Chemical kinetics is concerned with the rates of chemical reactions, that is, whether reactions proceed quickly or slowly. As we have already mentioned, some spontaneousCapable of proceeding without an outside source of energy; refers to a reaction in which the products are thermodynamically favored (product-favored reaction). reactions are extremely slow. An example is the Haber-process synthesisFormation of substances with more complicated sturctures than do their precursors. of ammonia:
- N2 + 3H2 → 2NH3 ΔGm°(298 K) = – 33.27 kJ mol–1
Even though a negative ΔGm° predicts that this reaction can occur at room temperatureA physical property that indicates whether one object can transfer thermal energy to another object., it is of little value unless chemists can find some way to speed it up. On the other hand we often want to slow down undesirable reactions, such as spoilage of food or decomposition of wood. Hence it is quite useful to know how factors such as temperature, concentrations of reactants and products, and catalysts will affect the rates of reactions. Moreover, studying these factors gives valuable information about the sequence of microscopic events by which a reaction occurs. Knowledge of when and where bonds are formed and broken as well as how molecular structures change during a reaction can be very useful in helping us to devise ways to speed up or slow down that reaction.
Chemical kinetics is concerned with the rates of chemical reactions, the dependence of those rates on temperature, concentration, and catalysts, and the microscopic mechanisms by which reactions occur. The rate of a reaction is defined in terms of the change in concentration of a reactant or product per unitA particular measure of a physical quantity that is used to express the magnitude of the physical quantity; for example, the meter is the unit of the physical quantity, length. time, and it usually decreases as the reaction progresses. The reaction rate ordinarily is proportional to the concentrations of reactants and/or products, each raised to a power called the order with respect to that reactant or product. When the concentration of a species which is not a reactant or product in the overall reaction affects the rate, that species is called a catalyst. An equation expressing the dependence of reaction rate on concentrations is called a rate equation or rate lawAn equation which describes the rate of a reaction as a function of the rate constant and the concentrations of reactants (and any other substances that affect the rate, such as products or catalysts); also called rate equation..
On a microscopic level, a reaction usually involves unimolecular processes, in which a single moleculeA set of atoms joined by covalent bonds and having no net charge. changes structure, or bimolecular processes, in which two molecules collide. One determinant of rate in this situation is frequencyThe rate at which a periodic event occurs; specifically, the rate at which the waves of electromagnetic radiation pass a point. of collisions as seen in the animation on this page. Collision of three or more molecules simultaneously is much less probable. In both uni- and bimolecular elementary processes an activation energyThe energy barrier over which a reaction must progress in order for reactants to form products; the minimum energy that reactants must have if they are to be converted to products. barrier must be surmounted before product molecules can be produced. The species at the top of a graph of energy versus reaction coordinate is called the activated complexIn the mechanism of a reaction, a species that lies at an energy peak and that can change either into products or into reactants; also called a transition state. or transition state. Because of this energy barrier only a small fraction of the molecules are energetic enough to reach the transition state, but that fraction increases rapidly as temperature increases, and so reaction rates are strongly dependent on temperature. The temperature dependence of the rate constantIn a differential rate equation, the proportionality constant that relates the rate with the concentrations of reactants and other species that affect the rate. The rate constant is the rate of reaction when all concentrations are 1 M. can be used to obtain the activation energy by means of an Arrhenius plot.
This animation displays the number of collisions that occur between the dark blue particle and a set of lighter blue particles. When hit, a lighter blue particle changes to black. A second hit changes a black particle to a red particle, so that the number of collisions can be easily counted. The frequency at which the dark blue particles hits light blue particles is an important determinant of how quickly a reaction will go.
Most reactions occur in two or more steps. Such a sequence of elementary processes is called a reaction mechanism, and the overall rate is determined by the slowest, or rate-limiting, step. The experimental rate law tells us the composition of the activated complex for the rate-limiting stepThe step in a reaction mechanism that by its relatively slow rate limits the overall rate of a reaction; also called rate-determining step., but often several mechanisms are possible which agree with the rate law. Other evidence must then be used to decide among these mechanisms.
A catalyst speeds up a reaction by changing the mechanism so that the activation energy is lowered. Many heterogeneous catalysts are of great industrial importance, but the most efficient catalysts known are the enzymes in living organisms. An enzyme operates by adsorbing a substrateThe molecule or ion that is a reactant in an enzyme-catalyzed reaction. molecule at an active siteThe location where catalysis occurs in an enzyme or other catalyst. whose structure is exactly right to stretch bonds which are to be broken or to hold atomsThe smallest particle of an element that can be involved in chemical combination with another element; an atom consists of protons and neutrons in a tiny, very dense nucleus, surrounded by electrons, which occupy most of its volume. in position while new bonds form. This almost ideal structure of the active site makes enzymes highly specific and extremely efficient catalysts.