Draw a general graph of a non-spontaneous reaction for an uncatalyzed reaction. Show a second plot exhibiting the reaction as it might appear in the presence of a catalyst. Clearly show DG and both DG++ (aka “double dagger”), and describe the difference between them.
The correct answer and explanation is :
In the graph above, the energy profiles of an uncatalyzed and a catalyzed reaction are shown. The uncatalyzed reaction (blue curve) exhibits a high activation energy (energy peak) compared to the catalyzed reaction (red dashed curve), which has a lower activation energy. Here’s a detailed explanation of the key components:
- Uncatalyzed Reaction:
- In an uncatalyzed reaction, the activation energy (the energy barrier that must be overcome for the reaction to proceed) is relatively high. This results in a larger difference between the energy of the reactants and the transition state, which corresponds to a higher ΔG++ (also known as the activation energy or Gibbs free energy of activation).
- ΔG (the overall Gibbs free energy change) is the difference in energy between the reactants and the products. In a non-spontaneous reaction, this value is positive, meaning that the products have higher energy than the reactants.
- Catalyzed Reaction:
- When a catalyst is added to the reaction, it provides an alternative pathway with a lower activation energy. This is reflected in the red curve, where the energy barrier is reduced, making it easier for the reactants to transform into products.
- ΔG++ in the catalyzed reaction is smaller because the catalyst lowers the energy of the transition state, but ΔG (the overall change in free energy between reactants and products) remains unchanged. The catalyst does not affect the overall thermodynamics of the reaction, just the rate at which equilibrium is reached.
Differences Between ΔG and ΔG++:
- ΔG represents the difference in Gibbs free energy between the reactants and the products, indicating whether the reaction is spontaneous or non-spontaneous. A positive ΔG indicates a non-spontaneous reaction (products are at a higher energy than reactants).
- ΔG++ refers to the activation energy of the reaction, the energy required to form the transition state from the reactants. The lower this value, the faster the reaction can occur.
The catalyst lowers ΔG++, making the reaction occur faster, but it does not affect ΔG, which remains positive for a non-spontaneous reaction.