Problem 208 - A Complete Resource Book in Chemistry for JEE Main

Question

Consider an endothermic reaction \(\mathrm{X} \longrightarrow \mathrm{Y}\) with the activation energies \(E_{b}\) and \(E_{f}\) for the backward and forward reactions respectively. In general, \(\quad\) [2005] (a) \(\mathrm{E}_{\mathrm{b}}<\mathrm{E}_{\mathrm{f}}\) (b) \(E_{b}>E_{f}\) (c) \(\mathrm{E}_{\mathrm{b}}-\mathrm{E}_{f}\) (d) there is no definite relation between \(E_{b}\) and \(E_{f}\)

Step-by-Step Solution

Answer

(a) E_b < E_f .

Step 1: Understand Endothermic Reactions

An endothermic reaction is a chemical reaction in which the system absorbs energy from the surroundings as heat. For an endothermic reaction, the energy required to convert reactants into products is greater than the energy released when the products revert back to reactants.

Step 2: Analyze the Relationship Between Activation Energies

In an endothermic reaction, the activation energy for the forward reaction ( E_f ) is generally higher than the activation energy for the backward reaction ( E_b ). This is because more energy is needed to drive the reaction forward due to the absorbance of heat.

Step 3: Compare E_b and E_f for an Endothermic Reaction

Since the forward reaction requires more energy input to proceed, we conclude that E_f > E_b . Therefore, option (a), indicating that E_b < E_f , is correct in the context of an endothermic reaction.

Key Concepts

Activation Energy

Activation energy is a crucial concept in understanding how chemical reactions occur. It is the minimum amount of energy required to initiate a chemical reaction. Think of it like pushing a boulder up a hill—activation energy is necessary to get the boulder moving over the hilltop. In chemical terms, activation energy determines how easily reactants can transform into products.

For an endothermic reaction, this energy is needed to ensure the reactants have enough energy to break bonds and form new ones to create products. In our original exercise, we see that the activation energy for the forward reaction (\(E_f\)) is greater than that for the backward reaction (\(E_b\)). This is because more energy needs to be absorbed to progress from reactants to products, making \(E_f\) higher than \(E_b\).

Understanding activation energy helps in predicting reaction rates and understanding why some reactions require heat or happen spontaneously. The larger the activation energy, the slower the reaction rate, as fewer molecules will have enough energy to surpass the energy barrier.

Forward Reaction

In any chemical reaction, a forward reaction refers to the process where reactants are converted into products. The direction of this forward reaction is essential in determining the overall change in energy. For endothermic reactions, like in the exercise given, the forward reaction absorbs energy from the surroundings.

The forward reaction in an endothermic process needs a significant amount of energy, more than is released when the reaction goes in the reverse direction. This makes the forward reaction less spontaneous unless additional energy is supplied.

- Endothermic forward reactions absorb heat- They have a positive enthalpy change (\(\Delta H > 0\)), indicating energy is taken in
- Often occur at elevated temperatures to provide the necessary activation energy In such reactions, we supply energy, often in the form of heat, to facilitate the transformation of reactants to products. The need for high activation energy (\(E_f\)) is characteristic of an endothermic forward reaction.

Backward Reaction

The backward reaction, often referred to as the reverse reaction, takes place when products revert to reactants. In the context of an endothermic process, this reaction releases energy, typically in the form of heat back to the surroundings.

The backward reaction is crucial for reaching chemical equilibrium, where the rates of the forward and backward reactions are equal, leading to a stable concentration of reactants and products over time.

- Backward reactions in endothermic processes are exothermic- They have a negative enthalpy change (\(\Delta H < 0\)), meaning they release energy- Tend to happen more readily at lower temperatures compared to endothermic forward reactionsIn our example, the activation energy (\(E_b\)) for the backward reaction is smaller than that for the forward reaction. This indicates that less energy is needed to trigger the reverse process, making it more feasible under typical conditions. The differences in activation energies between forward and backward reactions dictate how easily these processes can occur under specific conditions.