There are four factors that can affect how rapidly a chemical reaction can occur. Chemists change these four factors to speed up or slow down reactions. You might think that chemists would always want to speed up reactions to save time. However, some reactions give off so much heat as they occur that they could damage the reaction container or burn the desired product, so the reactants are added slowly, in small amounts, to slow down the release of heat. Also, the reason we put food in the freezer or refrigerator is to slow down the growth of mold.
Reaction Mechanisms and the Slow Step
Before we look more closely at the four factors to determine how they affect the speed of a reaction, we need to think about one more feature of a chemical reaction, its mechanism. The mechanism of a reaction is the step-by-step process by which a reaction occurs. A reaction mechanism is like the steps described in the instruction manual for putting together a piece of furniture or a model or a Lego kit. Some reaction mechanisms are very simple, involving only one step. But most reaction mechanisms involve two or more steps to make it from the starting materials to the finished products. Just as there can be a very slow, difficult step when putting together a piece of furniture or building a model, there is usually one step in each reaction mechanism that is more complicated and slower than all of the other steps in the mechanism. This slow step, officially known as the rate-determining step, holds up the completion of the entire reaction because it requires more energy to occur than any other step. In other words, the slow step of the mechanism has a higher activation energy (Ea) than any other step in the mechanism. This fact is important to chemists, because if chemists want to change the speed of the reaction, they need to make changes that affect the slow step, because if they can make the slow step of the mechanism go faster, the entire reaction will go faster.
The video below gives a silly, but clear example of a process (a mechanism) that has several steps. While watching the video, see if you can determine the rate-determining step. Hint: The rate-determining step changes in the middle of the video!
The overall process involves starting with candies and wrappers, and ending with wrapped candies. We can state that the overall process involves two steps, with each step occurring at its own speed:
1) the candies moving down the belt
2) the women wrapping up the candies.
At the start of the process, the slow step is the number of candies coming down the belt, because the two women are able to wrap up each candy and have some time to wait. But soon, the number of candies coming down the belt increases, and the women cannot wrap each candy; they have become the slow step.
Now let's look at an actual chemical reaction: (CH3)3CCl + OH- → (CH3)3COH + Cl- (overall reaction)
By carrying out several experiments, it was determined that this reaction has a two-step mechanism:
Step 1: (CH3)3CCl → (CH3)3C+ + Cl- (the slow step)
Step 2: (CH3)3C+ + OH- → (CH3)3COH (the fast step)
The first step is the slow step. This step involves a molecule of (CH3)3CCl falling apart, to lose a Cl- ion. The OH- ions have nothing to do with this step; they simply wait around for the (CH3)3CCl to fall apart. Once the (CH3)3CCl falls apart, the OH- then comes in to react. Notice that the (CH3)3C+ is formed in the first step and then used up in the second step, so that when you add the two steps together, you get the overall reaction. (CH3)3C+ is called a reaction intermediate.
Let's look at how each of the four factors that affect the rate of a reaction affect the slow step.
Changing the Concentration of Reactants
Remember that chemical reactions usually involve the collision of particles. So, if there is a higher concentration of particles, then there should be more collisions in a given amount of time, and the reaction should go faster. We need to be careful, though, because if we want to speed up a reaction that occurs in several steps, we need to specifically speed up the slow step to make the entire reaction go faster. In other words, the reaction will go faster only if we increase the concentration of particles that take part in the slow step.
Let's go back to the video to clarify this point. We stated that there were two steps in the "mechanism": the candies moving down the belt and the women wrapping the candies. By the end of the video, the women wrapping was the slow step. At that time, if we wanted to get more wrapped candies in a given amount of time, adding more candies would not help because the candies moving down the belt was the fast step. How could we get more wrapped candies? We could get more people to help wrap the candies. Looking at it in another way, we could increase the concentration of people to speed up the reaction.
If we wanted to speed up the reaction (CH3)3CCl + OH- → (CH3)3COH + Cl- we could increase the amount of (CH3)3CCl, because it is the only reactant that is in the slow step. There are no OH- ions in the slow step, so adding more OH- would not change the rate of the reaction.
The take home lesson: If you increase the concentration of reactants that are part of the slow step, you will increase the rate of reaction. If you increase the concentration of reactants that are not in the slow step, nothing will happen to the rate of the reaction.
This video shows how the rate of a reaction can change when the concentration of reactants is changed:
Adding a Catalyst
A catalyst is a chemical that is added to a reaction to speed up the reaction. A catalyst is neither a reactant nor a product because a catalyst is not used up by the reaction. The role of the catalyst is to take part in the slow step of the reaction and to lower the activation energy. If the activation energy is lowered, more molecules will have enough energy to be able to collide effectively to make products, so the reaction will go faster. At the end of the reaction, the catalyst will return to its original form.
In the following video, the red Co2+ ion acts as a catalyst. You will see that the Co ion takes part in the reaction because the color of the solution becomes green, signifying that the Co ion has changed in some way. At the end of the reaction, however, the red color returns, and we know that the Co2+ ion is back in its original form.
The reaction CH3OH + Cl- → CH3Cl + OH- is a one-step reaction that is very slow. The reaction rate can be greatly increased by adding H3O+ ions to the solution to create a two-step process:
Step 1: CH3OH + H3O+ → CH3OH2+ + H2O
Step 2: CH3OH3+ + Cl- → CH3Cl + H3O+
Notice that the two steps add up to the original, overall reaction, and that the H3O+ first appears on the left side of the reaction, undergoes a change in the first reaction, then reappears on the right side in the second reaction.
Changing the Temperature
Changing the temperature of a reaction will almost always cause a change in the reaction rate. Generally, heating a reaction will increase the rate because the higher temperature makes the particles move more rapidly, so there will be more effective collisions in a given period of time because more particles will have an energy greater than the activation energy. It is also true that cooling a reaction will slow it down. At lower temperatures, the particles are moving more slowly, and fewer particles have an energy greater than the activation energy, and so cannot collide with enough energy to cause bonds to break.
The following video shows the effects of temperature on the reaction rate:
Changing the Nature of the Reactants
In many cases, chemists have a chemical that they want to change to a new form. So chemists will look for various ways in which they can carry out that change, usually with the goal of finding the fastest, safest, least expensive way to do so. Thus, by changing the nature, or properties of one of the reactants, we can cause a desired change in a second reactant. As an example, let's assume that we want to remove the permanganate ion (MnO4-) from a solution because the permanganate ion is a very dark purple color. We could use either the oxalate ion (C2O42-) or we could use the iron (II) ion (Fe2+) to react with the permanganate and change it in to the nearly colorless Mn2+ ion. Watching the two videos will show us which reactant, oxalate or iron (II), creates a faster reaction to remove the purple color:
Oxalate with permanganate:
iron (II) with permanganate