First-order reaction

Definition

A first-order reaction is a type of chemical reaction where the rate is directly proportional to the concentration of one reactant. This means that if the concentration of that reactant doubles, the reaction rate also doubles. This straightforward relationship makes it easier to analyze and predict the behavior of these reactions, especially when using rate laws and integrated rate equations.

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}, reflecting the dependency on time.
2. The integrated rate law for a first-order reaction can be expressed as ln[A] = -kt + ln[A]₀, where [A]₀ is the initial concentration.
3. The half-life of a first-order reaction is independent of the initial concentration, making it unique compared to other reaction orders.
4. Common examples of first-order reactions include radioactive decay and some enzyme-catalyzed reactions.
5. Graphing ln[A] versus time produces a straight line for first-order reactions, confirming their order through linear regression analysis.

Review Questions

• How does the relationship between concentration and reaction rate in first-order reactions facilitate understanding their kinetics?
  • In first-order reactions, the direct proportionality between concentration and reaction rate simplifies the analysis of kinetics. This means that as you change the concentration of the reactant, you can predict how the rate will change. This clear relationship helps chemists develop models and equations that accurately describe the behavior of these reactions over time.
• What role does the half-life play in characterizing first-order reactions compared to other types of reactions?
  • The half-life in first-order reactions is a crucial metric because it remains constant regardless of initial concentration, which is not true for zero or second-order reactions. This consistent half-life allows for easier predictions about how long it will take for a certain percentage of reactants to convert into products. In contrast, in other reaction orders, half-lives vary significantly with changes in concentration, complicating their analysis.
• Evaluate how integrated rate laws provide insights into the progression of first-order reactions over time and their implications in real-world applications.
  • Integrated rate laws offer a mathematical framework to quantify how concentrations change throughout a first-order reaction. By utilizing these laws, scientists can predict how long it will take for reactants to diminish to specific levels or calculate remaining quantities at any point in time. This capability has practical implications in various fields, such as pharmacokinetics where understanding drug degradation or elimination rates can significantly influence dosage and treatment plans.

Related terms

Rate constant: A coefficient that links the reaction rate with the concentration of reactants in a rate law, unique for each reaction at a given temperature.

Half-life: The time required for the concentration of a reactant to decrease to half its initial value in a chemical reaction.

Integrated rate law: An equation that relates the concentrations of reactants and products over time, providing insight into the progression of a reaction.