Question

Draw the energy profile for the following exothermic reaction: $$ \mathrm{H}_{2}(g)+\mathrm{Cl}_{2}(g) \rightleftarrows 2 \mathrm{HCl}(g) $$ Label the axes Energy and Progress of reaction and indicate the reactants, products, transition state, activation energy, and energy of reaction.

Short answer

The energy profile shows reactants at higher energy, products at lower energy, with a peak for the transition state indicating activation energy, and negative energy of reaction for exothermic release.

Step-by-step solution

Understanding the Reaction

The reaction given is an exothermic reaction where hydrogen gas reacts with chlorine gas to form hydrogen chloride. In this type of reaction, energy is released to the surroundings, leading to products with lower energy than the reactants.

Drawing the Energy Axis

Draw a vertical axis on the left side, labeled as 'Energy'. This axis represents the energy level of the substances involved in the reaction.

Drawing the Progress of Reaction Axis

Draw a horizontal axis at the bottom, labeled as 'Progress of Reaction'. This axis shows how the reaction progresses from reactants to products over time.

Indicating Reactants and Products

Start the energy curve at a higher energy level on the left side for the reactants \( \mathrm{H}_2(\text{g})+\mathrm{Cl}_2(\text{g}) \). On the right side, draw the energy level lower for the products \( 2\mathrm{HCl}(\text{g}) \).

Drawing the Transition State

Draw a peak between the reactants and products to represent the transition state. This is the highest point on the curve, indicating the maximum energy state during the reaction.

Indicating Activation Energy

Label the energy difference between the energy of reactants and the peak (transition state) as 'Activation Energy'. This is the energy needed to start the reaction.

Indicating Energy of Reaction

Label the energy difference between the reactants and products as 'Energy of Reaction'. Since the reaction is exothermic, this value will be negative, reflecting the decrease in energy.

Finalizing the Energy Profile

Ensure all sections are labeled: Reactants, Transition State, Products, Activation Energy, and Energy of Reaction. This completes the energy diagram.

Key concepts

Energy Profile

An energy profile diagram is a visual representation of the energy changes during a chemical reaction. For exothermic reactions like the one in our exercise, the diagram typically features a curve that starts with the reactants at a higher energy level and ends with the products at a lower energy level, indicating energy release.
To construct this profile, we begin by labeling our axes. The vertical axis represents the energy level, while the horizontal axis reflects the progress of the reaction—the pathway the reactants take to become products. At the start, place the reactants' energy at the left, higher on the vertical axis. As the reaction proceeds, the curve ascends to a peak, then descends, reflecting the lower energy of the products.
This profile not only shows the transition from reactants to products but also highlights key concepts like activation energy and transition state, which we will delve into in the sections below.

Activation Energy

Activation energy is a crucial concept that describes the minimum energy required to initiate a chemical reaction. In an energy profile, this is represented by the energy difference between the reactants and the transition state's peak.

• This energy forms a barrier that the reactants must overcome to transform into products.
• The peak on the energy profile diagram represents the highest energy state reached during the reaction.

For exothermic reactions, although the products have lower energy than the reactants, a sufficient amount of energy is initially needed to achieve this highest state. Once this activation energy is provided, the reaction proceeds downhill energetically, releasing energy into the surroundings.
Even though the reaction releases energy, the initial input is crucial for breaking and reforming bonds, forming new products. Hence, this concept highlights the importance of energy barriers in chemical processes.

Transition State

The transition state in a chemical reaction is a temporary, high-energy state of the reactant molecules.

• It occurs at the peak of the energy profile diagram, representing the point where old bonds are breaking, and new bonds are forming.
• This fleeting arrangement is sometimes referred to as the activated complex.

In our exothermic reaction example, as hydrogen and chlorine atoms react, they first form a transition state before becoming molecules of hydrogen chloride (HCl).
The transition state is a crucial point where the reaction can either proceed to form products or revert to reactants. Its position at the top of the energy barrier is significant because it signifies the highest energy point of the reaction pathway. By understanding the transition state, chemists can better understand the steps involved in chemical reactions.

Energy of Reaction

The energy of reaction is a measure of the overall energy change that occurs during a chemical reaction. In an exothermic reaction, this value is negative, indicating that energy is being released.

• It is calculated as the difference in energy between the reactants and the products. In the energy profile, this is depicted as the vertical distance between the reactants and products lines.
• For our reaction, \[\Delta E = E_{ ext{products}} - E_{ ext{reactants}}\]where \( \Delta E\) denotes the energy change of the system.

A negative \( \Delta E\) reflects a release of energy, making the surroundings warmer. This released energy often appears as heat and can be harnessed for work or simply contribute to the entropy of the surroundings.
Understanding the energy of reaction is pivotal for scientists and engineers to predict the feasibility and efficiency of chemical reactions in various practical applications.