Hello and welcome to Teach Me to Science! This post discusses Reaction Steps which are a component of Kinetics in General Chemistry 2.
Why you should understand Reaction Steps: The idea that reactions take place in individual steps boggled my mind when I first began General Chemistry 2. However, understanding how reactions take place is a key idea in chemistry and helps piece together the big picture of how molecules interact to create chemical reactions.
Key Ideas
Reaction Steps
Elementary Steps
Rate Determining Steps
Rate laws of individual steps
Summary
Reaction Steps
Reactions occur in steps. Typically when you see a chemical reaction written down, it includes several chemicals reacting to produce several chemical products. However, most chemical reactions that take place are much more simple than that. They occur in a series of steps. Why? Because breaking down a reaction into several steps is more energetically favorable than trying to get multiple molecules to collide in the exact right orientation with enough energy. If the idea that molecules collide to react is confusing please review my post and video on collision theory.
Reaction steps are individual steps that the components of a reaction undergo. All of the reaction steps must add together to create the overall equation of the chemical reaction.
In the example to the left, the overall chemical equation is A goes to D. The steps below that describe the proposed reaction mechanism. Remember that chemicals that appear on both sides of an equation cancel each other out. These chemicals are considered spectators because they appear on both sides of the chemical reaction and are not an overall product of the reaction.
The products of the reaction steps are considered intermediates because they are not the final products of the reaction.
If you cancel out all of the molecules which appear on both sides of the equations for the reaction steps, you would end up with the overall reaction equation "A goes to D".
Elementary Steps
Elementary steps are the simplest reaction steps possible and cannot be broken down further into more reaction steps.
Rate Determining Steps
The rates of each step are different. Some are slow, some are fast, and one is the slowest. The rate-determining step in a reaction mechanism is the slowest step. Just like a relay team is only as good as their slowest runner, a reaction can only go as fast as its slowest reaction step.
Rate Laws of individual steps
If you've read my post or watched my youtube video on rate laws, you'll know that normally rate laws cannot be determined from the reaction coefficients. Individual reaction steps are the exception to this rule.
The rate laws of individual steps CAN be written using the coefficients in the reaction. This is because the individual reaction steps represent what is actually happening on a molecular scale. This means the coefficients accurately represent the ratio of how the molecules are interacting. On a larger scale, this is not always true.
Recall that the rate law only relies on the speed of the reactants. In the first reaction step, A is raised to an imaginary reaction order of 1. Likewise, B is raised to a reaction order of 2 which comes from the balanced equation. This is ONLY true for reaction steps. The overall rate law CANNOT be determined from the overall balanced equation.
Given that you know that the overall rate law of the reaction is the rate law given to the left, you can figure out which of the reaction steps is the rate-determining step.
Because the overall rate law is the same as the rate law from the first reaction step, you know the first step is the rate-determining step and is the slowest.
Summary
Reactions occur in steps.
Breaking down a reaction into several steps is more energetically favorable.
Reaction steps are individual steps that the components of a reaction undergo.
All of the reaction steps must add together to create the overall equation of the chemical reaction.
Molecules that appear on both sides of an equation cancel each other out and are called Spectators.
The products of reaction steps are considered intermediates because they are not the net products of the reaction.
Elementary steps are the simplest reaction steps possible and cannot be broken down further into more reaction steps.
The rate-determining step in a reaction mechanism is the slowest step.
I hope you've found this post on Reaction Steps helpful. If you have any questions feel free to comment on this post, email me, or comment on the video I've linked below.
- Saren
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