The study and control of chemical reaction rates are the subjects of chemical kinetics. The rate of a chemical reaction depends on the nature of the reaction and on variables of temperature and pressure, pH and reactant concentrations. The presence of substances called catalysts also has major effects on the rates of chemical reactions. Catalysts are often important, or even crucial, because many reactions occur too slowly in their absence to be of benefit or practical use.
Rates of Chemical Reactions
The rate of a chemical reaction is the change in concentration of a reactant or of a product of the reaction, per unit change of time. Because chemicals react, and products form, in molar ratios specific to a reaction, the rate of a reaction must be specified in terms of individual reactants and products. For example, in the reaction: C3H8 + 3H2O = 3CO + 7H2, H2 (hydrogen gas) is formed at seven times the rate at which C3H8 (propane) is consumed.
A catalyst is a substance that increases the rate of a reaction but is not itself consumed in the reaction so that the initial and final concentrations of the catalyst are equal. The process of catalysis lowers the energy required for chemical reactions to occur. Reactions then proceed much faster, in some cases by many orders of magnitude. Catalysts do not change the equilibrium position of a reaction. Catalysts are of two types: homogeneous and heterogeneous.
A homogeneous catalyst is in the same phase as the reactants, usually either the liquid or the gas phase. An example is the decomposition of hydrogen peroxide, H2O2. The decomposition occurs in solution by a two-step process, using an iodide ion (I-) as a catalyst: H2O2 + I- = IO- + H2O; H2O2 + IO- = H2O + O2 + I-. The overall reaction is obtained by adding the two reactions together, with the result: 2H2O2 = 2H2O + O2. Iodide is consumed in the first reaction and regenerated in the second; therefore, it does not appear in the overall reaction.
A heterogeneous catalyst is in a different phase than the reactants, the catalyst frequently being a solid that provides a surface for arranging the reactants in a way that favors reaction. For example, ethylene (C2H4) reacts with hydrogen (H2), using solid platinum (Pt) as a catalyst, forming ethane, by the reaction: C2H4 + H2 + Pt = C2H6. H2 molecules are adsorbed onto the Pt surface, where they dissociate into hydrogen (H) atoms. The H atoms then move over the Pt surface, where they encounter adsorbed C2H4 molecules. The reaction is rapid because the energy required to break the H-H bond has already been expended.
Life would be impossible without organic catalysts called enzymes, which are large protein molecules. More than 2,000 enzymes are known, and every biochemical reaction is catalyzed by one specific enzyme. An example is “peptidase,” an enzyme that breaks peptide bonds in proteins. Peptidase does not work with starch, which is broken down by “amylase” in the mouth. Enzymes function by enfolding reactants into their complex structures, bringing the reactants close together and into a geometry ideal for reaction.