Chapter 3. Rate Laws
Success is measured not so much by the position one has reached in life, as by the obstacles one has overcome while trying to succeed.
—Booker T. Washington
3.1 Basic Definitions
A homogeneous reaction is one that involves only one phase. A heterogeneous reaction involves more than one phase, and the reaction usually occurs at the interface between the phases. An irreversible reaction is one that proceeds in only one direction and continues in that direction until one of the reactants is exhausted. A reversible reaction, on the other hand, can proceed in either direction, depending on the concentrations of reactants and products relative to the corresponding equilibrium concentrations. An irreversible reaction behaves as if no equilibrium condition exists. Strictly speaking, no chemical reaction is completely irreversible. However, for many reactions, the equilibrium point lies so far to the product side that these reactions are treated as irreversible reactions.
The molecularity of a reaction is the number of atoms, ions, or molecules involved (colliding) in a reaction step. The terms unimolecular, bimolecular, and termolecular refer to reactions involving, respectively, one, two, or three atoms (or molecules) interacting or colliding in any one reaction step. The most common example of a unimolecular reaction is radioactive decay, such as the spontaneous emission of an alpha particle from uranium-238 to give thorium and helium:
92U238 →90Th234+2He4
The rate of disappearance of uranium (U) is given by the rate law
–rU = kCU
The only true bimolecular reactions are those that involve the collision with free radicals (i.e., unpaired electrons, e.g., Br•), such as
Br• + C2H6 → HBr +C2H5•
with the rate of disappearance of bromine given by the rate law
–rBr· = kCBr·CC2H6
The probability of a termolecular reaction, where 3 molecules collide all at once, is almost nonexistent, and in most instances the reaction pathway follows a series of bimolecular reactions, as in the case of the reaction
2NO + O2 → 2NO2
The reaction pathway for this “Hall of Fame” reaction is quite interesting and is discussed in Chapter 9, along with similar reactions that form active intermediate complexes in their reaction pathways.