First-Order Reactions

Definition

First-order reactions are chemical reactions in which the rate of reaction is directly proportional to the concentration of one reactant. This means that as the concentration of the reactant decreases, the rate of the reaction slows down at a rate that depends only on that specific reactant. In this type of reaction, the half-life is constant, and the rate law can be expressed in a simple form that reflects this direct relationship.

5 Must Know Facts For Your Next Test

1. For first-order reactions, the rate constant (k) has units of s^-1, indicating a dependence on time.
2. The integrated rate law for a first-order reaction is given by ln[A] = -kt + ln[A]0, where [A]0 is the initial concentration and [A] is the concentration at time t.
3. Graphing ln[A] versus time yields a straight line with a slope of -k, allowing for easy determination of the rate constant from experimental data.
4. In first-order reactions, the half-life is independent of concentration, making it unique compared to zero-order and second-order reactions.
5. Common examples of first-order reactions include radioactive decay and certain types of enzyme kinetics.

Review Questions

• How can you determine if a reaction is first-order based on experimental data?
  • To determine if a reaction is first-order, you can plot the natural logarithm of the concentration of a reactant against time. If the resulting graph is a straight line, it indicates that the reaction follows first-order kinetics. The slope of this line will provide you with the negative value of the rate constant (k), confirming the first-order nature of the reaction.
• Discuss how the half-life for first-order reactions differs from that of zero-order and second-order reactions.
  • In first-order reactions, the half-life remains constant and does not depend on the initial concentration of reactants, making it unique among reaction orders. In contrast, for zero-order reactions, half-life is directly proportional to initial concentration and decreases as concentration decreases. For second-order reactions, half-life increases with decreasing initial concentration. This fundamental difference highlights how reaction order influences the behavior and predictability of a chemical reaction over time.
• Evaluate how understanding first-order reactions can impact real-world applications like pharmacokinetics and environmental chemistry.
  • Understanding first-order reactions is crucial in fields such as pharmacokinetics, where it helps predict how drugs are absorbed and eliminated from the body. Since many drugs follow first-order kinetics, knowing their half-lives allows healthcare providers to optimize dosages for effective treatment. Similarly, in environmental chemistry, knowledge of first-order degradation processes aids in predicting how pollutants break down over time, facilitating better management strategies for pollution control and environmental protection.

Related terms

Rate Law: An equation that relates the reaction rate to the concentrations of reactants, typically determined experimentally.

Half-Life: The time required for the concentration of a reactant to decrease to half of its initial value, which remains constant for first-order reactions.

Integrated Rate Law: An equation that expresses the concentration of a reactant as a function of time, allowing for the determination of reaction rates over time.