Elementary Steps

Definition

Elementary steps are the individual, simple reactions that make up a complex chemical reaction mechanism. Each elementary step represents a single molecular event, where reactants are transformed into products, and these steps are essential for understanding how reaction rates are determined. The combination of these elementary steps gives rise to the overall reaction mechanism, influencing both the rate and the path of the chemical process.

5 Must Know Facts For Your Next Test

1. Elementary steps can be unimolecular, involving one reactant, or bimolecular, involving two reactants colliding in a single step.
2. Each elementary step has an associated rate constant, which is crucial for calculating the overall reaction rate from the individual steps.
3. The stoichiometry of elementary steps directly reflects their molecularity, allowing for predictions about how changing reactant concentrations will influence reaction rates.
4. Elementary steps must add up to give the balanced overall equation for the reaction, ensuring conservation of mass and charge.
5. In mechanisms involving intermediates, these species appear in some elementary steps but do not appear in the overall balanced equation for the reaction.

Review Questions

• How do elementary steps contribute to determining the overall reaction mechanism?
  • Elementary steps are the building blocks of a reaction mechanism, and each step represents a specific molecular event. By analyzing these steps, we can identify how reactants transform into products and uncover the pathway of the reaction. Understanding each elementary step allows chemists to predict how changes in concentration or conditions may impact the overall reaction rate.
• Discuss how the molecularity of elementary steps influences the rate law for a chemical reaction.
  • The molecularity of elementary steps refers to the number of reactant molecules involved in each step. This directly affects the rate law because the overall reaction rate can be derived from the individual rate constants of each elementary step. If an elementary step is unimolecular or bimolecular, it will contribute differently to the overall rate law, reflecting how various concentrations of reactants impact the speed of reaction.
• Evaluate how catalysts alter elementary steps in a chemical mechanism and their effect on reaction rates.
  • Catalysts provide alternative pathways for reactions by lowering activation energy, which can change the rate constants of individual elementary steps. This means that while catalysts do not alter the overall thermodynamics of a reaction, they can significantly speed up reactions by enhancing certain elementary steps within the mechanism. This results in increased reaction rates without being consumed in the process, making them vital in industrial and biological processes.

Related terms

Reaction Mechanism: The detailed step-by-step sequence of elementary steps that describes how reactants convert into products during a chemical reaction.

Rate Law: An equation that relates the rate of a chemical reaction to the concentration of its reactants, reflecting how elementary steps contribute to the overall reaction rate.

Catalyst: A substance that increases the rate of a chemical reaction by providing an alternative pathway with lower activation energy, affecting the individual elementary steps in the mechanism.