Question

question (start on a new page)
the following graph shows a potential energy diagram for the reaction a + b → c + d.

1. is the forward reaction endothermic or exothermic? motivate your answer.
2. calculate δh for the forward reaction.
3. define activation complex.
4. what is the energy of the activated complex?
5. define activation energy.
6. a catalyst is now added to the reaction. explain what a catalyst does and why it is added.
7. how does the catalyst influence the reverse reaction?
8. redraw the graph to indicate the mechanism of how a catalyst will influence the reaction.

Explanation:

Step1: Determine reaction type

Since the potential energy of products is lower than that of reactants, the forward reaction is exothermic. $\Delta H=\text{Potential energy of products}-\text{Potential energy of reactants}<0$.

Step2: Define activation complex

The activation complex is the unstable, high - energy intermediate state that exists at the peak of the potential energy curve during a chemical reaction.

Step3: Find energy of activation complex

The energy of the activation complex is the potential energy at the peak of the curve. From the graph, it is around 60 (units not given).

Step4: Define activation energy

Activation energy ($E_a$) is the minimum energy that the reactants must possess to undergo a chemical reaction. It is the difference between the energy of the activation complex and the energy of the reactants.

Step5: Explain catalyst effect

A catalyst provides an alternative reaction pathway with a lower activation energy. It does not change the $\Delta H$ of the reaction. When added, it lowers the energy barrier for both the forward and reverse reactions, increasing the reaction rate.

Step6: Analyze reverse reaction influence

The catalyst affects the reverse reaction in the same way as the forward reaction. It lowers the activation energy of the reverse reaction, making it easier for the products to convert back to reactants.

Step7: Redraw graph for catalyst

To redraw the graph with a catalyst: Draw a new curve with a lower peak (lower activation energy) but the same starting and ending points (same $\Delta H$) for the forward and reverse reactions.

Answer:

1. The forward reaction is exothermic.
2. $\Delta H =$ (Potential energy of products) - (Potential energy of reactants) and is negative for the forward reaction.
3. The activation complex is the high - energy intermediate at the peak of the potential energy curve.
4. The energy of the activation complex is approximately 60 (units not given).
5. Activation energy is the difference between the energy of the activation complex and the energy of the reactants.
6. A catalyst lowers the activation energy of the reaction by providing an alternative pathway and does not change $\Delta H$.
7. The catalyst lowers the activation energy of the reverse reaction, making it easier to occur.
8. Redraw the graph with a lower - peaked curve (lower activation energy) but same starting and ending points for the forward and reverse reactions.