Question

The activation energy for a simple chemical reaction \(\mathrm{X} \rightarrow \mathrm{Y}\) is Ea for forward direction. The value of Ea for backword direction may be a. \(-\mathrm{Ea}\) b. \(2 \mathrm{Ea}\) \(\mathbf{c}_{*}>\) or \(<\mathrm{Ea}\) d. Zero

Short answer

The activation energy for the backward reaction is either greater or less than Ea, based on the reaction's energetics.

Step-by-step solution

Understand Activation Energy

Activation energy (E_a) is the minimum energy required for a reaction to occur. For a reaction X ightarrow Y, E_a represents the energy needed for the transformation from X to Y during the forward reaction.

Consider Reverse Reaction Energy

The activation energy for the reverse reaction is the energy needed to convert Y back to X. This depends on the energy difference between the reactants and products along with the forward activation energy.

Use the Potential Energy Diagram

In a potential energy diagram, E_a of the forward direction is from reactants to the peak, and E_a' for the backward reaction is from products to the peak. If the reaction is exothermic, E_a' is typically greater than E_a, and if it is endothermic, E_a' is typically less.

Consider Reaction Type

For exothermic reactions, activation energy for the reverse reaction is higher than the forward because energy is released as heat. For endothermic reactions, the reverse activation energy might be lower as extra energy is absorbed during the reaction.

Analyze the Options

Given the options, (-E_a) is not physically feasible since activation energy cannot be negative. Option 'b', 2E_a, might imply a much higher reverse action, uncommon unless very exothermic. 'd', zero activation energy, is not reasonable for non-trivial reactions. Thus option c) with E_a' being either greater or less than E_a is most plausible depending on reaction energetics.

Key concepts

Understanding Chemical Reactions

A chemical reaction involves a process where reactants are transformed into products, often involving a change in energy. This can be visualized as breaking the chemical bonds in reactants and forming new bonds in products. The activation energy, noted as \(E_a\), plays a crucial role in determining whether a reaction will occur. It acts as an energy barrier that reactants must overcome to convert into products. Without the necessary energy to surpass this barrier, the reaction may not proceed, or it may proceed at a slower rate. Understanding the role of activation energy helps to predict and control the direction and rate of chemical reactions.

Understanding Potential Energy Diagrams

A potential energy diagram is a visual representation of the energy changes during a chemical reaction. It plots energy on the vertical axis and the progress of the reaction on the horizontal axis. Such diagrams help us understand how energy varies as reactants transform into products.

Key features of these diagrams include:

• The reactants are usually placed on the left, and the products on the right.
• A peak or "energy hill" represents the transition state or activation energy \(E_a\).
• The difference in height between reactants and products shows the overall energy change of the reaction.

This diagram helps in visualizing whether a reaction is exothermic or endothermic and assists in understanding the energy needed for the reverse reaction as well.

Exothermic Reactions Simplified

In exothermic reactions, energy is released as reactants convert into products. This release often manifests as heat, making such reactions feel warm to the touch. On a potential energy diagram, the products are placed at a lower energy level than the reactants, indicating that energy has been released during the process.

Characteristics of exothermic reactions include:

• The activation energy for the reverse reaction is higher than for the forward reaction as products are more stable.
• Common examples include combustion reactions and many oxidation processes.
• These reactions can sometimes self-sustain once started, as the energy released helps surpass the activation energy for subsequent molecules.

Recognizing when a reaction is exothermic helps in predicting its behavior and stability during the transformation.

Endothermic Reactions Explained

Endothermic reactions absorb energy, often leading to a sensation of coolness. These processes require an input of energy to proceed, making the products positioned at a higher energy level than the reactants on a potential energy diagram.

Here are some features of endothermic reactions:

• They tend to have a lower activation energy for the reverse reaction because the products are less stable without continuous energy input.
• Examples include photosynthesis and the dissolving of certain salts in water.
• Such reactions often need sustained energy input to maintain the transformation from reactants to products.

Understanding endothermic reactions is essential for applications where energy absorption is useful, such as in cooling processes.