynamic feasibility of reactions.