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Reaction Mechanisms and The Rate Determining Step

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  • 0:05 Reaction Mechanism
  • 2:21 Rate-Determining Step
  • 3:48 Lesson Summary
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Lesson Transcript
Instructor: Amy Meyers

Amy holds a Master of Science. She has taught science at the high school and college levels.

This lesson explores what a reaction mechanism is and how it relates to the speed of a reaction. You'll discover how to pinpoint the rate-determining step and learn how to write a rate law based on the rate-determining step.

Reaction Mechanism

When you see a balanced chemical equation, it is tempting to think that the reaction takes place in one step. For example, in the reaction of A + B --> AB, it is easy to think that A and B just join and make AB in one step. But, this isn't necessarily true. It may take many steps to finally get to AB, and A and B may make an intermediate substance or two before they make the final product. When you write out the step-by-step sequence of reactions for a chemical reaction, it is called the reaction mechanism.

Let's look at the reaction of: H2 (g) + I2 (g) --> 2 HI (g). You might think that it is such a simple-looking reaction that there was only one step - an H combines with an I to make HI. This isn't what happens, though. Scientists aren't sure, but they believe that the actual reaction happens in either two or three steps.

Example A

  1. I2 --> 2 I
  2. 2 I + H2 --> 2 HI

The '2 I's cross out.

Example B

  1. I2 --> 2 I
  2. I + H2 --> H2I
  3. H2I + I --> 2 HI

Each 'I' in step one cancels out with an 'I' in step two and an 'I' in step three. The 'H2I's also cross out.

Notice that the 'I' in step one and the 'H2I' in step two aren't written in the net overall original equation. This is because they cancel out, just like in a math equation. The 'species,' such as the 'I's and 'H2I's, that appear in the steps but not in the net equation are called intermediates.

This reaction that we have been working with contains substances that are all in the gas phase. Even the intermediates of I and H2I are gases. Any reaction in which the reactants and products exist in a single phase is a homogeneous reaction.

Rate-Determining Step

If one step is slower than the rest of the steps, then it is a rate-determining step. The rate-determining step controls the reaction rate. Think of what happens to a freeway when there is an accident closing all but one lane. The freeway can only flow as fast as the cars in that one lane can go. The one lane is the determining rate of flow.

A rate law is an expression of the rate of reaction in terms of the concentration of the reactants. This means that it expresses the rate by taking the concentrations of reactants into consideration.

Rate = k[A], where k is a specific rate constant for the reaction and [A] is the concentration of the reactant.

You need to know which of the reaction steps in a reaction is the determining step in order to write the rate law. The law is written using the concentrations of the reactants in the rate-limiting step.

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