Demonstrations for Chapter 14: Chemical Kinetics

92. Iodine Clock Reaction is one of the classic illustrations of the time rate of change of concentration of reactants characteristic of chemical reactions. Here, a product of the reaction is elemental iodine which forms a deep blue complex in the presence of starch and acts as the visual indicator.

93. Rates of chemical reactions depend on the concentrations of the reacting materials. In this demonstration, the rate of disappearance of the highly colored Fe(SCN)3 (thiocyanate complex), formed by reacting ammonium thiocyanate (NH4SCN) with Fe(III) chloride, on mixing with Sn(II) chloride in aqueous solution. Two different concentrations of the thiocyanate are tested simultaneously.

94. Temperature dependence of the reaction (above) will be demonstrated.

95. Particle size plays a significant role in chemical reactions involving solids and/or liquids... especially viscous liquids. This demonstrations involves bringing together solid potassium permanganate, a powerful oxidizing agent, and glycerol (glycerin), an organic substance capable of being oxidized. Dust explosions provide other examples of the effects of particle size: coal dust and grain elevators - lycopodium powder: the spores of this common moss do not easily support combustion as a lump, but a dust is explosive.

96. Kinetic versus Thermodynamic Control. One reaction of special interest because of its importance to organic chemistry is the dehydration reaction. Sulfuric acid is a powerful dehydrating agent, removing water wherever it finds it. Here, the elements of water are removed from adjacent carbon atoms in sugar molecules, leaving a char of essential coal... "charcoal." Classroom discussion of kinetic control versus thermodynamic control will focus around the intermolecular dehydration of ethanol to yield diethyl ether (<140°C) and the intramolecular dehydration reaction to yield ethylene and water (>180°C):

C2H5OH CH2=CH2 + H2O          (favored at higher T)
2 C2H5OH C2H5OC2H5 + H2O        (favored at lower T)

97. Hydrogen Peroxide Decomposition. Peroxides are highly energetic combinations of atoms containing the -O-O- linkage. Most are very unstable and decompose, on initiation, very rapidly and with considerable release of energy. The decomposition of 30% solutions of hydrogen peroxide, a powerful bleaching (oxidizing) agent is demonstrated. Here the reaction is initiated in different ways: with finely divided particle of MnO2; with Fe(III).

98. Organic peroxides, organic molecules containing the same -O-O- linkage are also highly reactive, powerful oxidizing agents. In this reaction, cumene hydroperoxide, a reaction intermediate in the commercial synthesis of phenol and acetone, two important industrial chemicals used for everything from pharmaceuticals to polymers, is decomposed in the presence of a trace of acid (which is the initiator).

99. Oscillating reaction. Illustrates the complex mechanisms which often prevail in chemical reactions. A brief discussion of the Belousov-Zhabotinskii reaction, the most studied of thesde oscillating reactions, beghins with the net reaction, the oxidation of malonic acid by bromate ion:

2 H+ + 2 BrO3- + 3 CH2(COOH)2 2 BrCH(COOH)2 + 3 CO2 + 4 H2O

The 3-step cycle involves catalysis by cerium ions in a redox couple (which will be described in a later discussion of electrochemistry and redox reactions) looks something like this:

(1) BrO3- + 2 Br- + 3 CH2(COOH)2 + 3H+ 3 BrCH(COOH)2 + 3H2O
(2) BrO3- + 4 Ce3+ + 5 H+ HOBr + 4 Ce4+ + 2 H2O
(3) BrCH(COOH)2 + 4 Ce4+ + H2O + HOBr 2 Br- + 3 CO2 + 4 Ce3+ + 6 H+

Note as well

(4) BrCH(COOH)2 + H2O HOBr + CH2(COOH)2

100. Catalysts are of two kinds: accelerators and inhibitors. However, the word "catalyst" has taken on the general meaning of ... substances that speed up chemical reactions. Their action is associated with alterations in the pathway from reactants to products thereby changing (lowering, in the case of speeding up the reaction)the activation energy (or barrier to formation of reaction products. In this case, the reaction is the silver ion catalyzed oxidation of Mn(II) ions in the presence of S2O82- ions.

101. Catalytic Oxidation. Oxidation of ammonia on a platinum surface which serves as a catalyst is an illustration of important industrial processes involved in the formation of nitric acid, for example. Here the chemistry produces dinitrogen monoxide as the principal nitrogen oxide product.