For a reaction A\(\rightarrow\)B, the enthalpy of the reaction is −4.2 kJ mol−1 and the enthalpy of activation is 9.6 kJ mol−1. The correct potential energy profile for the reaction is shown in option
The Correct Option is C
Solution and Explanation
To solve this problem, we need to understand the basic concept of a potential energy profile for a chemical reaction. The reaction given is \( A \rightarrow B \), with a given enthalpy change and activation energy.
The key information provided is:
- The enthalpy change (\(\Delta H\)) is \(-4.2 \, \text{kJ mol}^{-1}\). This indicates that the reaction is exothermic, meaning the energy of the products (B) is lower than the energy of the reactants (A).
- The activation energy (\(E_a\)) is \(9.6 \, \text{kJ mol}^{-1}\). This is the energy barrier that must be overcome for the reaction to proceed.
A potential energy diagram for the reaction will display the following features:
- The reactants (A) starting at a higher energy level compared to the products (B), due to the exothermic nature of the reaction.
- The peak of the diagram corresponding to the transition state, which is higher than the energy of reactants by \(9.6 \, \text{kJ mol}^{-1}\).
- The energy difference between the reactants and products being \(4.2 \, \text{kJ mol}^{-1}\) in favor of the products, reinforcing the exothermic reaction nature.
Among the given options, we need to identify the diagram that correctly represents these features. The correct diagram will show:
- A decrease in energy from reactants to products, reflecting the \(-4.2 \, \text{kJ mol}^{-1}\) enthalpy change.
- An energy peak that is \(9.6 \, \text{kJ mol}^{-1}\) higher than the reactants, denoting the activation energy.
Comparing the features of each diagram provided with our expectations, the correct potential energy profile for this reaction is shown in the following image:
This diagram accurately displays the characteristics of an exothermic reaction with the given enthalpy and activation energy values. Therefore, this is the correct option.
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Concepts Used:
Enthalpy change
Enthalpy Change refers to the difference between the heat content of the initial and final state of the reaction. Change in enthalpy can prove to be of great importance to find whether the reaction is exothermic or endothermic.
Formula for change in enthalpy is:-
dH = dU + d(PV)
The above equation can be written in the terms of initial and final states of the system which is defined below:
UF – UI = qP –p(VF – VI)
Or qP = (UF + pVF) – (UI + pVI)
Enthalpy (H) can be written as H= U + PV. Putting the value in the above equation, we obtained:
qP = HF – HI = ∆H
Hence, change in enthalpy ∆H = qP, referred to as the heat consumed at a constant pressure by the system. At constant pressure, we can also write,
∆H = ∆U + p∆V
Standard Enthalpy of Reaction
To specify the standard enthalpy of any reaction, it is calculated when all the components participating in the reaction i.e., the reactants and the products are in their standard form. Therefore the standard enthalpy of reaction is the enthalpy change that occurs in a system when a matter is transformed by a chemical reaction under standard conditions.