Question

For an endothermic reaction, where \(\Delta \mathrm{H}\) represents the enthalpy of the reaction in \(\mathrm{kJ} / \mathrm{mol}\), the minimum value for the energy of activation will be (a) less than \(\Delta \mathrm{H}\) (b) zero (c) more than \(\Delta \mathrm{H}\) (d) equal to \(\Delta \mathrm{H}\)

Short answer

The minimum value for the energy of activation is more than \( \Delta H \). (Option c)

Step-by-step solution

Understanding the Problem

We are given options for the minimum energy of activation for an endothermic reaction where the enthalpy change, \( \Delta H \), is known. We need to determine which option correctly describes the energy of activation compared to \( \Delta H \).

Defining Endothermic Reaction

In an endothermic reaction, energy is absorbed, resulting in products having a higher energy than reactants, and \( \Delta H \) (enthalpy change) is positive.

Energy of Activation

The energy of activation is the minimum energy needed for the reactants to reach the transition state from which products can be formed. It represents the energy barrier of the reaction.

Analyzing and Comparing

For an endothermic reaction, to surpass the energy barrier and form products with higher energy, the energy of activation must exceed \( \Delta H \). Therefore, the minimum energy required must be more than \( \Delta H \) to provide the energy necessary to reach the transition state.

Conclusion

Based on the above analysis, for an endothermic reaction, the minimum energy of activation is typically more than \( \Delta H \), which corresponds to option (c).

Key concepts

Energy of Activation

In any chemical reaction, the energy of activation is key because it is the gateway that reactants must pass through to transform into products. Think of the energy of activation as a hurdle or barrier; it is the minimum energy that reacting molecules need to achieve the transit through the transition state and onwards to become the desired products.

The height of the energy barrier represents how much energy must be supplied for the reaction to proceed. For endothermic reactions where energy is absorbed, the energy of activation must be reasonably high. This is because reactants need additional energy not only to reach the transition state but to proceed to a final state that is less stable (higher energy) than the starting point.

In essence, for endothermic reactions, since product formation involves absorbing energy, the energy of activation is generally greater than the enthalpy change \(\Delta H \), ensuring that enough energy is supplied for the reactants and transition state to break the energetic barrier.

Enthalpy Change

Enthalpy change, symbolized as \( \Delta H \), is an important concept in understanding chemical reactions, especially endothermic ones. It represents the total heat absorbed or released during a reaction carried out at constant pressure.

For endothermic reactions, \( \Delta H \) is always positive. This means that energy is being absorbed from the surroundings, and the products of the reaction are at a higher energy state than the reactants. The positive value of \( \Delta H \) corresponds to the net energy input required to convert reactants into products.

Understanding \( \Delta H \) is essential because it gives insight into the overall energy demand of a reaction and complements the concept of energy of activation, together describing how energy is involved in reaching product formation.

Transition State

The transition state is a temporary and high-energy state that occurs when reactants are converted into products during a chemical reaction. Imagine a peak that reactants must climb to get to the other side, which is the end of the reaction.

In technical terms, the transition state represents the configuration of atoms at the maximum energy point along the reaction path. It is the summit of the energy barrier that reactants need to overcome to form products.

• The energy required to reach this state is known as the energy of activation.
• The transition state is fleeting and cannot be isolated.
• Successfully attaining the transition state is crucial for a reaction to proceed.

Understanding the transition state helps clarify why some reactions require significant energy of activation and how catalysts can reduce this energy requirement, thereby accelerating the reaction.

Chemical Kinetics

Chemical kinetics is the study of the rates of chemical reactions and the factors that affect these rates. It provides valuable information about how quickly a reaction occurs and how it proceeds.

Factors influencing reaction rates include:

• Concentration: Higher concentrations generally increase reaction rates due to more frequent collisions between reactants.
• Temperature: As the temperature rises, reaction rates typically increase since molecules have more kinetic energy and collide more forcefully.
• Catalysts: Substances that lower the energy of activation, facilitating a quicker transition to the transition state.

Chemical kinetics is essential for controlling industrial chemical processes, optimizing reaction conditions, and understanding complex reaction mechanisms. By analyzing the kinetics, scientists can design better catalysts, improve reaction efficiencies, and predict how various conditions will affect the reaction outcomes.