Rate law

Definition

Rate law is a mathematical expression that relates the rate of a chemical reaction to the concentration of its reactants. It helps in understanding how changes in concentration can affect the speed of a reaction and is fundamental in determining reaction order and rate constants. Rate laws can provide insights into the mechanism of a reaction, which is crucial for predicting how reactions behave under various conditions.

5 Must Know Facts For Your Next Test

1. The general form of a rate law is: rate = k [A]^m [B]^n, where k is the rate constant, [A] and [B] are the concentrations of reactants, and m and n are their respective orders.
2. Rate laws are determined experimentally and cannot be deduced from the balanced chemical equation alone.
3. The overall order of a reaction is the sum of the exponents in the rate law equation, providing insights into how sensitive the reaction rate is to changes in reactant concentration.
4. Zero-order reactions have a constant rate that does not depend on the concentration of reactants, while first-order reactions show a linear relationship between rate and concentration.
5. Complex reactions may have different rate laws for different steps, and identifying these can help clarify the overall reaction mechanism.

Review Questions

• How does the concept of order of reaction relate to rate law and what implications does it have for understanding reaction mechanisms?
  • The order of reaction indicates how the rate changes with varying concentrations of reactants, as expressed in the rate law. For instance, if a reaction is first-order with respect to a reactant, doubling its concentration will double the reaction rate. Understanding this relationship helps chemists infer details about the reaction mechanism, such as which steps are likely to be slow or fast based on their respective orders.
• Discuss how experimental determination of rate laws differs from deriving them from stoichiometric coefficients in balanced equations.
  • While stoichiometric coefficients provide information on how many molecules react or are produced, they do not accurately reflect the dynamics of how a reaction proceeds. Rate laws must be determined experimentally because they reveal how the actual rates depend on reactant concentrations, which can vary independently from stoichiometry. This discrepancy highlights that kinetics and thermodynamics are interconnected but distinct concepts.
• Evaluate the significance of knowing rate constants when analyzing complex reactions involving multiple steps and intermediates.
  • Knowing rate constants for each step in a complex reaction allows chemists to model the overall kinetics accurately and understand how each elementary step contributes to the total reaction rate. By comparing these constants, one can identify rate-determining steps and gain insights into potential intermediates formed during the process. This level of analysis can be crucial for designing more efficient catalysts or optimizing industrial chemical processes.

Related terms

Reaction Rate: The change in concentration of a reactant or product per unit time, indicating how fast a reaction occurs.

Order of Reaction: The power to which the concentration of a reactant is raised in the rate law, which indicates how the rate is affected by changes in concentration.

Rate Constant: A proportionality constant in the rate law equation that is specific to a given reaction at a particular temperature, reflecting the intrinsic speed of the reaction.