Reaction rate constant

Definition

The reaction rate constant is a proportionality factor in the rate law equation that relates the rate of a chemical reaction to the concentration of the reactants. This constant is specific to a particular reaction at a given temperature and helps determine how quickly the reactants are converted into products. Its value is influenced by factors such as temperature and catalysts, making it essential for understanding reaction kinetics.

5 Must Know Facts For Your Next Test

1. The reaction rate constant (k) has units that depend on the overall order of the reaction, such as M^-1s^-1 for second-order reactions or s^-1 for first-order reactions.
2. The value of k increases with rising temperature due to increased kinetic energy among molecules, leading to more frequent and effective collisions.
3. In reactions involving catalysts, the presence of a catalyst lowers the activation energy, resulting in a higher reaction rate constant.
4. The reaction rate constant is not influenced by the concentration of reactants; instead, it is intrinsic to the reaction itself under specific conditions.
5. For elementary reactions, the order of the reaction corresponds directly to the stoichiometric coefficients in the balanced equation, affecting how k is determined.

Review Questions

• How does temperature affect the value of the reaction rate constant?
  • Temperature has a significant impact on the reaction rate constant because an increase in temperature raises the kinetic energy of molecules. As molecules move faster, they collide more frequently and with greater energy, making it easier for them to overcome activation energy barriers. This generally results in an increased value of the reaction rate constant (k), indicating that reactions proceed at a faster rate at higher temperatures.
• Compare and contrast how the reaction rate constant differs in first-order versus second-order reactions.
  • In first-order reactions, the reaction rate constant (k) has units of s^-1, meaning that the rate depends linearly on only one reactant's concentration. In contrast, second-order reactions have a reaction rate constant with units of M^-1s^-1, indicating that their rates depend on the concentrations of either two reactants or one reactant squared. This difference in order affects how we calculate k and how changes in concentration influence reaction rates.
• Evaluate how catalysts affect both the activation energy and reaction rate constant in chemical reactions.
  • Catalysts play a crucial role by lowering the activation energy required for a chemical reaction to occur. By providing an alternative pathway for the reaction, catalysts increase the frequency of successful collisions between reactants. As a result, this leads to a higher reaction rate constant (k), reflecting an increased rate of reaction without being consumed in the process. This catalytic effect demonstrates how external factors can modify fundamental kinetic properties of reactions.

Related terms

Rate Law: An equation that expresses the relationship between the rate of a chemical reaction and the concentration of its reactants, often in the form of rate = k[A]^m[B]^n.

Activation Energy:

The minimum amount of energy required for reactants to undergo a chemical reaction, which affects the rate constant and overall reaction rate.

Order of Reaction:

The sum of the powers of the concentration terms in the rate law, indicating how the rate depends on the concentrations of reactants.