Glossary
Activation Energy
The minimum amount of energy required for reactants to transform into products, representing the energy barrier that must be overcome for a reaction to occur.
Example:
Lighting a match requires a small amount of heat to overcome the activation energy and start the combustion reaction.
Catalyst
A substance that increases the rate of a chemical reaction without being consumed in the process, typically by providing an alternative reaction pathway with a lower activation energy.
Example:
Enzymes in biological systems act as catalysts, speeding up vital biochemical reactions like digestion without being used up.
Elementary Steps
The individual, simple molecular events that constitute a complex reaction mechanism, each representing a single collision or rearrangement.
Example:
In the reaction of NO₂ with CO, the first elementary step might be two NO₂ molecules colliding to form NO and NO₃.
Intermediate
A species that is produced in one elementary step of a reaction mechanism and then consumed in a subsequent elementary step, thus not appearing in the overall balanced equation.
Example:
In the atmospheric formation of smog, the hydroxyl radical (OH•) often acts as an intermediate, quickly reacting after its formation.
Overall Balanced Chemical Equation
The net chemical equation obtained by summing all elementary steps in a reaction mechanism and canceling out any species that appear on both reactant and product sides.
Example:
For the decomposition of H₂O₂, the overall balanced chemical equation is 2H₂O₂ → 2H₂O + O₂ after combining the elementary steps.
Rate Law
An equation that expresses the relationship between the rate of a chemical reaction and the concentrations of the reactants, determined experimentally or from the rate-determining step of an elementary reaction.
Example:
For a reaction with a rate law of Rate = k[A]², doubling the concentration of A would quadruple the reaction rate.
Rate-Determining Step
The slowest elementary step in a reaction mechanism, which limits the overall rate of the reaction and dictates the form of the experimental rate law.
Example:
If you're building a LEGO castle, the step where you have to sort through thousands of tiny pieces to find the right ones is likely the rate-determining step.
Reaction Mechanisms
The detailed sequence of elementary steps that describe how reactants are transformed into products in a chemical reaction.
Example:
Understanding the reaction mechanism for ozone depletion helps scientists develop strategies to protect the Earth's atmosphere.
Transition State
A high-energy, unstable molecular arrangement that exists momentarily at the peak of the activation energy barrier during a chemical reaction.
Example:
Imagine a gymnast at the very peak of a handstand before flipping over; that precarious moment is analogous to a transition state in a reaction.