Rate law

Definition

Rate law is an equation that relates the rate of a chemical reaction to the concentration of its reactants. This concept is critical for understanding how different factors affect the speed of a reaction, as it shows how the rate is influenced by the concentrations of the reactants raised to a power, known as the order of the reaction. Rate laws help in determining the mechanism of a reaction and how equilibrium is achieved as reactions proceed forward and reverse.

5 Must Know Facts For Your Next Test

1. The rate law is typically expressed as: rate = k[A]^m[B]^n, where k is the rate constant, [A] and [B] are the concentrations of reactants, and m and n represent their respective orders.
2. The overall order of a reaction is the sum of the individual orders with respect to each reactant in the rate law.
3. Rate laws can be determined experimentally by measuring the change in concentration of reactants or products over time.
4. In some reactions, a catalyst can change the rate law by altering the value of the rate constant k without changing the overall stoichiometry.
5. Rate laws are essential for predicting how changes in concentration or temperature will affect reaction rates and help in understanding dynamic equilibria.

Review Questions

• How does the rate law help in understanding reaction mechanisms?
  • The rate law provides insights into how different reactants contribute to the overall speed of a reaction. By analyzing the order of each reactant within the rate law, chemists can infer possible elementary steps in the reaction mechanism. This understanding allows for predictions on how altering reactant concentrations will impact the rate, helping to piece together a more detailed picture of how complex reactions occur.
• Discuss the significance of determining the rate constant 'k' in relation to temperature changes.
  • The rate constant 'k' is crucial because it not only influences the rate of reaction but also varies with temperature according to the Arrhenius equation. By determining 'k' at different temperatures, one can understand how thermal energy affects molecular interactions and transition states. This knowledge helps chemists manipulate conditions to optimize reaction rates for desired outcomes or industrial applications.
• Evaluate how changes in concentration affect equilibrium based on the principles established by rate laws.
  • Changes in concentration can shift the position of equilibrium according to Le Chatelier's principle. The rate law shows that as concentrations of reactants increase, the rate of forward reaction typically increases, leading to more products being formed. However, if products accumulate, this can also increase the reverse reaction rate. Understanding these dynamics through rate laws enables chemists to predict and control shifts in equilibrium effectively.

Related terms

Reaction Rate: The speed at which reactants are converted into products in a chemical reaction, usually expressed as a change in concentration over time.

Equilibrium Constant: A numerical value that expresses the ratio of concentrations of products to reactants at equilibrium, providing insight into the position of equilibrium in a reversible reaction.

Order of Reaction: The exponent to which the concentration of a reactant is raised in the rate law, indicating how the rate of reaction depends on that reactant's concentration.