Question

Draw the potential energy diagram for an endothermic reaction. Indicate on the diagram the activation energy for both the forward and reverse reactions. Also indicate the heat of reaction.

Short answer

Draw two axes for potential energy vs. reaction progress. Mark the reactants' energy lower than the products'. Plot forward and reverse activation energy curves peaking at the transition state, with forward Ea below reverse Ea. Finally, indicate the positive heat of reaction (ΔH) from reactants to products.

Step-by-step solution

Drawing the Axes

Begin by drawing two axes on graph paper or a blank sheet of paper. The vertical axis should be labeled 'Potential Energy' and the horizontal axis 'Reaction Progress'. Make sure the axes are sufficiently long to accommodate the entire diagram.

Indicating the Reactants and Products Energy Levels

Since the reaction is endothermic, the potential energy of the products will be higher than that of the reactants. Mark a point on the vertical axis below the middle to indicate the potential energy of the reactants. Then mark a point higher up to represent the potential energy of the products.

Drawing the Activation Energy for the Forward Reaction

The activation energy is the energy needed to initiate the reaction. Draw a curve starting from the reactants' energy level, peaking up to indicate the transition state, and then descending towards the products' energy level. The peak of this curve represents the highest energy point - the activated complex of the forward reaction.

Labeling the Activation Energy for the Forward Reaction

Label the energy gap between the reactants' energy level and the peak of the curve as the 'Activation Energy for the Forward Reaction (Ea forward)'. This is the minimum energy required for the reactants to transform into the transition state before forming products.

Drawing the Activation Energy for the Reverse Reaction

Although the diagram illustrates an endothermic reaction, it’s important to show that the reverse reaction is exothermic. Draw a curve from the products' energy level, that peaks at the same activated complex, but then descends down to the reactants' energy level. The peak of this curve is the same transition state for both the forward and reverse reactions.

Labeling the Activation Energy for the Reverse Reaction

Label the energy gap between the products' energy level and the peak of the curve as the 'Activation Energy for the Reverse Reaction (Ea reverse)'. This is the activation energy required for the reverse reaction to proceed.

Indicating the Heat of Reaction

The heat of reaction (ΔH) is the difference in potential energy between the products and reactants. Indicate this on the diagram by drawing a horizontal arrow pointing from the reactants' energy level to the products' energy level and label it 'ΔH (endothermic)'. For an endothermic reaction, ΔH is positive, meaning that the system absorbs heat from the surroundings.

Key concepts

Activation Energy

In chemical reactions, the activation energy is akin to a hurdle that reactants must overcome to transform into products. It's a crucial concept in understanding reaction kinetics.

Imagine you're pushing a boulder uphill before it can roll down the other side. The climb represents the activation energy, the peak correlates to the transition state, and the roll down is the reaction proceeding to form products. In endothermic reactions, drawing this 'hill' on a potential energy diagram involves marking a high point (peak) that shows how much energy the reactants need to absorb to reach the transition state. This 'peak' towers above both the reactants and products, signaling the energy input requirement for the reaction initiation. Conversely, for the reverse reaction, which is exothermic, the peak stays the same, but the downhill slope ends lower, at the original reactants' level, implying energy release upon reverting back.

Reaction Progress

The reaction progress can be visualized as a journey from starting materials to final products, plotted on a horizontal axis in a potential energy diagram. This progression provides a visual story of the reaction path.

As reactants transform into products, they go through various changes, represented graphically as a curve on the diagram. The initial point marks the reactants' energy level, and as the curve reaches its zenith—the transition state—it then descends, mapping the energy decrease until the plot ends at the products' stage. The span of the curve along the 'Reaction Progress' axis visualizes the entire reaction mechanism, capturing the dynamic nature of chemical transformations.

Heat of Reaction

Heat of reaction, denoted as ΔH, is the thermal exchange accompanying a chemical reaction. In simple terms, it tells us whether heat flowed into the system (endothermic) or out to the surroundings (exothermic).

On a potential energy diagram, the ΔH is represented by a horizontal arrow or line stretching from the reactants to the products. This visual illustrates the net energy change. For an endothermic reaction, the arrow points upward, implying that energy is absorbed, while in an exothermic process, it would point downwards. Understanding ΔH is essential, as it indicates the energetics of a reaction, providing insight into the feasibility and conditions necessary for a reaction to occur.

Transition State

The transition state of a reaction is a fleeting configuration where bonds are in the act of breaking and forming — it is the apex of the activation energy barrier. This highest-energy state is key to understanding the reaction pathway.

Depicting the transition state on a potential energy diagram involves locating the peak on the curve connecting reactants and products — a spot that signifies it has reached the highest potential energy during the reaction. This state is unstable and exists for just a moment before the system relaxes into products or falls back to reactants. The transition state isn't isolated or directly observed but is inferred from the energy requirements and reaction kinetics.

Exothermic Reaction

An exothermic reaction is essentially the opposite of an endothermic one; it releases energy into its surroundings, often in the form of heat. These reactions give a cozy feeling when sitting by a fire or the warmth from a hand-warmer in winter.

On a potential energy diagram, exothermic reactions show the products with less potential energy than the reactants, with a downward arrow indicating the exothermic heat of reaction (ΔH). The practical implications of exothermic processes are significant—they drive our engines, power our stoves, and keep us warm, all by releasing stored chemical energy as heat.