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 a single reactant. This means that if the concentration of that reactant doubles, the rate of the reaction also doubles. Understanding this concept helps in analyzing how reaction rates change with concentration and in deriving integrated rate laws, as well as in categorizing reactions based on their molecularity.

5 Must Know Facts For Your Next Test

1. In first-order reactions, the rate can be expressed mathematically as rate = k[A], where k is the rate constant and [A] is the concentration of the reactant.
2. The integrated rate law for a first-order reaction is given by ln[A] = -kt + ln[A]₀, which shows how concentration changes over time.
3. The half-life of a first-order reaction is independent of its initial concentration and is given by t₁/₂ = 0.693/k.
4. Graphing the natural logarithm of the concentration of the reactant against time yields a straight line with a slope of -k for first-order reactions.
5. First-order reactions often describe processes like radioactive decay and certain enzyme-catalyzed reactions in biochemistry.

Review Questions

• How does changing the concentration of a reactant affect the rate of a first-order reaction?
  • In a first-order reaction, the rate of reaction is directly proportional to the concentration of one reactant. This means if you double the concentration, the rate also doubles. This relationship allows us to predict how alterations in concentration will impact the speed at which the reaction occurs, making it crucial for understanding kinetics.
• What is the significance of the integrated rate law for first-order reactions in experimental data analysis?
  • The integrated rate law for first-order reactions provides a mathematical framework to analyze experimental data by allowing chemists to plot ln[A] against time. A straight line indicates that the reaction follows first-order kinetics, making it easier to determine the rate constant k from the slope. This method helps validate kinetic models and understand how different factors influence reaction rates.
• Evaluate how knowledge of half-life can be applied to real-world situations involving first-order reactions, such as drug metabolism.
  • Understanding half-life in first-order reactions is crucial for applications like pharmacokinetics. For example, knowing that a drug has a specific half-life helps medical professionals predict how long it will remain effective in the body before its concentration drops significantly. This knowledge aids in determining dosing intervals and adjusting medication regimens for effective treatment while minimizing side effects.

Related terms

Rate constant: A proportionality factor in the rate law equation that is specific to a particular reaction at a given temperature.

Half-life:

The time required for the concentration of a reactant to decrease to half of its initial value in a first-order reaction.

Molecularity: The number of reactant molecules involved in an elementary step of a reaction, which can help classify reactions as unimolecular, bimolecular, etc.