Glossary
Activation Energy ($E_a$)
The minimum amount of energy required for reactants to transform into products in a chemical reaction.
Example:
A spark provides the necessary activation energy to ignite a mixture of hydrogen and oxygen, leading to the formation of water.
Alternate Reaction Pathway
A new series of steps or a different transition state provided by a catalyst, which has a lower activation energy than the uncatalyzed pathway.
Example:
Enzymes in our bodies create an alternate reaction pathway for complex biochemical reactions, allowing them to occur rapidly at body temperature.
Catalyst
A substance that increases the rate of a chemical reaction by providing an alternate reaction pathway with a lower activation energy, without being consumed in the overall reaction.
Example:
In the production of ammonia via the Haber-Bosch process, iron acts as a catalyst to speed up the reaction between nitrogen and hydrogen gases.
Elementary Steps
Individual molecular events that constitute the overall reaction mechanism, each representing a single collision or decomposition.
Example:
The decomposition of ozone in the stratosphere involves several elementary steps, including reactions with chlorine atoms.
Energy Diagram
A graphical representation that illustrates the energy changes during a chemical reaction, showing the relative energies of reactants, products, and transition states.
Example:
By analyzing an energy diagram, a chemist can visually compare the activation energies of catalyzed and uncatalyzed reactions.
Enthalpy (ΔH)
A thermodynamic property representing the total heat content of a system at constant pressure; it indicates whether a reaction is exothermic or endothermic.
Example:
The negative enthalpy change (ΔH) for combustion reactions indicates that they release heat to the surroundings.
Rate-Determining Step
The slowest elementary step in a reaction mechanism, which limits the overall rate of the reaction.
Example:
In a multi-step synthesis, identifying the rate-determining step is crucial for optimizing reaction conditions to maximize product yield.
Reaction Mechanism
The sequence of elementary steps by which a chemical reaction occurs, detailing the bond breaking and forming processes.
Example:
Understanding the reaction mechanism for ozone depletion helps scientists develop strategies to protect the Earth's atmosphere.
Regeneration (of catalyst)
The process by which a catalyst, consumed in an earlier step of a reaction mechanism, is reformed in a later step, ensuring its overall concentration remains constant.
Example:
In catalytic converters, precious metals facilitate the conversion of harmful gases, undergoing regeneration so they can continue to process exhaust fumes.