Rates of Reaction

Understanding Rates of Reaction

• A reaction rate refers to the speed at which reactants are converted into products.
• It’s measured by the change in concentration of a reactant or a product over a specific period of time.
• Reaction rates can be influenced by various factors, including temperature, concentration of reactants, particle size, and the presence of a catalyst.

Factors Affecting Reaction Rates

• Temperature: Increasing the temperature increases the kinetic energy of the particles, leading to more frequent and energetic collisions, thereby accelerating the reaction rate.
• Concentration of reactants: In a higher concentration, particles are closer together, leading to more collisions per unit time, thus increasing the reaction rate.
• Particle size: Smaller particles have a larger surface area to volume ratio, leading to more collisions and a faster reaction rate.
• Presence of a catalyst: Catalysts lower the activation energy needed for a reaction, leading to an increased reaction rate without being consumed in the process.

Rate Equations and Orders of Reaction

• The rate equation shows the relationship between the rate of a reaction and the concentrations of the reactants. It takes the form rate = k[A]m[B]n, where k is the rate constant, [A] and [B] are concentrations of reactants, and m and n represent the order of reaction with respect to the individual reactants.
• The overall order of a reaction is the sum of the individual orders (m + n in the above equation).
• Zero order reactions have constant rates which are unaffected by the concentrations of reactants.
• First order reactions have rates that are directly proportional to the concentration of one of the reactants.
• Second order reactions have rates that are either proportional to the concentration of two different first order reactants, or proportional to the square of the concentration of one reactant.

Activation Energy and the Collision Theory

• Activation energy is the minimum energy required for a reaction to occur when particles collide. Only a fraction of collisions have this energy, illustrated by Maxwell-Boltzmann distribution curves.
• The Collision Theory states that for a reaction to occur, particles must collide with sufficient energy (at least equal to the activation energy) and an appropriate orientation.
• The presence of a catalyst provides an alternate reaction pathway with a lower activation energy, increasing the proportion of successful collisions and thus increasing the reaction rate.

Rate-Determining Step

• In a multi-step reaction, the rate-determining step is the slowest step that determines the rate of the overall reaction.
• The rate equation is determined by the concentration of the reactants involved in the rate-determining step.