654 crp:ative chemistry 10. What conclusions may be based on the following experimental data? Moistened iron filings rust when confined in a closed vessel with air, the volume of the air being decreased in the process. The air remaining does not support combustion or life. 11. What is combustion? Give two or three examples. What discovery led to an understanding of the true nature of combustion? 12. What is spontaneous combustion? Give some examples. How would you demonstrate it? 13. Combustion is the opposite of photosynthesis. Explain what is meant by this statement. 14. What is the nature of a flame? 15. Discuss carbon monoxide as to (a) conditions which produce it, (b) its properties, and (c) why it is dangerous. 16. Discuss the composition of safety matches. 17. Discuss the composition of "strike anywhere" matches. 18. Discuss the principle employed in fireworks. 19. Discuss fire extinguishers as to (a) the principle of each type, and (b) the conditions for which each type is best suited. 20. Give a general rule for the effect of an increase in temperature on the speed of a chemical reaction. 21. What is the nature of an explosion? 22. What materials are used in the foamite type of fire extinguisher? 23. Prepare a list of applications to which glass fabrics are best suited.
UNIT IX SECTION 4 AIR AND WATER ARE TRANSFORMED INTO MANY USEFUL MATERIALS BY SYNTHESIS IN THE PRESENCE OF CATALYSTS When man tries to prepare compounds found in nature, he often finds it necessary to utiHze very high temperatures or pressures; but it is typical of nature that its reactions take place relatively rapidly at ordinary temperatures and pressures. The secret of chemical reactions in nature seems to lie in catalysis. A catalyst is a substance which needs to be present in only very small amounts either to activate molecules in such a way that reactions which would otherwise take place so slowly as to give no appreciable results take place quite rapidly or, less frequently, to produce a marked decrease in the rate of a chemical reaction. It is also characteristic of catalysis that the catalyst does not become a part of the products of the reaction. The study of many processes taking place in nature led scientists to the conclusion that nearly every chemical change can be catalyzed. Some of the greatest developments of modern chemistry are the results of attempts to find the proper catalysts for desired reactions. Synthesis is the building of more complex substances from simpler ones. The preparation of water from oxygen and hydrogen is an example of synthesis. In many cases desired syntheses will take place only in the presence of the proper catalysts. The preparation of high explosives and fertilizers from air and water are typical examples of processes which involve the use of catalysts. Catalysts Are Effective in Very Small Concentrations. It is customary to prepare oxygen in the laboratory by heating potassium chlorate, KCIO3. The addition of a small amount of manganese dioxide, Mn02, will cause this reaction to take place much more rapidly and at a lower temperature; in this case the manganese dioxide is the catalyst. The more of the catalyst that is present, the more rapid will be a given chemical reaction, but many reactions are noticeably accelerated by astonishingly small quantities of catalysts. 655