Energy profiles: Everything at a Glance

Energy profiles are complete descriptors of thermodynamics, kinetics, and equilibrium. Thermodynamics tells about the feasibility of a reaction whereas, chemical kinetics informs about the speed at which the reaction proceeds. The energy profile is a diagrammatic description of all the prerequisite information about kinetics and thermodynamics.

Energy profile is an ID of a reaction!

In order to understand the correlation of energy profile with kinetics and thermodynamics, let us first review what are reactions and what happens during a reaction.

What are Reactions?

When a series of events results in a change in the composition of any chemical substance, the process is regarded as a chemical reaction. The study of this whole process of compositional change (reaction) is categorized as either thermodynamic or kinetic depending on the aspects being studied.

Thermodynamics is concerned with;

Whether that process will occur or not (feasibility).
What set of conditions would be required to bring that chemical change?
The energy absorbed or released during this compositional change.

Chemical kinetics is concerned with;

How fast or how slow a thermodynamically feasible reaction will take place?
By which route would the reaction take place (mechanism)?

What Happens During a Reaction?

Composition changes as a result of old bond breakage and new bond formation.
As different identity is produced, the energies of products will differ from reactants.
This difference of energies results in heat exchange with the surrounding (exothermicity or endothermicity), the thermodynamic concern.

In order to understand how energy profiles deal with the above-mentioned parameters, let’s understand what energy profiles are:

Energy Profiles

“An energy profile is a graphical representation of the energies of involved commodities, plotted as a function of reaction coordinates (axis depicting reaction progress from reactants to product formation)”

A complete energy profile must depict the energies of all involved commodities such as;

Reactant (at starting point of reaction).
Products (at the ending part of reaction coordinate).
Reaction intermediates (as per their appearance in the reaction mixture).
Transition state (high energy activated complex being formed against each reaction step).
All possible pathways for the reaction.

Outcomes of Energy Profiles

The energy profile displays kinetic, equilibrium, and thermodynamic parameters such as;

Kinetics

Activation energy (difference of energies of the transition state and reactants for any reaction (forward/backward)).
From the Arrhenius equation, energy profiles help yield rate constants, at any temperature.
The reaction mechanism i.e. they display the lowest energy pathways using associated reaction intermediates.

Equilibrium

Both forward and backsword rate constants can be calculated for the energy profile leading to the determination of equilibrium constant for any reversible reaction.

Thermodynamics

Feasibility of a chemical reaction i.e. by calculating delta G form equilibrium constant using Van’t Hoff equation.
Thermicity/Enthalpy associated with each reaction step by subtracting reactants energy from that of the product.
By calculating delta G from equilibrium constant, and enthalpy from delta E, the entropy of a system can be estimated.

Therefore, it can be said that reaction chemistry can completely be described by an energy profile.

Do Energy Profiles cover Catalytic Reactions?

The energy profiles not only describe the catalytic or retarding reactions but also elaborate the mechanistic pathways.

A catalyst provides a reaction with a new mechanism having a lower energy pathway with lowered activation energy. As already described, the detailed energy profile contains all the possible reaction pathways. Therefore, catalytic or retarding reactions are depicted in energy profiles with lower or higher activation energy barriers, respectively.

Accordingly, the detailed energy profile of a reaction has the potential of:

Behavior depicting the thermodynamic and kinetic picture of the reactions.
Predicting the feasibility of non-spontaneous reaction to catalysis.
Retarding potency of retarders and associated mechanistic pathways.

Hence,

“Energy profile of a reaction is regarded as the identity card of a reaction”

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Concepts Berg

How do enzymes affect the energy profile?

Enzymes are catalysts. Therefore, they will affect energy profiles by lowering the activation energy.

What are the parts of an energy profile (complete)?

In point to point description of energy as a function of the reaction’s progress, a minimum of two plateaus can be distinguished that are; reactant energy at the start of reaction coordinate, and product energy at the end. In between these two transition state(s) appears a maxima, and the intermediate(s) appear as minima.

The number of maxima and minima appearing between reactant and product baselines totally depends on the number of steps involved in the mechanism of that reaction. The number of maxima is equal to the number of arrows appearing in the reaction mechanism and the number of minima is one less than the number of arrows e.g. equivalent to the number of intermediates.

Therefore, an energy profile is complete when it contains reactant product plateaus, all transition state maxima, and all intermediate minima corresponding to the reaction mechanism as its parts.

What is the difference between an endothermic and an exothermic energy profile?

In an exothermic profile, the reactant(s) will be at a higher energy state than the product(s). The exact opposite is true for an endothermic graph/profile.

Is it possible for a reaction to have zero activation energy?

Not for real reactions.

Transition states that are very short-lived and get converted to the stable product immediately after their formation can show zero activation energy but in actual practice, these conversions would be so fast that they could not be studied within a real-time span. Therefore, these conversions are not regarded as reactions.

So, it can be said that it is not possible for real reactions to be associated with zero activation energy, even very fast nuclear reactions have activation energy equal to the binding energy of the nuclei.