Assume you have the reaction [tex]X(g) \rightarrow Y(g)[/tex], and that you start with pure [tex]X[/tex] and allow the reaction to come to equilibrium.

Plot a graph of rate versus time which shows how the forward and reverse reaction rates change from the time the reaction starts until equilibrium is attained.



Answer :

Certainly! When analyzing the progress of a chemical reaction, especially one that reaches equilibrium, it is valuable to understand how the rates of the forward and reverse reactions evolve over time.

### Steps to create the plot:

1. Define the Initial Conditions:
- Initially, we start with pure [tex]\([X]\)[/tex]. Hence, the concentration of [tex]\(X\)[/tex] is high, and the concentration of [tex]\(Y\)[/tex] is zero.

2. Forward and Reverse Rates:
- Forward Reaction Rate: The forward rate depends on the concentration of [tex]\(X\)[/tex]. It can be represented by [tex]\( \text{Rate}_{\text{forward}} = k_f [X] \)[/tex], where [tex]\( k_f \)[/tex] is the forward rate constant.
- Reverse Reaction Rate: The reverse rate depends on the concentration of [tex]\(Y\)[/tex]. It can be represented by [tex]\( \text{Rate}_{\text{reverse}} = k_r [Y] \)[/tex], where [tex]\( k_r \)[/tex] is the reverse rate constant.

3. Change in Concentrations Over Time:
- Initially, because we have only [tex]\(X\)[/tex], [tex]\([X]\)[/tex] will start to decrease, and [tex]\([Y]\)[/tex] will start to increase as the reaction proceeds.
- As [tex]\(X\)[/tex] is converted to [tex]\(Y\)[/tex], the forward rate will decrease (since there is less [tex]\(X\)[/tex]), and the reverse rate will increase (since more [tex]\(Y\)[/tex] is being formed).

4. Equilibrium:
- At equilibrium, the forward reaction rate equals the reverse reaction rate, meaning that the concentrations of [tex]\(X\)[/tex] and [tex]\(Y\)[/tex] become constant.
- The point where the two rates meet on the graph indicates equilibrium.

### Key Points to Plot:

- Time Points: Choose specific time points to measure the rates.
- Forward Reaction Rate: Starts high and decreases over time.
- Reverse Reaction Rate: Starts at zero and increases over time.
- Equilibrium: Where the forward rate equals the reverse rate.

### Step-by-Step Detailed Solution:

1. Starting Conditions:
- Use arbitrary values for the rate constants, say [tex]\( k_f = 0.5 \)[/tex] and [tex]\( k_r = 0.3 \)[/tex].
- Let’s assume [tex]\([X]\)[/tex] starts at 1 mole/L and [tex]\([Y]\)[/tex] starts at 0 mole/L.

2. Formulate the Concentrations Over Time:
- For simplicity, we can assume an exponential decay for [tex]\(X\)[/tex]: [tex]\([X] = e^{-k_f t}\)[/tex]
- For [tex]\( Y \)[/tex]: [tex]\([Y] = 1 - e^{-k_f t}\)[/tex], assuming stoichiometric conversion and initial amount of [tex]\(X\)[/tex] is 1 mole.

3. Calculate the Rates Over Time:
- Forward Rate: [tex]\( \text{Rate}_{\text{forward}} = k_f [X] = 0.5 e^{-0.5 t} \)[/tex]
- Reverse Rate: [tex]\( \text{Rate}_{\text{reverse}} = k_r [Y] = 0.3 (1 - e^{-0.5 t}) \)[/tex]

4. Plot the Rates Over Time:
- Choose a range of time, e.g., from [tex]\(0\)[/tex] to [tex]\(20\)[/tex] units.
- Plot the forward rate and reverse rate on the same graph.

### Graph Interpretation:

- The forward reaction rate starts at a maximum value and declines over time.
- The reverse reaction rate starts at zero and increases as more product ([tex]\(Y\)[/tex]) is formed.
- The point where the forward and reverse rates meet is the equilibrium point.

Here’s a detailed interpretation of how you'd create the graph:

```plaintext
Forward Reaction Rate Plot (Assuming k_f = 0.5):
Rate_forward = 0.5 exp(-0.5 t)

Reverse Reaction Rate Plot (Assuming k_r = 0.3):
Rate_reverse = 0.3 (1 - exp(-0.5 t))
```

### Final Observations:

When you plot these rates with time (t) on the x-axis:
- At [tex]\( t = 0 \)[/tex], the forward reaction has its highest rate.
- As [tex]\( t \)[/tex] increases, the forward rate diminishes, and the reverse rate rises.
- Both rates will eventually intersect at an equilibrium point where the rates balance each other.

### Summary

- The forward rate decreases exponentially.
- The reverse rate increases as the forward rate decreases.
- Equilibrium is achieved when the forward rate equals the reverse rate.

This depiction provides a thorough understanding of the kinetics of a reaction progressing toward equilibrium.