An exothermic chemical reaction proceeds by two stages reactants `overset("Stage I")(rarr) "intermdediate" overset("Stage II")(rarr)` products, The activation energy of stage `I` is `50 kJ mol^(-1)`. The overall enthalpy change of the reaction is `-100 "kJ mol"^(-1)`. Which diagram could represent the energy level diagram for the reaction ?

Options

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B.
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D.

Explanation

To solve the problem, we need to analyze the information given about the exothermic reaction that occurs in two stages. Let's break it down step by step.

### Step 1: Understand the Reaction Stages
The reaction proceeds in two stages:
1. **Stage I**: Reactants convert to intermediates.
2. **Stage II**: Intermediates convert to products.

### Step 2: Identify Given Data
- The activation energy (Ea) for Stage I is **50 kJ/mol**.
- The overall enthalpy change (ΔH) for the reaction is **-100 kJ/mol** (indicating an exothermic reaction).

### Step 3: Draw the Energy Level Diagram
1. **Initial Energy Level**: Start with the energy level of the reactants.
2. **First Peak**: Draw a peak representing the transition state for Stage I (the energy level will rise due to activation energy). The peak will be at **50 kJ** above the reactants.
3. **Intermediate Level**: After Stage I, the energy level drops to the intermediate state.
4. **Second Peak**: Draw another peak for Stage II, which represents the transition state from intermediate to products. The height of this peak will depend on the activation energy for Stage II.
5. **Final Energy Level**: The energy level of the products will be lower than that of the reactants by **100 kJ** (since ΔH is -100 kJ).

### Step 4: Calculate the Energy Levels
- If we assume the energy of the reactants is at **0 kJ**, then:
- The energy of the intermediates after Stage I will be at **50 kJ**.
- The energy of the products will be at **-100 kJ** (0 - 100 = -100 kJ).

### Step 5: Determine Activation Energy for Stage II
- The activation energy for Stage II can be calculated as the difference between the energy of the intermediates and the peak of Stage II.
- If we assume the peak for Stage II is at **X kJ**, then:
- Activation energy for Stage II = X - 50 kJ.

### Step 6: Identify the Correct Diagram
- The correct energy level diagram must show:
- Two peaks (one for each stage).
- The energy of products lower than the reactants by 100 kJ.
- The activation energy for the backward reaction (from products to intermediates) must be greater than that for the forward reaction (from reactants to intermediates).

### Conclusion
Based on the analysis, the correct energy level diagram will show:
- Two peaks.
- The energy of the products lower than the reactants by 100 kJ.
- The activation energy for the backward reaction greater than for the forward reaction.