of a chemical reaction, r
, quoted in units of mol dm-3
, is given by the expression
r = k [A]0 [B]1 [C]2
where [A], [B] and [C] are the concentrations of the reactants in mol dm-3 and k is the rate constant.
For example, for the reaction between propanone (acetone) and iodine in the presence of hydrogen ions, the equation is
r = k [CH3COCH3]1 [I2]0 [H+] 1
If the concentration of a reactant is raised to the power 0 (making it equal to 1), it has no effect on the rate - it is zero order. It does not really have to be included in the equation at all. If it raised to the power 1 (i.e. not changed at all), it affects the rate and is first order. If it raised to the power 2 (i.e. squared), it has an even stronger effect on the rate and is second order.
Zero order reactants decrease in concentration at a constant rate throughout a reaction. First order reactants decrease at a rate that continually slows, with a constant half-life. Second order reactants decrease at a rate that continually slows and has an increasing half-life.
The order of a reactant is related to its role in the actual chemical process. Reactions take place in steps, forming intermediate compounds. The slowest step determines the overall rate of the reaction and is indeed known as the rate determining step. Only those reactants which are involved in or before this step will appear in the rate equation.
Reactant-orders cannot be predicted theoretically; we must determine them by experiment.
The rate constant, k, depends on temperature and on the orientation of the molecules involved and, for many reactions, is approximately given by the Arrhenius equation:
k = A e-EA/RT
where A is a pre-exponential factor relating to molecular orientation, EA is the activation energy of the reaction, R is the universal gas constant and T is the temperature. e, of course, is just e. As the temperature increases, EA/RT decreases, so e is being raised to a less negative number. Thus k increases, so the reaction goes faster.
With thanks to the editors for their patience.