Question

Phosphorus pentachloride is used in the electronics industry to manufacture computer chips. Draw the energy profile for the following reaction: $$ \mathrm{PCl}_{5}(g)+\text { heat } \rightleftarrows \mathrm{PCl}_{3}(g)+\mathrm{Cl}_{2}(g) $$

Short answer

The energy profile shows an upward curve indicating an endothermic reaction where products have higher energy than reactants.

Step-by-step solution

Understand the reaction system

The reaction given is a reversible decomposition reaction of phosphorous pentachloride (\(\mathrm{PCl}_5\)) where heat is applied. It breaks down into phosphorous trichloride (\(\mathrm{PCl}_3\)) and chlorine gas (\(\mathrm{Cl}_2\)).

Identify the reaction type

This is an endothermic reaction because heat is required to convert \(\mathrm{PCl}_5\) into \(\mathrm{PCl}_3\) and \(\mathrm{Cl}_2\). In endothermic reactions, the reactants have less energy than the products, and heat is absorbed.

Determine the energy profile components

The energy profile will need to show initial energy (reactants), activation energy, peak energy (transition state), final energy (products), and the overall energy change (\(\Delta H\)).

Draw the energy profile diagram

- Start by drawing a diagram with the reaction progress on the x-axis and energy on the y-axis.- Plot the energy of \(\mathrm{PCl}_5\) as the starting point on the y-axis.- Draw an upward curve to a peak representing the activation energy (energy required to reach the transition state).- From the peak, draw the curve down to the energy level of \(\mathrm{PCl}_3\) and \(\mathrm{Cl}_2\).- The final level should be higher than the initial energy, indicating an endothermic reaction.- Label \(\Delta H\) as the difference between the final and initial energy levels, showing that \(\Delta H\) is positive.

Key concepts

Endothermic Reaction

An endothermic reaction is a type of chemical process where heat is absorbed from the surroundings. This means that the products have higher energy than the reactants, resulting in a positive change in enthalpy (ΔH). For the reaction \( ext{PCl}_5(g) + ext{heat} \rightleftharpoons ext{PCl}_3(g) + ext{Cl}_2(g)\), heat is a necessary component to drive the process forward.

• In endothermic reactions, when heat is provided, it breaks chemical bonds in the reactants, leading to the formation of new products.
• The absorption of energy often leads to cooling of the reaction environment as heat is taken in.
• Common examples outside this reaction include melting ice and photosynthesis.

This property of absorbing energy makes endothermic reactions essential in various industrial processes where heat needs to be controlled.

Energy Profile Diagram

An energy profile diagram is a graphical representation of the energy changes during a chemical reaction. This diagram helps visualize the energy status of reactants and products through the course of the reaction. For the reversible decomposition of \( ext{PCl}_5\), an energy profile diagram would reveal the energy landscape of the process.

• The y-axis of the diagram represents energy, while the x-axis indicates the reaction progress.
• The diagram starts at the energy level of the reactants, \( ext{PCl}_5\) in this case.
• An upward curve symbolizes the energy needed to overcome the activation energy threshold and reach the transition state.
• From the peak, the line proceeds down to the energy level of the products, \( ext{PCl}_3\) and \( ext{Cl}_2\).
• The final energy level is higher than the initial, reflective of the endothermic nature of the reaction.

Understanding these diagrams is crucial for chemists trying to manipulate reactions for industrial applications or laboratory experiments.

Activation Energy

Activation energy is the minimum energy required to initiate a chemical reaction. It is the barrier that reactants must overcome to be transformed into products. For the decomposition of phosphorus pentachloride, the activation energy is illustrated by the peak in the energy profile diagram.

• This energy is required to reach the transition state, where existing bonds are breaking and new bonds are forming.
• Activation energy is considered a catalyst's target, which can lower this barrier, making reactions occur more readily.
• The magnitude of activation energy influences the rate of reaction; higher activation energy means a slower reaction rate.

Chemical reactions with high activation energies often require additional energy input, typically in the form of heat, to proceed at practical rates, as in the case of endothermic reactions.

Reversible Reaction

A reversible reaction is a chemical process where the reactants convert to products and vice versa under suitable conditions. The decomposition of \( ext{PCl}_5\) to form \( ext{PCl}_3\) and \( ext{Cl}_2\) is an example of such a reaction.

• Reversible reactions can achieve equilibrium, where the rate of the forward reaction equals the rate of the reverse reaction.
• These reactions are represented with a double arrow (\(\rightleftharpoons\)), indicating the bi-directional nature of the process.
• In their energy profiles, reversible reactions have similar energy barriers for both directions, though not always identical.

Understanding reversible reactions is fundamental for achieving chemical balance in systems, impacting industries like pharmaceuticals and environmental engineering, where control over reaction direction and extent is crucial.