Interpreting Energy Level Diagrams for the ESAT

Features of an Energy Level Diagram

In chemical reactions, energy is either absorbed from or released to the surroundings as chemical bonds are broken and formed. An energy level diagram, also known as a reaction profile, provides a visual representation of these energetic changes. The vertical y-axis represents the total enthalpy (chemical energy) of the substances, typically measured in $kJ\ mol^{-1}$, while the horizontal x-axis represents the progress of the reaction (sometimes called the reaction coordinate).

There are three critical levels to identify on any energy level diagram:

1. The reactant level: The initial energy of the starting materials.
2. The product level: The final energy of the substances formed.
3. The peak: The highest energy point on the curve, representing the transition state or activated complex.

Exothermic Reactions and Negative Enthalpy

An exothermic reaction is one in which energy is transferred from the chemical system to the surroundings. This occurs because the energy released when new bonds form in the products is greater than the energy required to break the bonds in the reactants.

In an exothermic energy level diagram:

• The energy level of the products is lower than the energy level of the reactants.
• The enthalpy change, $Δ H$, is negative (e.g., $Δ H < 0$).
• An arrow representing $Δ H$ points downwards from the reactant line to the product line.

Common examples of exothermic processes include combustion, neutralisation between acids and alkalis, and many oxidation reactions.

Endothermic Reactions and Positive Enthalpy

An endothermic reaction absorbs energy from the surroundings. This happens when the energy required to break existing bonds is greater than the energy released during the formation of new bonds.

In an endothermic energy level diagram:

• The energy level of the products is higher than the energy level of the reactants.
• The enthalpy change, $Δ H$, is positive (e.g., $Δ H > 0$).
• An arrow representing $Δ H$ points upwards from the reactant line to the product line.

Typical endothermic reactions include thermal decomposition, such as the breakdown of calcium carbonate, and the reaction between citric acid and sodium hydrogencarbonate.

Activation Energy and the Transition State

Even in highly exothermic reactions, a certain amount of energy is usually needed to initiate the process. This is the activation energy ($E_a$). It represents the minimum energy that colliding particles must possess to react successfully by breaking the original bonds.

On a diagram, $E_a$ is the vertical distance measured from the energy of the reactants to the peak of the energy curve. It is always a positive value because energy must be put into the system to reach the transition state. The transition state is the point where reactant bonds are at their limit of breaking and product bonds are beginning to form.

The Impact of Catalysts on Energy Profiles

A catalyst is a substance that increases the rate of a chemical reaction without being consumed. It achieves this by providing an alternative reaction pathway that has a lower activation energy. When representing a catalyst on an energy level diagram:

• The reactant and product levels remain exactly the same.
• The enthalpy change ($Δ H$) remains unchanged.
• The peak of the curve is lower than the original peak, representing the reduced $E_a$.

Worked Example: Calculating Enthalpy and Activation Energy

Consider a reaction where the energy of the reactants is $150\ kJ\ mol^{-1}$, the energy of the products is $400\ kJ\ mol^{-1}$, and the peak of the energy curve is at $550\ kJ\ mol^{-1}$.

1. Identify the reaction type: Because the energy of the products ($400\ kJ\ mol^{-1}$) is higher than the energy of the reactants ($150\ kJ\ mol^{-1}$), the reaction is endothermic.
2. Calculate the enthalpy change ($Δ H$): Use the formula $Δ H = H_{products} - H_{reactants}$. In this case, $400 - 150 = +250\ kJ\ mol^{-1}$.
3. Calculate the activation energy ($E_a$): Use the formula $E_a = H_{peak} - H_{reactants}$. In this case, $550 - 150 = 400\ kJ\ mol^{-1}$.
4. Identify the effect of a catalyst: If a catalyst were added, the $Δ H$ would remain $+250\ kJ\ mol^{-1}$, but the $E_a$ would be less than $400\ kJ\ mol^{-1}$.