Kinetics

• Orders of reaction are used to relate the rate of a reaction to the reacting species.
• If changing the concentration of a reactant A has no effect on the rate of the reaction, then the reaction is zero order with respect to A.
• If doubling the concentration of a reactant A doubles the rate of the reaction, then the reaction is first order with respect to A. The rate can be expressed as:

  rate = k [A] where k is the rate constant and [A] is the concentration of reactant A in mol l -1
• If doubling the concentration of a reactant A increases the rate of the reaction fourfold, then the reaction is second order with respect to A. The rate can be expressed as:

  rate = k [A]^2
• The order of a reaction with respect to any one reactant is the power to which the concentration of that reactant is raised in the rate equation.
• The overall order of a reaction is the sum of the powers to which the concentrations of the reactants are raised in the rate equation.
• The order of a reaction can only be determined from experimental data.
• The rate equation and the rate constant, including units, can be determined from initial rate data for a series of reactions in which the initial concentrations of reactants are varied. These can be zero, first, second or third order.
• Reactions usually occur by a series of steps called a reaction mechanism.
• The rate of reaction is dependent on the slowest step, which is called the ‘rate determining step’.
• Experimentally determined rate equations can be used to determine possible reaction mechanisms.
• The rate of a chemical reaction normally depends on the concentrations of the reactants