Illustration of change in activation energy caused by a catalyst. Made by me in Inkscape by tracing the original image by Vinay.bhat, who holds the copyright but has released all rights.

Illustration of change in activation energy caused by a catalyst.

A reaction rate is the speed at which reagents are consumed and products are created. Chemical Kinetics is the study of the rate of chemical reactions. Because the rate of a chemical reaction depends on its pathway, different kinetics may apply for the same overall transformation if it can occur through a variety of mechanisms. Path dependence fundamentally distinguishes the propositions of Chemical Kinetics from those of Chemical Thermodynamics. Chemical Thermodynamics depends solely on the comparison of the initial and final state of the system and surroundings, without recourse to discussion of the reaction pathway, unlike Chemical Kinetics. The use of a catalyst, for example, will change the kinetics of a reaction, but it will not change the position of equilibrium, which is a thermodynamic function.

Chemical Kinetics problems involving number crunching are rare on the MCAT. However, conceptual questions involving Chemical Kinetics are very common on the test. Chemical kinetics lends itself especially well to the MCAT passage format where passages presenting data can lead to questions involving inference of reaction order, for example, or where the application of kinetics principles allows one to judge among various reaction mechanisms. In the construction of multiple choice questions, the MCAT writers are also fond of questions that make sure students don't confuse the propositions of Chemical Kinetics, which depend on the reaction pathway, with the concepts of Chemical Thermodynamics, which do not depend on the mechanism.

WikiPremed Resources




Chemical Kinetics Practice Items
Problem set for Chemical Kinetics in PDF format

Answer Key
Answers and explanations

Chemical Kinetics Images
Image gallery for study with links to larger teaching JPEGs for classroom presentation

Question Drill for Chemical Kinetics
Conceptual Vocabulary Self-Test

Basic Terms Crossword Puzzle

Basic Puzzle Solution

Learning Goals

Proficiency 

Understand how the rate of a chemical reaction is measured.

Comprehend how to describe the relationship between the rate of a reaction, the concentration of reagents, and the rate constant k in a rate equation.

If presented with a rate equation, be prepared to determine the reaction order.

If presented with a graph of the logarithm of concentration versus time for first order reactions (or reciprocal concentration versus time for second order reactions), be prepared for questions involving determination of the rate constant.

Grasp the concept of half-life and be prepared to solve basic quantitative problems.

Be able to apply the Arrhenius equation to interpret changes in rate constant with temperature.

Be adept at inferring the reaction mechanism from experimental data generated by a kinetic study.

Understand the behavior of catalysts in interacting with reagents to lower activation energy.

If you asked, be able to describe the concepts underlying kinetic versus thermodynamic control of various competing reactions.

Suggested Assignments

Study the basic terms for chemical kinetics to get a feel for the topic using the question server. Complete the fundamental terms crossword puzzle. Here is the solution to the puzzle.

Perform the practice items for chemical kinetics. Here is the answer key for the practice items.

Read pp. 32-46 in ExamKrackers Chemistry. Perform practice items 17-23 on pg 47.

Review the web resources for chemical kinetics.

Conceptual Vocabulary for Chemical Kinetics

Chemical kinetics
Chemical kinetics is the study of reaction rates in a chemical reaction.
Reaction rate
The reaction rate for a reactant or product in a particular reaction tells you how fast a reaction takes place.
Catalysis
Catalysis is the increase in rate of a chemical reaction by means of a substance called a catalyst.
Order of reaction
The order of reaction with respect to a certain reactant is defined, in chemical kinetics, as the power to which its concentration term in the rate equation is raised.
Rate-determining step
The rate-determining step is a chemistry term for the slowest step in a chemical reaction.
Reaction intermediate
An intermediate in a chemical reaction is a molecular entity with a lifetime appreciably longer than a molecular vibration that is formed from the reactants and reacts further to give the products of a chemical reaction.
Reaction mechanism
A reaction mechanism is the step by step sequence of elementary reactions by which overall chemical change occurs.
Reaction coordinate
A reaction coordinate is an abstract one-dimensional trajectory representing progress along a reaction pathway.
Transition state
The transition state of a chemical reaction is a particular configuration along the reaction coordinate defined as the state corresponding to the highest energy along this reaction coordinate.
Activated complex
An activated complex is a transitional structure in a chemical reaction that results from an effective collision between molecules and that persists while old bonds are breaking and new bonds are forming.
Activation energy
Activation energy, also called threshold energy, is a term defined as the energy that must be overcome in order for a chemical reaction to occur.
Rate law
The rate law or rate equation for a chemical reaction is an equation which links the reaction rate with concentrations or pressures of reactants and constant parameters.
Transition state theory
Transition state theory is a conception of chemical reactions or other processes involving rearrangement of matter as proceeding through a continuous change in the relative positions and potential energies of the constituent atoms and molecules.
Reaction step
A reaction step of a chemical reaction is defined as an elementary reaction, constituting one of the stages of a reaction in which a reaction intermediate is converted into the next reaction intermediate in the sequence between reactants and products.
Homogeneous catalysis
Homogeneous catalysis describes catalysis where the catalyst is in the same phase as the reactants.
Heterogeneous catalysis
Heterogeneous catalysis describes catalysis where the catalyst is in a different phase to the reactants.
Elementary reaction
An elementary reaction is a chemical reaction in which one or more chemical species react directly to form products in a single reaction step and with a single transition state.
Autocatalysis
A single chemical reaction is said to have undergone autocatalysis if the reaction product is itself the catalyst for that reaction.
Hammond's Postulate
Hammond's postulate states that if a transition state and intermediate occur consecutively during a reaction process and have nearly the same energy content, their interconversion will involve only a small reorganization of the molecular structures.
Molecularity
Molecularity in chemistry is the number of colliding molecular entities that are involved in a single reaction step.
Arrhenius equation
The Arrhenius equation is a simple, but remarkably accurate, formula for the temperature dependence of a chemical reaction rate, more correctly, of a rate coefficient.
Reaction inhibitor
A reaction inhibitor is a substance that decreases the rate of, or prevents, a chemical reaction.
Catalytic cycle
A catalytic cycle in chemistry is a term for a multistep reaction mechanism that involves a catalyst.
Collision theory
Collision theory assumes that for a reaction to occur the reactant particles must collide, but only a certain fraction of the total collisions, the effective collisions, cause the transformation of reactant molecules into products.
Advanced terms that may appear in context in MCAT passages
Biocatalysis
Biocatalysis can be defined as the utilization of natural catalysts, such as protein enzymes, to perform chemical transformations on organic compounds.
Potential energy surface
A potential energy surface is a representation of interactions in simple chemical and physical systems in which the total energy of an atom arrangement can be represented as multidimensional surface with atomic positions as variables.
Steady state
A steady state is a situation in which all state variables are constant in spite of ongoing processes that strive to change them.
Katal
The katal is the SI unit of catalytic activity.
Eyring equation
The Eyring equation in chemical kinetics, which relates the reaction rate to temperature, follows from transition state theory, contrary to the Arrhenius equation, which is empirically based.
Temperature jump
A temperature jump is a piece of apparatus useful in the study of chemical kinetics, involving the discharging of a capacitor through a small volume solution containing the reaction to be studied.