Glossary
Catalysts
Substances that increase the rate of a chemical reaction without being consumed in the overall process, typically by providing an alternative reaction pathway with a lower activation energy.
Example:
Enzymes in biological systems are highly efficient catalysts, speeding up biochemical reactions like the breakdown of food without being used up themselves.
Elementary Step
A single molecular event that occurs in a reaction mechanism, representing a direct collision or rearrangement of molecules.
Example:
In the reaction of two NO₂ molecules to form N₂O₄, the direct collision of two NO₂ molecules to form N₂O₄ is an elementary step.
Intermediates
Species that are produced in one elementary step of a reaction mechanism and then consumed in a subsequent step, meaning they do not appear in the overall balanced equation.
Example:
In the catalytic destruction of ozone, ClO• is an intermediate; it's formed when chlorine atoms react with ozone and then consumed when it reacts with oxygen atoms.
Overall Reaction
The net chemical change that results from summing all the elementary steps in a reaction mechanism, with intermediates and catalysts canceling out.
Example:
If a reaction mechanism involves steps A+B→X and X+C→D, the overall reaction is A+B+C→D, showing the final transformation.
Rate Law Derivation (from Mechanism)
The process of determining the mathematical expression that relates the rate of a reaction to the concentrations of reactants, based on the proposed elementary steps and the rate-determining step of a mechanism.
Example:
Given a multi-step mechanism, an AP Chemistry student must perform a rate law derivation (from mechanism) to predict how changes in reactant concentrations will affect the overall reaction speed.
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 making a sandwich, and the slowest part is toasting the bread, then toasting is the rate-determining step for the entire sandwich-making process.
Reaction Mechanism
A series of elementary steps that describe how a chemical reaction occurs at the molecular level, detailing the sequence of bond breaking and forming.
Example:
The decomposition of ozone (O₃) in the stratosphere involves a multi-step reaction mechanism where O₃ first breaks apart, and then the resulting oxygen atom reacts with another O₃ molecule.
Steady-State Approximation
An assumption that the concentration of a reaction intermediate remains constant over time because its rate of formation is approximately equal to its rate of consumption.
Example:
When a complex industrial chemical process reaches a stable operating point, the concentration of short-lived byproducts might be treated using the steady-state approximation to simplify calculations.