Chapter 17: Problem 15
Consider the energy profile for a reversible endothermic reaction. Is \(E_{\text {act }}\) greater for the forward or the reverse reaction?
Short Answer
Expert verified
The activation energy is greater for the reverse reaction.
Step by step solution
01
Understanding Reaction Profiles
An energy profile is a graph that shows the change in energy as the reactants are converted to products. In a reversible reaction, there is an activation energy for the forward reaction \(E_{\text{act, forward}}\) and for the reverse reaction \(E_{\text{act, reverse}}\).
02
Characteristics of Endothermic Reactions
For an endothermic reaction, the products are at a higher energy level than the reactants. This means that energy is absorbed during the reaction, resulting in a positive change in enthalpy (\(\Delta H > 0\)).
03
Identifying Activation Energies
In an energy profile, the activation energy for the forward reaction \(E_{\text{act, forward}}\) is the energy difference from the reactants to the peak of the energy curve. For the reverse reaction, \(E_{\text{act, reverse}}\) is the energy difference from the products to this peak.
04
Comparing Activation Energies
For an endothermic reaction, the products are higher in energy than the reactants. Therefore, the activation energy for the reverse reaction \(E_{\text{act, reverse}}\) includes the energy difference of the forward reaction's activation energy and the \Delta H of the reaction: \(E_{\text{act, reverse}} = E_{\text{act, forward}} + \Delta H\). Since \(\Delta H > 0\), it follows that \(E_{\text{act, reverse}} > E_{\text{act, forward}}\).
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Key Concepts
These are the key concepts you need to understand to accurately answer the question.
Activation Energy
When thinking about chemical reactions, it's helpful to know the concept of activation energy. Activation energy, often symbolized as \(E_{\text{act}}\), is the minimum amount of energy needed for a reaction to occur. Imagine it as a hurdle that reactant molecules need to hop over to transform into products. Without sufficient energy to surpass this hurdle, the reaction won't take place.
In the context of a reversible endothermic reaction, we have two important activation energies to consider: \(E_{\text{act, forward}}\) for the forward reaction and \(E_{\text{act, reverse}}\) for the reverse reaction. In an endothermic reaction, where the products hold more energy than the reactants, additional energy is absorbed from the surroundings.
This absorbed energy means that while \(E_{\text{act, forward}}\) is already required to start the reaction, the \(E_{\text{act, reverse}}\) needs to be even larger. This is because it's like going back up the hill from a higher starting point. With endothermic reactions, hence, the activation energy for the reverse reaction is greater than that for the forward reaction.
In the context of a reversible endothermic reaction, we have two important activation energies to consider: \(E_{\text{act, forward}}\) for the forward reaction and \(E_{\text{act, reverse}}\) for the reverse reaction. In an endothermic reaction, where the products hold more energy than the reactants, additional energy is absorbed from the surroundings.
This absorbed energy means that while \(E_{\text{act, forward}}\) is already required to start the reaction, the \(E_{\text{act, reverse}}\) needs to be even larger. This is because it's like going back up the hill from a higher starting point. With endothermic reactions, hence, the activation energy for the reverse reaction is greater than that for the forward reaction.
Reversible Reaction
Chemical reactions are often reversible,
which means that they don’t just go from reactants to products but can also go back from products to reactants.
In our case of a reversible endothermic reaction, we have dynamic scenarios where both the forward and reverse reactions happen. The term 'reversible' essentially means the reaction can move in both directions, reaching what we call equilibrium when both rates are balanced.
A good way to picture this is as a two-way street:
The balance between the forward and reverse reactions helps determine the conditions at which a chemical balance, or equilibrium, is achieved. This delicate balance is what allows the system to "decide" how much of each substance will be formed at any given moment.
In our case of a reversible endothermic reaction, we have dynamic scenarios where both the forward and reverse reactions happen. The term 'reversible' essentially means the reaction can move in both directions, reaching what we call equilibrium when both rates are balanced.
A good way to picture this is as a two-way street:
- One lane is the forward reaction (turning reactants into products, consuming energy as it's endothermic),
- and the opposite lane is the reverse reaction (turning products back into reactants, trying to lower the energy level).
The balance between the forward and reverse reactions helps determine the conditions at which a chemical balance, or equilibrium, is achieved. This delicate balance is what allows the system to "decide" how much of each substance will be formed at any given moment.
Energy Profile
An energy profile is a pictorial depiction of how energy changes during the progress of a chemical reaction. Think of it as a map that shows the energy journey from reactants to products.
For a reversible endothermic reaction, the energy profile typically features a curve with a peak, where the highest point represents the activation energy for the forward reaction. The height from the baseline (reactants) to this peak is \(E_{\text{act, forward}}\). Beyond this peak, the energy line descends towards products, which still lie above the reactants due to the added energy requirement of the endothermic process.
However, when considering the reverse reaction, starting from products, the peak represents \(E_{\text{act, reverse}}\). In this scenario, an energy profile also helps illustrate the change in enthalpy \((\Delta H)\), showing the energy absorbed in the process.
The energy profile is a great tool for visualizing and comparing the energy required for both directions of a reversible reaction, clearly showing that for endothermic reactions, more energy is needed to convert products back to reactants.
For a reversible endothermic reaction, the energy profile typically features a curve with a peak, where the highest point represents the activation energy for the forward reaction. The height from the baseline (reactants) to this peak is \(E_{\text{act, forward}}\). Beyond this peak, the energy line descends towards products, which still lie above the reactants due to the added energy requirement of the endothermic process.
However, when considering the reverse reaction, starting from products, the peak represents \(E_{\text{act, reverse}}\). In this scenario, an energy profile also helps illustrate the change in enthalpy \((\Delta H)\), showing the energy absorbed in the process.
The energy profile is a great tool for visualizing and comparing the energy required for both directions of a reversible reaction, clearly showing that for endothermic reactions, more energy is needed to convert products back to reactants.
