Chemical Kinetics on the MCAT

Oftentimes in our daily lives, we want to get to our goals and desires as fast as possible. Probably the most relatable example is in all of us where we all want to get to medical school and become a physician ASAP: however, as with all things in life, most things are slow and take time to achieve, step by step. 

The parallel with chemical reactions is uncanny: in order for a chemical reaction to progress and eventually form the products, they must undergo certain steps in order to reach the resulting outcome!

This chapter overview should give a basic overview of chemical kinetics including the various theories, mechanisms, and equations related to the topic. Let’s go!

Chemical Kinetics on the MCAT: What You Need to Know

Topics on general chemistry will be tested on the Chem/Phys and the Bio/Biochem section of the MCAT and can appear both as passage based and fundamental discrete questions. 

Here at MCAT Mastery, we calculated around a 5-7 question average that will cover chemical kinetics. These don’t necessarily have to be asked in the context of general chemistry as these questions can also have applications in organic and biochemistry!

Introductory general chemistry accounts for 30% of the content covered in the Chemical and Physical Foundations of Biological Systems and 5% of the content covered in the Biological and Biochemical Foundations of Living Systems.

Important Sub-Topics: Chemical Kinetics

Try to study and review this material in conjunction with similarly related topics such as enzymes, thermodynamics, and equilibrium!

If  you can study these topics and chapters within the same time frame, such as within the same week or two, you’ll see the overlap present and will have a better chance of internalizing the content reviewed as there’s significant overlap!

1. Basics of Reaction Mechanisms

Before getting into the fundamentals of reaction mechanisms, it’s key to differentiate the difference between kinetics and spontaneity. Kinetics refers to how fast a reaction will occur while spontaneity — a thermodynamic property — refers to how likely a reaction will occur. 

As mentioned above, a chemical reaction can’t simply have the reactant particles collide to form the product (although this does have some merit). In actuality, chemical reactions must follow a reaction mechanism.

Though there are two steps as shown above, really the only important one to pay attention to is the slow step which is also known as the rate determining step. As its name implies, how fast a reaction will occur depends on this step. It’s basically analogous to the limiting reagent in stoichiometry problems!

As we’ll see later as well, this step will be the only one considered when determining the rate law of a reaction which is basically an equation we can use to calculate the reaction rate (i.e. how fast a reaction will occur). 

2. Theories and Factors Affecting Chemical Kinetics

Within chemical kinetics, there are 2 main factors that are used to explain how chemical reactions progress: 1) collision theory and 2) transition state theory. Though the latter is really more accurate in terms of a theoretical approach, a synthesis of both is necessary to completely explain chemical kinetics! 

Collision theory basically states that when reactant particles collide with each with enough energy, the product(s) — this minimum amount of required energy is called the activation energy (Ea).

While this is a good start for explaining chemical kinetics, the transition state theory is viewed more favorably by chemists as it better explains the thermodynamics that is also in play!

As stated by its name, the main component of this theory is the idea of a transition state which is a high energy molecular configuration along the reaction coordinate of a chemical reaction. This may be a little hard when put into words and is better understood via a visual!

There are a couple of important takeaways from this: notice how the difference in energy levels between the reactants and the transition state is equal to the activation energy. In this way, we still have an application of collision theory. 

Likewise, when viewed this way, notice how also the transition state can either form either the reactant or product. This is important when considering equilibrium as now we can also see how reversible reactions relate to chemical kinetics!

3. Understanding Reaction Rate and Rate Laws

In general, when the word reaction rate comes to mind, it’s fair to say that it simply refers to how fast a chemical reaction takes place! For a more precise definition, we can actually define the reaction rate as how fast the reactant(s) disappears and/or how fast the product(s) appears.

This should make sense as during the course of a chemical reaction, the reactants will disappear as they react together to form products. We can actually model this as an 

equation given the chemical equation below:

There are a couple of things to note about the equation: notice how there’s a negative sign in front of reactants A and B — this is to indicate that the reactants are disappearing in order to form products. 

Additionally, notice also how the concentrations of the reactants and products are divided by their stoichiometric coefficients. This is to ensure that there’s an equal standard to compare the rate of appearance/disappearance. 

You might notice that this equation looks pretty similar to the equation for the equilibrium constant. However, there is one important difference: the power of the reactants must be determined experimentally!

The powers 9 times out of 10 won’t be the same as the stoichiometric coefficients. You must actually measure the reaction rates with varying concentrations. Afterward, you see how changing the concentration affects the reaction rate. You’ll often see it displayed on a table as shown below:

In order to find the power, more specifically called the order, follow the steps shown below:

a. To find the power of reactant A, find 2 trials where reactant B remains constant and reactant A.

b. Then, compare how changing [A] affects the reaction rate! Below are some of the main relationships to look out for. 

• [A] Doubles ⇒ Rate Remains Constant: Zero Order (x = 0)

• [A] Doubles ⇒ Rate Doubles: First Order (x = 1)

• [A] Doubles ⇒ Rate Quadruples: Second Order (x = 2)

c. Do the same for reactant B (and for any other reactants). 

Using the example above, we see that doubling reactant A (while keeping reactant B) constant) quadruples the reaction rate making it a second order. Likewise, when we double reactant B (while keeping reactant A constant), the reaction rate also doubles, making it a first order.

To find the total order of the rate law, simply add the exponents together. Shown below is the completed rate law!

Finally, another important note to mention is that the rate constant varies with temperature. While of course the value remains constant at a certain temperature, the rate constant changes as you change the temperature. 

Important Definitions and Key Terms

Below are some high yield definitions and key terms to refer to when reviewing concepts and ideas about chemical kinetics in general chemistry!

Additional FAQs - Chemical Kinetics on the MCAT

Additional Reading Links (Coming Soon!) – Study Notes for Chemical Kinetics on the MCAT

Additional Reading: General Chemistry MCAT Topics:

• Atomic Structure on the MCAT
• Periodic Table on the MCAT
• Bonding and Chemical Reactions on the MCAT
• Acids and Bases on the MCAT
• Electrochemistry on the MCAT
• Equilibrium on the MCAT
• Solutions on the MCAT
• Stoichiometry on the MCAT
• The Gas Phase on the MCAT
• Thermochemistry on the MCAT
• Redox Reactions General Chemistry MCAT