Rate Equations

• Rate equations express the rate of a reaction in terms of the concentrations of the reactants. Each reactant concentration is raised to the power of its order in the reaction.
• Zero-order reactions have a constant rate. This means the rate is uninfluenced by the concentration of any reactants.
• First-order reactions directly depend on the concentration of one reactant. If the concentration is doubled, the rate also doubles. The unit of rate constant (k) in this case is s-1.
• Second-order reactions have rates proportional to the square of the concentration of one reactant, or to the product of two reactant concentrations. In this scenario, if concentration is doubled, the reaction rate quadruples. The unit of rate constant (k) is M-1 s-1.
• Overall order of a reaction is the sum of the orders of all the reactants in the rate equation.
• The rate constant, k, is the proportionality constant in the rate equation. Its value depends on temperature and the presence of a catalyst.
• A large value of k corresponds to a fast reaction and small value of k corresponds to a slow reaction.
• The half-life (t) of a first-order reaction is a constant. It is given by t = 0.693/k.
• Rate equations can be determined by a series of experiments and observing the effect of changing concentrations on the rate of reaction.
• Carry out initial rates method by running several experiments with different initial concentrations of reactants, keeping all other variables constant.
• The order with respect to each reactant is found by comparing the initial rates of reaction. Determine the orders by seeing how much the rate changes when concentration changes.
• Always carry rate experiments at constant temperature. Changes in temperature can affect the speed of reaction by changing the rate constant.
• The Arrhenius equation links the rate constant, k, to the activation energy of the reaction and the temperature at which it is carried out.
• The Arrhenius equation is commonly rearranged to a linear form to plot graphs from which activation energy can be calculated.
• The Maxwell-Boltzmann distribution explains the concept of activation energy in terms of the distribution of molecular energies in a gas. The area under the curve equals the total number of molecules, and the area to the right of the activation energy represents the proportion of molecules with sufficient energy to react.