First-order reactions

Definition

First-order reactions are chemical reactions where the rate is directly proportional to the concentration of one reactant. This means that if you double the concentration of that reactant, the reaction rate also doubles. Understanding first-order reactions is crucial because they display a specific relationship between concentration and time, allowing us to predict how quickly a reaction will proceed.

5 Must Know Facts For Your Next Test

1. In a first-order reaction, the units of the rate constant (k) are typically s^{-1}, indicating that the reaction rate decreases exponentially over time.
2. The integrated rate law for first-order reactions allows you to determine the concentration of a reactant at any given time, which is useful for predicting outcomes in experiments.
3. The half-life of a first-order reaction does not depend on the initial concentration; it remains constant regardless of how much reactant you start with.
4. Graphically, a plot of ln[A] versus time yields a straight line with a slope of -k, confirming the first-order nature of the reaction.
5. First-order reactions are commonly found in processes like radioactive decay and certain enzyme-catalyzed reactions.

Review Questions

• How does changing the concentration of a reactant affect the rate of a first-order reaction?
  • In a first-order reaction, increasing the concentration of the reactant will lead to a proportional increase in the reaction rate. This means that if you were to double the concentration of that reactant, the rate at which the reaction occurs would also double. This direct relationship is what characterizes first-order kinetics, making it easier to predict how changes in concentration will influence overall reaction speed.
• Describe how you would determine if a reaction follows first-order kinetics using experimental data.
  • To determine if a reaction follows first-order kinetics, you would collect data on reactant concentrations at various time intervals. Then, you could plot ln[A] (the natural logarithm of the concentration) against time. If the resulting plot yields a straight line with a negative slope, this confirms that the reaction is first-order. Additionally, examining whether the half-life remains constant across different starting concentrations can further validate this classification.
• Evaluate the implications of constant half-life in first-order reactions and how this characteristic influences real-world applications.
  • The constancy of half-life in first-order reactions has significant implications in various fields. For instance, in pharmacokinetics, understanding that drug elimination follows first-order kinetics allows healthcare professionals to predict how long it will take for medication levels to decrease by half. This knowledge is critical for determining dosing schedules and ensuring effective treatment plans. Moreover, in environmental science, recognizing that pollutants may decay at constant rates can aid in assessing long-term impacts and remediation strategies.

Related terms

Rate constant: A proportionality constant in the rate equation that is unique for each reaction at a given temperature.

Half-life: The time required for half of the reactant to be consumed in a reaction; it remains constant for first-order reactions.

Integrated rate law: An equation that relates the concentration of a reactant to time; for first-order reactions, it can be expressed as ln[A] = -kt + ln[A]₀.