Question

The following reaction is endothermic: $$ \mathrm{CF}_{2} \mathrm{Cl}_{2}(g) \longrightarrow \mathrm{CF}_{2} \mathrm{Cl}(g)+\mathrm{Cl}(g) $$ Draw an energy diagram that shows the relative energies of the reactants, products, and the activated complex. Label the diagram with molecular representations of reactants, products, and a possible structure for the activated complex.

Short answer

The energy diagram for this endothermic reaction starts with the reactants at a lower energy level. As energy is absorbed, the system moves to a higher energy state called the activated complex, which is the highest point on the diagram. Finally, it moves to an even higher energy level where the products are located. The molecular structures are drawn next to each label.

Step-by-step solution

Identify reactants and products

The given equation is: \( CF_{2}Cl_{2}(g) \longrightarrow CF_{2}Cl(g) + Cl(g) \). Here, \( CF_{2}Cl_{2} \) is the reactant and \( CF_{2}Cl\) and \( Cl\) are the products.

Sketch the Energy Diagram

Start by drawing a hump-shaped curve on a graph. The horizontal axis represents the progress of reaction from reactants to products, while the vertical axis represents the energy. Mark the curve's initial point as 'Reactants' at a certain energy level.

Illustrate the Activated Complex

The highest point on the curve is the activated complex or the transition state. This is the most unstable state where old bonds are breaking and new bonds are forming. Label the top of the hump as 'Activated Complex'.

Indicate Products

Since this is an endothermic reaction, the energy of the products is higher than the energy of the reactants. Draw the 'Products' label higher on the energy axis than the 'Reactants'.

Draw Molecular Structures

Finally, represent the reactants, products, and the activated complex using their molecular structures next to the respective labels.

Key concepts

Energy Diagram

An energy diagram is a graphical representation that shows the energy changes during a chemical reaction. In the context of an endothermic reaction, it's crucial to understand that overall, energy is absorbed. This is depicted in the diagram by showing reactants at a lower energy level than the products. In such a diagram:

• The horizontal axis represents the progress of the reaction, moving from reactants to products.
• The vertical axis stands for the energy level. As you move up the axis, the energy increases.

For the reaction \( CF_{2}Cl_{2}(g) \rightarrow CF_{2}Cl(g) + Cl(g) \), start by marking the energy level of the reactants on the left. Draw the progression path as a curve that rises to a peak before descending slightly to the level of the products, which is higher than that of the reactants due to the endothermic nature of the process. The peak of this curve represents the maximum energy point, known as the activated complex. This diagram aids in visualizing the energy absorbed as bonds break and new bonds form.

Activated Complex

The activated complex, often referred to as the transition state, is a fleeting and critical point in the reaction pathway. It represents the highest energy state that the reactants must pass through to convert into products. In the energy diagram, this state is represented by the peak of the curve.At this stage:

• Old chemical bonds are breaking while new ones start forming.
• It has a unique molecular structure different from both the reactants and products, often represented as \( CF_{2}Cl oot Cl \).
• This structure is highly unstable and can quickly transform into the products or revert to reactants.

The energy required to reach this activated complex from the reactants is known as the activation energy. Understanding this peak helps in comprehending why certain reactions require energy to proceed, especially in endothermic reactions.

Molecular Structures

In chemical reactions, understanding the molecular structures of all involved species is vital for a comprehensive grasp of the reaction dynamics. For the given reaction \( CF_{2}Cl_{2} \rightarrow CF_{2}Cl + Cl \):The reactant, \( CF_{2}Cl_{2} \), consists of two fluorine atoms and two chlorine atoms bonded to a central carbon atom. As the reaction proceeds to products, one chlorine atom breaks away, resulting in:

• \( CF_{2}Cl \): Consists of one chlorine and two fluorine atoms bonded to carbon.
• \( Cl \): A lone chlorine atom.

During the activated complex (transition state), both Cl atoms are in the process of detaching, illustrated as \( CF_{2}Cl \boot Cl \).This understanding of molecular structures aids in visualizing the reaction process, showing how molecules rearrange, and highlights the specific changes in bonds that require energy in endothermic reactions. The structural transformations offer insight into the reactivity and the feasibility of chemical reactions.