Reaction Rates

The Basics of Reaction Rates

  • Reaction rate refers to how fast or slow a reaction proceeds.
  • The speed of a reaction is measured by the change in concentration of a reactant or product per unit time.
  • Reaction rates can change based on the concentration of reactants, temperature, surface area, presence of a catalyst, and the nature of reactants and products.

Rate Equations

  • The rate equation links the rate of reaction to the concentrations of the reactants.
  • It’s usually in the form: Rate = k[A]^m[B]^n
  • In the equation, k is the rate constant, [A] and [B] are the concentrations of the reactants, and m and n are the orders of reaction.
  • The overall order of a reaction is the sum of the individual orders (m + n).
  • The rate constant, k, changes with temperature.

Determining Orders of Reaction

  • Orders of reaction are determined experimentally by observing how the rate changes when the concentration of one reactant is altered, keeping others constant.
  • Order can be zero, first or second. A zero order reaction doesn’t depend on the concentration of the reactant, a first order reaction depends on the concentration to the power of 1, while a second order reaction depends on the concentration squared.
  • Zero order: the rate doesn’t change as you change the concentration of the reactant.
  • First order: the rate doubles as the concentration is doubled.
  • Second order: the rate quadruples as the concentration is doubled.

The Arrhenius Equation

  • The Arrhenius equation links the rate constant (k) to the temperature of the reaction and the activation energy (Ea).
  • The equation is: k = Ae^(-Ea/RT)
  • In the equation, A is the pre-exponential factor or frequency factor which is related to the collision frequency of the reactants, Ea is the activation energy, R is the gas constant and T is the Absolute temperature.

Catalysts and Reaction Rates

  • A catalyst is a substance that increases the rate of a chemical reaction by providing an alternative reaction pathway with a lower activation energy.
  • Catalysts remain chemically unchanged by the end of the reaction. They can be reused.
  • In the reaction between ozone and nitrogen dioxide, the nitrogen dioxide acts as a catalyst and speeds up the rate of reaction.

Relevance to Ozone Chemistry

  • Understanding reaction rates and the factors that affect them is crucial in understanding how atmospheric ozone is formed and degraded.
  • Reactions in the ozone layer, like the reaction of ozone with nitrogen dioxide, happen in three steps - initiation, propagation and termination. In each of these steps, reaction rates play a key role.
  • Knowing and understanding these rates is crucial for modelling ozone loss and for developing effective strategies to protect the ozone layer.