Reaction Mechanism: Videos & Practice Problems

A reaction mechanism details the sequence of elementary steps that constitute the overall chemical process at the molecular level. Each elementary step represents a single event in this sequence. The reaction mechanism is determined by canceling out catalysts and reaction intermediates, which are identified as compounds that appear as reactants in one step and products in another. The molecularity of an elementary step refers to the number of reactant molecules involved; it can be unimolecular (one reactant), bimolecular (two reactants), or termolecular (three reactants). Additionally, within a reaction mechanism, the slowest step is known as the rate-determining step, which dictates the overall reaction rate. The coefficients of reactants in this step correspond to the reaction orders in the rate law, providing a connection between the mechanism and the kinetic properties of the reaction.

Reaction Mechanism Overview

A reaction mechanism is a step by step sequence of elementary steps by which an overall chemical change occurs. Now when we use a term elementary step, an elementary step is just one step in a series of reactions that show the progress of a reaction at the molecular level. Now here we're going to have a reaction mechanism overview. So we're going to say here step one and Step 2 represent our two elementary steps. This would be elementary step one and elementary step 2.

In elementary step one we have

Cl gas + O 3 gas → ClO gas + O 2 gas

An elementary step two we have

ClO gas + ClO gas → Cl gas + Cl 2 gas

Now both elementary steps together, well that represents our reaction mechanism. So these two elementary steps together are our reaction mechanism and we can say here that our overall reaction can be created by canceling out what we call our catalyst and reaction intermediates.

So if we take a look here, we're going to say that a catalyst is a compound shown as a reactant in the first step and then as a product in the final step. So here the first step is just elementary step one, and the final step is elementary step 2. Now if this mechanism, if another mechanism we see has, let's say 4 elementary steps, then the first step would still be the initial step and it will be the very last one, step four. That would be our final step. OK, but in this case, since there's only two, this is the first step. This is the second step.

Now our reaction intermediate. This is a compound that first appears as a product in one elementary step, and then later as a reactant in another elementary step. Now these are cancelled out and by canceling them out, what's left behind comes down to form our overall reaction. So if we looked at our overall reaction, this would come down here. This came down here, this came down here, and then this came down over here.

So keep this in mind when you're looking at a reaction mechanism. A reaction mechanism is just all of the elementary steps together. If we were to cancel out our catalysts and our reaction on our reaction intermediates, that should give you the overall reaction for the equation. OK. So just keep this in mind, these key terms when it comes to any type of reaction mechanism and elementary step.

Reaction Mechanism Example

Consider the following reaction mechanism for the formation of nitrobenzene. Alright so identify the reaction intermediate or intermediates and catalysts or catalysts. So here if we take a look all of this together represents our reaction mechanism. Let's each individual reaction represents our elementary step, right? So here this would be elementary step one. This would be elementary Step 2 and elementary Step 3.

All right. So remember, a reaction intermediate appears first as a product, then later as a reactant. So if we take a look here, let's try to find them. Let's see. So here H2N3 appears as a product, and then later we see it appear as a reactant. So that would definitely be a reaction intermediate. But is that the only one? We would say no, that's not the only one. Who else does that? Here's another C6H6N2 plus here C6H6N2 plus here that is also a reaction intermediate. It appears first as a product and later as a reactant.

Do we see any others? Yes, we see this HSO4 minus appears as a product and then later as a reactant. Now do we have any catalysts? If we look, we do remember catalysts will appear in the very first step as a reactant, then it will appear in the very last elementary step as a product. So here this would be our catalyst and that would be our only catalyst involved in this entire reaction mechanism.

So just keep in mind a reaction intermediate appears first as a product and then later as a reactant. A catalyst appears as one of the first reactants, and then later in the very last elementary step, it appears as one of the products. So as long as you can remember that there was a difference between a reaction intermediate and a tablet.

Molecularity in Reactions

Now, molecularity is associated with a number of moles for reactant molecules within an elementary step. Here we take a look. We have one mole of carbon dioxide gas reacting with one mole of water liquid to produce one mole of carbonic acid. If we were to take a look here, we'd say that we have a total of 2 moles of reactants.

Now the number of moles of reactants gives us different names for the molecularity. If we had only one mole of a reactant then the molecularity would be called unimolecular. If there are two moles of reactants like we have in this example, then that would be called bimolecular. Now I know when we're naming covalent compounds in the past when we say 2:00 we'd say die like carbon dioxide. But for molecularity we don't use the prefix dye, we use the prefix by SO2 moles of reactants equals bimolecular.

If we add 3 moles of reactants total, then that would be termolecular. When we talked about numerical prefixes in the past, three would be try. But there's no such thing as trimolecular. So again when it's 3 moles of reactants it's termolecular. So when we talk about molecularity, just count the total number of reactant molecules and you'll be able to determine the molecularity of any elementary step.

Molecularity in Reactions Example

The elementary reaction of two moles of nitrogen dioxide reacting with one mole of fluorine gas to produce 2 moles of NL₂F gas is an example of a blank type of reaction. All right, so here we're talking about molecularity. So we need to determine the total number of reactant moles involved.

So here we have two moles of nitrogen dioxide and one mole of fluorine gas. Together that totals 3 moles of reactants. So unimolecular means we only have one mole of reactants. Bimolecular would mean two, Tetra would mean four.

So the answer is either B or D. So it's either trimolecular or termolecular. And remember from our earlier videos when it comes to molecularity, 3 moles of reactants is referred to as termolecular. Trimolecular does not exist. So here the answer would be option D.

Rate Law Determination

Now we're going to say here in a reaction mechanism, one of the elementary steps is classified as a slow step. And when we say slow step, we're going to say that is the rate determining step that limits the overall rate of a chemical reaction. Because remember, you're only as fast as your slowest step.

Now we're going to say for a slow step, we're going to say that the coefficients of the reactants are equal to the reaction orders of the rate law. Remember we talked about calculating rate laws beforehand by using a chart and a list of values.

But now we can look at the reaction mechanism, locate our slow step, use its coefficients and that it can give us the reaction orders, right? So this is just another way of connecting rate law to our chemical reactions, in this case our chemical reactions, our elementary steps that comprise a reaction mechanism.

Rate Law Determination Example

Here it says consider the following elementary steps. What is the rate law of the reaction mechanism? So when this reaction mechanism we have three elementary steps, the first one is a fast step, the second one is a slow step and the last one, the third step is also a fast step.

Now if we take a look, it says step one, we need to locate the slow step of the reaction mechanism. So the slow step is this. Right here we're going to say as long as the reactants are not intermediate, their coefficients equal the reaction orders of the rate law. So looking at the reactants here, we'd say K rate equals K NO2. The coefficient here is a 1 to the one and then N2O5 to the one. But we have an issue. One of our reactants is an intermediate. This intermediate, this intermediate. Remember, intermediates cancel out, so we could have stopped at step one if the reactants were not intermediates. But because they are, we have to continue to Step 2.

Now if a reactant is an intermediate, which we saw, you're going to cancel out with the product intermediate and we did that with the Fasttab before, now we're going to say step three. If Step 2 happens, use the elementary step possessing the product intermediate. Here we're going to say for that elementary step, the coefficients of the reactants still equal the reaction orders, and for that elementary step, the coefficients of the products equal the inverse, the inverse of the reaction orders. So let's see what that means.

Alright, so if we come back up here, my NO2 reactant got cancelled out by this NO2 product and we kind of have to make up for what we just lost. So this here took away the reactant that I needed. So I'm going to take both of its reactant and its product to make up for that loss. So if we if we if we look at everything, what do I have now? Well here I have an N2O5 that I originally had as a reactant and now I have a newly gained N2O5 also as a reactor. So how many total moles of N2O5 do I have? 21 from here that I just gained and one here that I originally had. So in essence their coefficient is 2 because I have two moles of it.

Coming down here we say that our new rate law is equal to KN2O5 to the two because again, I have two moles of it, one that I originally had plus one that I gained later on. And then let's look at what else we took. We also took this one mole of NO3, but here it is not a reactant, it is a product. We're going to say here bringing it down to our rate law. If it were reacting, it'd be NO3 to the one. But because it is a product, we said that it equals the inverse. So instead of being one, it's to the -1.

This can be a bit tricky. This happens anytime one of our reactants is an intermediate have to do this making up process what we just lost. So here the rate law would be rate equals KN2O5 to the two and NO3 to -1 right? So this would be our new and official rate law.

Consider the following elementary steps:
NO₂ (g) + NO₂ (g) → NO₃ (g) + NO (g)     [SLOW]
NO (g) + CO₃ (g) → N (g) + CO₄ (g)        [FAST]
What is the rate law of the reaction mechanism?

• A. rate = k [NO₂]
• B. rate = k [NO₂]²
• C. rate = k [NO][CO₃]
• D. rate = k [CO₃]
• E. rate = k [NO₄]

Answer: (B)

The following reaction of 2 Br₂ (g) + 2 NO (g) → N₂ (g) + 2 Br₂O (g) has the following rate law:Rate = k [Br₂][NO]². The proposed mechanism for the reaction is:
Br₂ (g) + NO₂ (g) → N (g) + Br₂O (g)            [SLOW]
N (g)  +  NO (g) → N₂ (g) + O (g)                  [FAST]
O (g)  +  Br₂ (g) → Br₂O (g)                          [FAST]
Which of the following statements is/are false?
a) The rate determining step is bimolecular.
b) There are three elementary steps in the reaction mechanism.
c) The mechanism possesses a catalyst.
d) O is the only reaction intermediate in this reaction mechanism.
e) This is not a valid mechanism for the reaction.

• A. A and B
• B. A, B, and C
• C. A, C, and D
• D. C and D
• E. C, D, and E

Answer: (D)

Here’s what students ask on this topic:

What is a reaction mechanism?

A reaction mechanism is a step-by-step sequence of elementary reactions by which overall chemical change occurs. It's essentially the detailed pathway that atoms and molecules follow as they undergo transformation from reactants to products during a chemical reaction. Each step in a mechanism is characterized by its own transition state and has an associated energy barrier that must be overcome for the reaction to proceed. These steps include the breaking and forming of chemical bonds, and they often involve the generation of reactive intermediates, such as free radicals, ions, or molecules in an excited state. The mechanism provides insight into the molecular-level events that dictate the speed (rate) and outcome (selectivity and yield) of reactions. Understanding reaction mechanisms is crucial for predicting the course of chemical reactions and for designing new reactions in research and industrial applications.

By what mechanism is the following reaction likely to occur?

To provide an accurate answer to your question, I would need to know the specific reaction you're referring to. However, I can explain the general mechanisms by which reactions occur.

Chemical reactions can proceed through various mechanisms, and these are often categorized by the type of process they involve, such as:

1. Substitution reactions - where one atom or group of atoms in a molecule is replaced by another atom or group of atoms.
2. Addition reactions - where two or more molecules combine to form a larger molecule.
3. Elimination reactions - where a molecule splits into two or more smaller molecules.
4. Rearrangement reactions - where the structure of a single molecule is rearranged to form a different isomer.

The specific mechanism of a reaction depends on the reactants, the conditions under which the reaction is carried out, and the type of reaction. For instance, organic reactions may proceed through mechanisms such as SN1, SN2, E1, E2, or through various radical or concerted processes. Inorganic reactions might involve coordination, redox, or acid-base mechanisms.

Understanding the specific reaction details, including the structure of the reactants and the reaction conditions, is essential to determine the exact mechanism.

What is molecularity?

Molecularity in chemistry refers to the number of molecules that come together to react in a single step of a chemical reaction. It's a term used to describe elementary reactions, which are reactions that occur in a single step and can't be broken down into simpler events. The molecularity of a reaction is determined by the number of reactant molecules involved. For example:

• A unimolecular reaction involves a single molecule undergoing a change to form products, such as a decomposition reaction where one molecule breaks apart into two or more products.
• A bimolecular reaction involves two reactant molecules colliding and reacting to form products, which is common in many organic reactions.
• A termolecular reaction involves three molecules coming together in a single step, which is quite rare due to the low probability of three molecules colliding at the same time.

Molecularity is always a whole number and cannot be zero. It's important to note that molecularity is only applicable to elementary reactions, not to the overall reaction, which may occur in multiple steps.

How do you determine molecularity?

Molecularity of a reaction refers to the number of molecules or species that come together to react in a single step. It's determined by examining the reaction mechanism, which is the step-by-step sequence of elementary reactions that leads to the formation of the final product.

To determine molecularity, you look at each elementary step in the mechanism and count the number of reactant species involved. Here are the possible outcomes:

• Unimolecular: If there's only one reactant molecule involved in an elementary step, the reaction is unimolecular. An example is the decomposition of a molecule into two products.
• Bimolecular: If two reactant molecules collide and react in a single step, the reaction is bimolecular. This is a common scenario in many chemical reactions.
• Termolecular: This is less common due to the low probability of three molecules colliding at the same time. However, if this happens, the reaction is termed termolecular.

Remember, molecularity is only applicable to elementary reactions, not the overall reaction, because the overall reaction may involve several steps with different molecularities.