First-order reaction

Definition

A first-order reaction is a type of chemical reaction where the rate of reaction is directly proportional to the concentration of one reactant. This means that if you double the concentration of that reactant, the rate of reaction will also double. This relationship allows for a straightforward mathematical representation, making it easier to analyze the kinetics and stoichiometry involved in chemical processes.

5 Must Know Facts For Your Next Test

1. In a first-order reaction, the unit of the rate constant is typically s^-1, indicating that the reaction rate decreases exponentially over time.
2. The integrated rate equation for a first-order reaction is expressed as ln([A]₀/[A]) = kt, where [A]₀ is the initial concentration, [A] is the concentration at time t, k is the rate constant, and t is time.
3. Graphing ln[A] versus time yields a straight line with a slope of -k, which provides a visual method to determine the rate constant from experimental data.
4. First-order reactions are commonly observed in processes such as radioactive decay and certain enzyme-catalyzed reactions.
5. The half-life of a first-order reaction is independent of its initial concentration, which is a unique characteristic compared to other order reactions.

Review Questions

• How does the rate law apply to first-order reactions, and what implications does this have for predicting reaction rates?
  • In first-order reactions, the rate law expresses that the reaction rate is directly proportional to the concentration of one reactant raised to the first power. This means that changes in the concentration of that specific reactant will linearly affect the rate of reaction. Understanding this relationship allows chemists to predict how variations in concentration will influence how quickly a reaction occurs.
• Explain how the integrated rate equation for first-order reactions can be used to calculate the concentration of reactants over time.
  • The integrated rate equation for first-order reactions, ln([A]₀/[A]) = kt, can be used to determine how the concentration of a reactant decreases over time. By rearranging this equation, one can isolate [A] and calculate its concentration at any given time t if k and [A]₀ are known. This approach is especially useful for analyzing experimental data and understanding reaction kinetics.
• Evaluate the significance of half-life in first-order reactions and discuss how it differs from other types of reactions in terms of dependence on initial concentrations.
  • The concept of half-life in first-order reactions is significant because it provides a consistent measure of time for a given fraction of reactant to decompose, irrespective of its initial concentration. This contrasts with zero or second-order reactions, where half-life varies depending on how much reactant you start with. The unique property of having a constant half-life makes it easier to predict behavior in first-order kinetics, particularly in fields like pharmacokinetics and radioactive decay.

Related terms

Rate Law: An equation that relates the rate of a reaction to the concentration of its reactants, including their orders in the reaction.

Half-Life: The time required for half of the reactant in a first-order reaction to be consumed, which remains constant regardless of initial concentration.

Integrated Rate Equation: A mathematical expression that describes how the concentration of reactants changes over time in a first-order reaction.