I don't understand how to get a rate determining step. I have an energy diagram for a two-step conversion where the first step Ea is about 18 units. The energy then goes down about 4 units and rises again about 10 units to the peak of the second step. Why is the rate-determining step the first one?

Question

I don't understand how to get a rate determining step. I have an energy diagram for a two-step conversion where the first step Ea is about 18 units. The energy then goes down about 4 units and rises again about 10 units to the peak of the second step. Why is the rate-determining step the first one?

Thanks from Sheryl

The step then goes down about 15 units. This is an endothermic reaction. Is that the reason the first step is rate determining?

Ea determines the rate - determining step. Don't bother looking this up. I managed to find it.

Sheryl

Then which peak is the rate determining step in your graph and why that peak and not the other one?

Answer 1

To determine the rate-determining step in a reaction, you need to identify the step that has the highest activation energy (Ea). The rate of a reaction is limited by the slowest step, which is typically the one with the highest activation energy.

In your case, you mentioned that the energy diagram shows a two-step conversion. The first step has an activation energy of about 18 units, while the second step goes down about 4 units and then rises again about 10 units to reach the peak.

Based on this information, it is likely that the first step with an activation energy of about 18 units is the rate-determining step. The reason for this is that the rate of the reaction is determined by the slowest step, which in this case is the step with the highest activation energy.

The second step, which has a peak but lower activation energy, is not the rate-determining step because it occurs after the rate-determining step. Even though it requires some energy to reach the peak, it does not control the overall rate of the reaction.

The fact that the reaction is endothermic, with the energy going down about 15 units, does not necessarily mean that the first step is the rate-determining step. The energy diagram provides information about the energy changes during the reaction but does not determine the rate-determining step. The determining factor is still the height of the activation energy barrier relative to the other steps in the reaction mechanism.

To fully understand the energy diagram and determine the rate-determining step, it is important to consider the reaction mechanism, the energy changes in each step, and the activation energy of each step.